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UniversiV Micronlms Intemationcil Q u i n n , Ja m e s ali e n
POWDERY MILDEW OF BEGONIA
The Ohio State University
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Universi^ Micronlms intemarioncii 300 N Z555 AD., AN N ARBOP VII J8106'313) 761-4700 POWDERY MILDEW OF BEGONIA
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy in the Graduate School
of The Ohio State University
by
James A. Quinn, B.S., M.S.
The Ohio State University
1980
Reading Committee: Approved By
M.O. Garraway C.R. Krause C.C. Powell Jr. t'L-1 cPmM Charles C. Powell Jr. Department of Plant Pathology ACKNOWLEDGEMENTS
I would like to thank my adviser, Dr. Charles C. Powell Jr.,
for suggesting this problem to me. His suggestions in the course
of these experiments were helpful and perceptive. The original
ideas, if not the procedures for the host range and chemical
control studies, were his. I would also like to thank the members
of my advisory committee, Drs. M.O. Garraway, C.R. Krause, Curt
Leben and R.A. Spotts. Dr. Garraway was always available
for a long, if one-sided chat, often putting aside his own work to discuss mine. Dr. Krause generously lent me full use of
his labs and assisted me with photography and microscopy. I would also like to thank Jim Mikkelsen and Mikkelsen's Incorporated
and The Ohio Florist's Association for the financial support that was a necessary ingredient of this study. Mikkelsen's Inc. also
supplied all Rieger begonias used in these experiments free of
charge. Lastly, thanks to all those whose friendship and support made my stay here at The Ohio State University so enjoyable,
not the least among whom is my wife Millie Quinn. March 29, 1954 Born Gary, Indiana
June, 1972 Graduated, Springfield North High School, Springfield, Ohio.
December, 1975 Graduated, B.S. cum laude, Ohio Uni versity, Athens, Ohio.
January 1976-August 1977, Teaching Assistant in Botany, Ohio University, Athens.
March, 1978 M.S., Ohio University.
January 1978-August 1980, Research Associate, The Ohio State University.
August, 1980 Ph.D., The Ohio State University
Fields of Study
Major Field: Plant Pathology
Studies in mycology and electron microscopy. Dr. J.P. Braselton, Ohio University.
Studies in epidemiology and chemical control with Dr. Charles C, Powell Jr., The Ohio State University. TABLE OF CONTENTS
Acknowledgements...... Vita ...... iii List of Tables...... vil List of Figures...... %iii Introduction...... 1 Chapter one. The Host 5 History and origin of Rieger begonias...... 5 Culture of Rieger begonias...... 6 A key to disorders on begonias...... 10 Chapter two. The Pathogen 14 Identification...... 14 Life Cycle...... 17 Double Petri plates...... 18 Chapter three. Host-Parasite Interactions 28 Intrageneric host range studies...... 28 Intergeneric host range and resistance of begonias to 0. begoniae...... 30 Factors affecting race formation in powdery mildews.... 44 Other host-pathogen interactions...... 46 Chapter four. Effects of the Environment 52 Literature Review...... 52 Effects of temperature on 0. begoniae...... 72 Effects of temperature on germination and appressorium formation on glass over time...... 76 Shriveling of conidia over time on glass...... ^9 Effects of temperature on haustorium formation...... 86 Growth of hyphae as effected by temperature and humidity. 90 Effects of temperature and relative humidity on amount of sporulation...... 95 Effects of relative humidity on conidial characteristics of conidia still attached to conidiophores...... 99 Effect of temperature and relative humidity on number of visible colonies of 0 . begoniae...... 102 Prevention of begonia mildew on whole plants by 28C...... 106 Eradication of Oidium begoniae using heat...... 108 Economics of heat eradication of 0. begoniae...... 130 Effects of light on 0. begoniae...... 132 Use of light induced synchronous morphogenesis of Oidium begoniae and of heat to prevent powdery mildew infections...... 143 Effects of free water on Oidium begoniae conidia...... 146
iv Effects of drought stress on 0. begoniae infections 150 Effects of nutrition on number of colonies of Oidium begoniae...... 152 General Summary: Prevention and eradication of Oidium begoniae using environmental manipulations...... 157 Chapter five. Chemical Control 165 Chemical control of powdery mildews, history and impor tance ...... 165 Summary of names, formulas, use and efficacy of fungi cides used against powdery mildews...... 166 Resistance to fungicides...... 181 Control of powdery mildews by dormant season sprays of surfactants...... 182 Chemical control of powdery mildew of begonias, intro duction...... 182 Prevention of powdery mildew of Rieger begonias with new systemic fungicides, 1978...... 185 Prevention of powdery mildew of Rieger begonias with new systemic fungicides, 1979...... 190 Prevention of powdery mildew of Rieger begonias with triademefon soil drenches 1979-1980...... 193 Prevention of powdery mildew of Rieger begonias with chemicals found in the home and with Exhalt 800...... 197 Eradication of powdery mildew of Rieger begonias with new systemic fungicides, 1978...... 200 Eradication of powdery mildew of Rieger begonias with new systemic fungicides, 1979...... 202 Prevention and eradication of powdery mildew of Rieger begonias with volatilized systemic fungicides, 1979..204 Effects of fungicides on fungal morphology...... 208 Chapter six. Biological Control and gynecology of Powdery Mildew of Begonia 231 Introduction...... 231 Determination of genera and relative numbers of fungi found on begonia leaves in the greenhouse and in double Petri plates...... 233 Effects of fungicides on begonia phylloplane fungi.... 242 Effect of temperature on growth of phylloplane fungi on cornmeal agar...... 250 Efficacy of some phylloplane fungi against 0. begoniae.252 Chapter seven. Epidemiological Modeling and Integrative Control of Oidium begoniae on Begonia 257 Comparison of r values...... 258 Eradication of infections with temperatures and fungicides...... 264 Integrated Control of Oidium begoniae...... 267 The future: A new model proposed for calculating infection rates from growth chamber studies...... 270 Appendix A Physiological Model of concrol of 0. begoniae 275
List of References...... 283 LIST OF TABLES
Table 1. Percent germination and appressorium formation of Oidium begoniae conidia on glass slides at 21C over time 20
Table 2. Host range of begonia powdery mildew and suscepti bility of B. X hiemalis 'Schwabenland Red to pow dery mildews from other hosts 30
Table 3. Intraspecific host range of 0 idium begoniae and general information on begonias 34
Table 4. Penetration by Oidium begoniae on selected species of begonia 3b
Table 5. Shriveling and encapsulation of haustoria of Oidium begoniae associated with selected cultivars of begonia after one week and compared with hyphal length 38
Table 6. Growth of 0 id ium begoniae in micrometers after 7 days on selected species of begonia at various temperatures 41
Table 7. Interactions between some host dependent factors and infection of Rieger begonias by Oidium begoniae. 40
Table 8. Cardinal temperatures for some powdery mildews 57
Table 9. Cardinal relative humidities for germination of conidia of powdery mildews 59
Effect of proportions of three salts on development of E. graminis on wheat seedlings 68
Germination of conidia of 0. begoniae on glass slides at different temperatures over time 77
Appressorium formation of conidia of 0. begoniae on glass slides at different temperatures over time 77 Table 13. Percent shriveling of conidia of Oidium begoniae at various temperatures over time at 100% relative humidity on glass slides. 80
Table 14. Percent shriveling of conidia of Oidium begoniae at various temperatures over time at 60% relative humidity on glass slides. 80
Table 15. Percent conidia germinated, with appressoria or not shriveled on glass slides at selected temperatures and relative humidities after 24h. 84
Table 16. Percent conidia germinated, with appressoria or not shriveled on excised B. x hiemalis leaves at selected temperatures and relative humiaities after 24h. 85
Table 17. Growth in micrometers of primary 0. begoniae haus- torial complexes over time. 89
Table 18. Percent shriveling and encapsulation of haustoria over time at 21, 24 and 280. 89
Table 19. Growth of hyphae of 0. begoniae on B . x hiemalis at 15,21,24 and 28C in micrometers as time passes.
Effect of relative humidity on hyphal length in micrometers of 0. begoniae on B. x hiemalis at 21C. 93
Amount of sporulation per leaf of 0. begoniae on B. X hiemalis at selected temperatures after 9 days. 97
Effect of 7 days of selected relative humidities on conidia intact on conidiophores as shown by conidial dessication and later conidial ability to germinate and form appressoria after a 24h period of detachment. 101
Table 23. Effect of selected temperatures and relative humi dities on number of visible colonies of Oidium begoniae race 2 on B . x hiemalis after 1 week.
Table 24. Monthly mean temperatures on Socotra Island.
Table 25. Effect of eradicative heat treatments on conidial characteristics of attached conidia of Oidium begoniae on Begonia x hiemalis. Effect of eradicative heat treatmnets on haustorial shriveling and encapsulation of Oidium begoniae in Begonia x hiemalis. 117
Effects of eradicative heat treatments on hyphal length of Oidium begoniae on excised Begonia x hiemalis leaves. 119
Effect of 28c on sporulation of Oidium begoniae on excised Begonia x hiemalis leaves. 120
Percent of conidiophores and conidia of Oidium begoniae on Beeonia s hiemalis with indicated stage of development by hour of day. 137
Effect of continuous light and dark on hyphal length of Oidium begoniae after 5 days growth on Begonia x hiemalis. 137
Effect of changing onset of light on percent stage of development of conidiophores and conidia. 139
Effect of continuous lifhc and dark on hyphal length of Oidium begoniae after 5 days growth on Begonia x hiemalis. 141
Conidia of Oidium begoniae collected by a Burkart spore trap set amongst infected B. x hiemalis plants in a greenhouse in March, 1978 at different hours of the day. 142
Temperature of Begonia x hiemalis 'Schwabenland Red' leaves in sun or under shade; readings taken March 3, 1980. 142
Effects of 29 C given at various periods before or after inoculation on number of visible Oidium begoniae colonies on B . x hiemalis after 7 days at 21 C. 144
Number of visible colonies formed from conidia suspended in water for various periods of time and afterwards sprayed onto B . x hiemalis leaves. 148
Percent dessication, germination and appressorium formation of conidia floating on water or on a dry surface. 148
Effect of boric acid root drenches on percent of Begonia x hiemalis 'Schwabenland Red' leaves infected with Oidium begoniae race 1. 154 ix Table 39. Effaces of various nutrient regimes on number of visible colonies of Oidium begoniae per Begonia X hiemalis 'Schwabenland Red' leaf in double Petri plates. 156
Table 40. Reported resistant strains of powdery mildews to mildicides 181
Table 41. Fungicides tested against Oidium begoniae in the literature and their efficacy. 184
Table 42. Preventive ability of systemic fungicides expressed as percent leaf surface and percent microscope fields with infections of Oidium begoniae. 187
Table 43. Relative amount of Botrytis cinerea on leaves treated with selected fungicides. 189
Table 44. Fungicide spray trials for control of powdery mildew on begonias; Spring 1979. Protective ability tnvo weeks after application on excised Begonia x hiemalis leaves. 191
Table 45. Effect of triademefon soil drench (100 ml of 2.5 gram a.i./lOOL solution) on Oidium begoniae and on Begonia X hiemalis plants after 50 days. 194
Table 46. Effect of triademefon soil drench (100 ml of 2.5 gram a.i./lOOL solution) on Oidium begoniae and on number of leaves of Begonia x hiemalis after 70 days. 194
Table 47. Efficacy of homeowner remedy type chemicals in control ling Oidium begoniae on Rieger elatior begonias. 198
Table 48. Effects of selected eradicative fungicides on percent leaf surface covered and percent germinability of 14 day old colonies, 7 days after fungicide application. 201
Table 49. Colonies per leaf formed from conidia obtained from leaves on which eradicative fungicides had been used. 203
Table 50. Prevention of powdery mildew of begonia using volatilized fungicides. 205
Table 51. Eradication of 0. begoniae with volatilized fungi cides as measured by conidial characteristics one week after treatment. 205
Table 52. Effect of protectant fungicides on germination of conidia of Oidium begoniae on Begonia x hiemalis. 209" Table 53. Effect of triforine as a protectant fungicide on hyphal length of 0 . begoniae on B . x hiemalis.
Table 54. Effect of eradicative fungicide treatments on haustorial characteristics of 0 . begoniae on B. X hiemalis 7 days after treatment.
Table 55. Effect of eradicative fungicide treatments on dessication of conidia from treated colonies, 7 days after treatment.
Table 56. Radial growth of colonies of begonia phylloplane fungi in centimeters after 9 days on commeal agar amended with fungicides.
Table 57. Radial growth of begonia phylloplane fungi in centi meters after 7 days on cornmeal agar amended with fungicides.
Table 58. Effects of preventive spray of dinocap (Karathane 20WD 6 oz/lOOgal) on numbers and genera of begonia phylloplane fungi in the greenhouse and double Petri plates.
Table 59. Number and genera of phylloplane fung’ isolated from 0. begoniae inoculated Begonia x hiemalis 'Schwabenland Red' leaves in double Petri plates after leaves were treated with eradicative and preventive sprays "f benomyl (Benlate 50WP, 8 02/ 100 gal).
Table 60. Effect of preventive fungicide treatments on number and genera of begonia phylloplane fungi on whole plants in the greenhouse. 249
Table 61. Effect of temperature on radial growth (ram) of some phylloplane fungi on co m m e a l agar. 251
Table 62. Efficacy of antagonists as sprays against number of Oidium begoniae colonies. 254
Table 63. Comparative r values of 0. begoniae on different cultivars or on treated B. x hiemalis leaves. 260
Table 64. Comparative f values for different eradicative treatments, (f = proportion of eradicated colonies). 266 LIST OF FIGURES
Figure 1. Life cycle of Oidium begoniae. 3
Figure 2. A. Conidia produced in chains (Sphaerotheca fuliginea). B. Conidia produced singly (Oidium begoniae) 19
Figure 3. Lobate appressorium formed at end of germ cube arising from conidium of Oidium begoniae.
Figure 4. Whole mount of haustorial complex from one day old infection at 21 C.
Figure 5. Transmission electron micrograph of mature haus torium of Oidium begoniae in epidermal cell of Begonia x hiemalis.
Figure 6 . Conidium of Oidium begoniae after 24 h at 21 C. Two germ tube stage.
Figure 7. Doubled Petri plates with water in lower plate and infected B. x hiemalis leaf in upper plate. 25
Figure 8 . Shriveled, encapsulated haustorium in Begonia x richmondensis incubated at 21 C. 37
Figure 9. Longitudinal section of leaf of Begonia 'curly zip' leaf, which has a hypodermal layer. 37
Figure 10. Hyphal lengths after 7 days of three species of begonia. 42
Figure II. Effects of rain on relative numbers of powdery mildews. 60
Figure 12. Diurnal periodicity of conidium release by three powdery mildews.
Figure 13. Number of visible colonies over time at selected temperatures. Figure 14. Excised begonia leaf in double Petri plate. Double Petri plate is in a plastic bag in which relative humidity is maintained by anhydrous calcium sulfate. 82
Figure 15. Conidial characteristics of 0. begoniae over time at 28 and 32 C. 112
Figure 16. Haustorial turgidity and encapsulation after exposure to high temperatures following 7 days of 21 C. 113
Figure 17. Growth of hyphae at eradicative temperatures. 114
Figure 18. Encapsulation of haustoria in progress in whole mount stained in Trypan blue. 121
Figure 19. Thick section of encapsulated Oidium begoniae haustorium in Begonia x hiemalis and treated with 21 C for 7 days followed by 14 days of 28 C. 122
Figure 20. Transmission electron micrograph of early stage of encapsulation, showing vessicular network between the haustorial matrix and host cytoplasm. 124
Figure 21. Transmission electron micrograph of encapsulated and shriveled haustorium. 126
Figure 22. Transmission electron micrograph of encapsulated penetration peg in tangential section. 126
Figure 23. Diurnal development of conidiophores and conidia of Oidium begoniae. 138
Figure 24. Ungerminated conidia included inside water conden sation mark 24 h after inoculation. 149
Figure 25. Results of triademefon soil drench. 196
Figure 26. Club-shaped hyphal ending of Oidium begoniae one week after protective treatment with triforine. . 212
Figure 27. Oidium begoniae haustorium one week after eradicative treatment with dinocap. 212
Figure 28. Oidium begoniae haustorium one week after eradicative treatment of triademefon applied as a drench. 212 Figure 29. Streptorayces sp. surrounded by and presumed internal in a hypha of Oidium begoniae.
Figure 30. Pénicillium sp. growing on surface of Begonia x hiemalis leaf among conidia of Oidium begoniae. 237
Figure 31. Verticillium sp. on comm e a l agar. 238
Figure 32. Inhibition of Cladosporium sp. by various fungicides on c o m meal agar.
Figure 33. Comparison of r values (1/t (logg(x^/l-x^) - logg X q /1-Xq ))) of various treatments of Oidium begoniae
Figure 34. Comparison of effects of temperature on germination, hyphal growth and sporulation. INTRODUCTION
When I was offered the opportunity to do work on control of powdery mildew on Rieger begonias, little did I realize how attractive this project was. First, there was a definite need for the project^as the disease was devastating crops to the point that the Rieger begonias were losing some of their marketability in the United States. To a certain extent this was caused by grower carelessness (e.g. lack of preventive spray program),but lack of knowledge was also a factor. Secondly, this project had but one aim and that was control of the disease.
The hypothesis, the experimental methods and all the strategies involved in finding ways of controlling the disease were left to me. This open-ended approach was indueive of creative solutions to control of the pathogen.
The result was an integrated approach to the management of this disease, including studies of identification and life cycle of the pathogen, host range, host resistance, effects of the environment, eradication with heat, biological control and synecology, and chemical control. The traditional means of controlling the disease, disease free stock plants and protective chemical sprays,were not shown to be incorrect. In fact, these studies underline the correctness of these measures. Many of 1 2 the experiments here are eradicative in nature and are applicable only when the correct preventive measures are not carried out.
Figure 1 diagrams the life cycle of the most important pathogen causing powdery mildew of begonia, Oidium begoniae. In this diagram stages in the life cycle are numbered 1-7 and steps can be blocked at points indicated by lines intercepting the arrows, as indicated by large letters A-G- The strategies investigated in this dissertation can be summarized by this diagram as follows:
The life cycle begins when a conidium (c) lands on a leaf (stage 1) and germinates (stage 2). Temperature (Chapter 4'), free water
(Ch. 4), and some preventive chemicals such as dinocap (Ch. 5), totally inhibit germ-tube formation (G). Relative humidity (Ch. 4), continuous darkness and pyrimidine fungicides (Ch. 5) lessen germination (intercept line A). Appressorium formation (A)(stage 3) was inhibited Cline B) by the same factors as germination. Pene tration and haustorium (h) formation Cstage 4) was inhibited (line
C) by 30 C and race-specific resistance (Ch. 3). Next, hyphae (H) form (stage 5). Pyrimidine fungicides (Ch. 5), host resistance
(Ch. 3), and 28 C stopped growth of hyphae within 7 days and inhi bited sporulation (stage 6) and branching (B) at line E. If wind is present, conidia (C) will be blown into the air (stage 7). Lack of wind or susceptible host tissue may cause inhibition at line G.
The culture of the host is outlined in Chapter one. In Chapter two identification and the life cycle of the pathogen is recounted.
Subsequent chapter (3-6) deal with assessments of vulnerabilities 1
,§
/ Figur© 1 . LHc cycle of Oidium begoniae (see texf) of the fungus life cycle to disruption as outlined in fig. 1 .
Chapter 7 attempts to summarize the previous chapters in a quantita
tive way. 1. THE HOST
The first begonia was discovered in 1690 by Charles Plumier.
He named the genus after his patron Michel Begon ( 7). The
1200 or more species of Begonia are found throughout the tropical
world (e.g. see Table 3). Countless hybrids and mutations have
also been bred and discovered The size of the plants varies
from small herbs to shrublike; foliage is elliptical to maple-
leafed in shape and is often attractively colored (e.g. Begonia
rex-culturum) ; flowers are small to large, single or double,
white or brilliantly colored.
The Begoniaceae is a family which contains three genera. The
vast majority of species are in the genus Begonia. Characteristics
of this family include being monoecious dicots; mostly herbaceous ;
usually perennial; more or less succulent ; leaves alternate,
assymetric, stipuled; seeds minute and many; flowers unisexual,
radially or bilaterally symmetrical; tepals separate; stamens many; ovary inferior, usually 3 celled and 3 winged with axile
placentation (3).
Rieger begonias: history and origin
Rieger begonias, used in the majority of the studies
reported herein, are also known as elatior begonias (15),
Hiemalis begonias and Holland begonias. The botanical name
given these triploid, fibrous rooted begonias is Begonia x
hiemalis Fotsch (6). They are derived from crosses between
5 the diploid, fibrous rooted Begonia socatrana Hook and the tetra-
ploid, tuberous Begonia x tuberhybrida Voss. The first cross was made by John Heal in 1883 and was exhibited as the variety
'John Heal' in 1885 (16). The Rieger elatior begonias were
introduced by Otto Rieger of Nurtingen, Germany from 1954-1965
Rieger had inherited a greenhouse built by his grandfather
(2). His work with Bemdt, ' the father of begonias', aroused
his interest in these plants. Rieger worked first on Lorraine begonias (B. socatrana x B . dregeii) and then on elatior types.
The first of his hybrids to be shown was Blooming Baroque at the
1954 anniversary show in Ludwigsburg. Subsequent releases such as the Schwabenland varieties are known for their longlasting,
intensely colored flowers. Perhaps 85% of the begonias produced
in Europe were Schwabenland Reds in 1975 (16). In the United
States the propagation of these plants is licensed by Mikkel-
sens Inc. of Ashtabula, Ohio.
Culture of Rieger begonias
Soil: A well aerated mix, high in organic matter and with an acid pH (5.5) is recommended. Plants in loam: Sphagnum
peat moss: perlite mixes of 3:1:1, 2:1:1, 1:1:1 and 0:1:1 all did well in experiments at The Ohio State University (8).
Mikkelsen's Inc. advises use of 6 Sphagnum peat: 3 perlite:
1 clay (2). Growers can modify this mix with bark and other
substances,but the medium should be well aerated and well drained.
Within 2-3 days of planting, Dexon and Benlate have been 7
routinely added as a soil drench to inhibit root and crown
infecting fungi. This practice has probably contributed to the
resistance of some strains of Botrytis cinerea and Oidium
begoniae to benomyl.
Temperature and Light: Photoperiod and temperature accounts for seasonal variations in vegetative growth. Schwab
enland Red has maximal shoot development in the spring and fall
under natural greenhouse conditions. This variation can be eliminated by growing stock plants with short days for the
3-4 weeks before cuttings are taken (4). High temperatures
(26.7 C or higher) should be avoided during these weeks as
they nullify the treatment. According to Mollgaard, European growers say stock plants are best kept at 18-20 C in summer and 16-28 C in winter (10) .
After leaf cuttings of Schwabenland type Rieger begonias are taken, long day treatments are begun to facillitate vegetaive growth. ShorY days, though, are continued for the first week after stem cuttings are taken on Aphrodite cultivars, after which
the long day treatment should be started(5). The long day
treatments are continued after sale of the cuttings by the propagator to the wholesaler. This long day treatment is carried out at 21-23 C (2,9). When photoperiods of natural sunlight are insufficient, 20-50 ft.-candles of light at night is supple mented. In Ohio supplemental lights are used to enhance vege
tative growth from Sept. 1 to April 15. The longer the light period the fewer the sprouts and the more the root and sprout
length (9). However, the more intense the light the less the
sprout length (17). During the summer,shade is used because of susceptibility to sunscalding of leaves and flowers. However,
it is better to grow the plants in full sun if the greenhouse
temperature can be kept near 21-23 C at all times (17),
This last point about shading is valid during the last
5-7 weeks before sale,when flower bud initiation and flowering occurs. During this time the plants are grown under short day conditions (11 h or more of darkness),because Begonia x hiemalis is a slightly photoperiodic plant. Four days of 11-
12 h of dark a day induces flowering (14). In Ohio this means
that a black cloth must be used to block out light from March
15 to Sept. 15 from 5 am to 8 am. The temperature should be lowered to 17-18 C,while flower production is desired. Once
the amount of flowers desired form, hold at 16 C or below (2).
High temperatures hasten flowering and cause flowers to be small (14). Greater intensity of light gives more flowers and more compact growth (17). At 18.5 3000 ft.-candles can be
tolerated. At 23 C, 2000 ft.-candles and at 26 C, 1000 ft.-
candles can be tolerated (2).
Rieger begonias are "light feeders" (2,8). Mikkelsen's
recommends 50-70ppm each of nitrogen, potash and phosphorous with
every watering. Overfeeding causes leaf brittleness, less flowers, and excessive vegetative growth. The use of 1/2 teaspoon/pot of 14-14-14 Osmocote, while noc recommended, may do some good.
Thorough watering is recommended to prevent accumulation of
salts that may damage the fine root system. The minimum
percent leaf content for healthy looking leaves of some of the nutrients are N, 4.7; P, 0.2, Ca 0.5; K, 0.95; Mg, 0.25%; and
B, 13-14ppm (11,13).
Keeping relative humidity at 50-60% is helpful in preventing
Botrytis infections and suppresses powdery mildew to a small extent
(see pages 99 -100). Good spacing between pots helps in this
plus gives the plants better shapes. In winter increasing
CO2 content of the air is beneficial. In summer the use of
cyclocel to control growth in shaded benches is of some benefit.
Cyclocel is added as a 1:30 solution either as a spray or as
a drench, about 2-4 0 2 ./5 inch pot. A key to disorders on begonias
The following key contains descriptions of nutrient deficiencies as given by Nelson, Krauskopf and Mingus (12).
All other descriptions are from personal observations of the author.
1. Nutrient deficiencies (after 12)
A. Chlorosis is a dominant symptom. 1. Chlorosis interveinal a. Interveinal chlorosis on older leaves followed by light tan necrotic spots within chlcrotic areas which expand until leaf dies...Mg.• b. Interveinal chlorosis on younger leaves...Fe. 2. Chlorosis not interveinal. a. Lower leaves uniformly yellow then purplish yellow and finally necrotic...N. b. Margins of canopy leaves yellow, then murky green- brown, and finally necrotic; all symptoms spread towards the leaf center...Ca. B. Chlorosis not a dominant symptom 1. Necrosis begins along the margin of lower leaves and progresses inward...K 2. Plants stunted but normal green...? 3. Rust color, striations and cracks develop on young leaf petioles and peduncles perpendicular to their axes, internodes shortened and lateral shoots prolific; young leaves brittle crinkled around rust color spots which turn necrotic; chlorosis and necrosis spreading inward from the margin of young leaves...B.
2. Cultural Problems A. Discoloration or marginal necrosis a major symptom. 1. Leaves have coppery color followed by marginal necrosis and leaf death. Upper leaves in canopy often affected first. Can be associated with long internodes and few flowers...sunscalding. 2. Marginal or veinal necrosis, stunting of plants and leaves; flowers marginally burned, especially at high temperatures...high salts or chemicals. Dimethoate, malthion, pardthion, diazinon, lindane, dinocap and sulfur all have been shown to cause such symptoms. B. Discloration and marginal necrosis not a major symptom. 1. Moist, orange-brown, rank-odored leaves rapidly wilt or drop off. Survivors often have sunscald symptoms... 11 Temperature above 95 F for prolonged period. 2. Leaves lose glossiness and succulence and wilt... Very low relative humidity or drought. (Such leaves may be more susceptible to sunscald and disease too.). 3. Vigorous growth with few flowers...Overfertilization, photoperiod too long, high temperatures, over-watering. 3. Diseases and Insects A. White, powdery fungus spreads over entire leaves from small colonies. Attacks upper and lower surfaces of leaves, flowers and stems... powdery mildew. B. White, small, wingless insects found mostly on underside of leaf, in leaf axil, on flowers and on dying leaves. Leaves often feel flaccid. Insects often covered with white, cottony fungus...mealy bugs. C. Chlorotic V-shaped lesions often covered with numerous small black spots; oily, tiny cleared spots seen nearby when held up to the light. Later stages include leaf necrosis, browning of the petioles and stems, wilting and plant death...bacterial blight caused by Xanthomonas begoniae. Clean up debris, destroy infected plants, avoid splashing water from plant to plant. D. Dark red color of leaves seen especially along veins. Nematodes found when veins are opened and examined under a dissecting microscope...Foliar nematode (Aphlenchoides fragariae). A yellow veinal chlorosis is associated with the less common Aphlenchoides ritzemabosi. E. Young plants have soft rot and collapse. Greyish-brown fungal tufts seen near soil line. Associated with high humidity... Botrytis blight caused by Botrvtis cinerea. Other diseases associated with Begonia spp: Agrobacterium tumefaciens (crown gall, very rare); Armillaria mellea (root rot, rare, reported from California); Gloeosporium sp. (leaf spot, never seen by the author); Meloidogyne sp. (root knot), OmphalIda flavida (leaf spot, reported from Puerto Rico) , Pénicillium bacilliosporium (probably a secondary invader), Phyllosticta sp. (leaf spot), Pythium sp. (root and stem rot, probably the most important of these lesser studied begonia diseases), Rhizoctonia solani (root and stem rot), Sclerotinia sclerotiorum (stem rot). Sclerotium rolfsii (stem rot), Sphaeropsis begoniicola (reported from Alabama on leaves), Thielaviopsis basicola (reported as a root rot on B . semperflorens in Ohio), Verti cil lium albo-atrum (wilt), spotted wilt virus.(1) Other insects include aphids, thrips, white flies and the diamond back moth. Slugs and mites are occasionally a problem. Most insect problems are not usually severe on Rieger begonias (2) Notes Co Chapter one.
1. Anonymous, 1960. Index of Plant Diseases In the United States. Crops Research Division Agricultural Research Service, United States Department of Agriculture (Agriculture handbook no. 165) Washington D.C.
2. Anonymous, mid-1970s. Rieger Begonias. Mikkelsens Inc., Ashtabula, Ohio.
3. Bailey, L.H. and E.Z. Bailey, revised by staff of the L.H. Bailey Hortorium, Ithaca N.Y., 1976. Hortus Third. Macmillan Pub. Co. and Collier Macmillan Pubs. New York and London.
4. Cohl, ri.A. and B.C. Moser, 1976. Effect of photoperiodic manipulation on seasonal variation in bud and shoot regenera tion of Rieger begonia cuttings. Hort Science 11; 376-377.
5. Cohl, H.A. and B.C. Moser, 1976. Environmental control of shoot initiation by Rieger begonia leaf cuttings. Hort Science 11: 378-379.
6 . Fotsch, K.A. 1933. Die Begonien. Stuttgart.
7. Graf, A.B. 1968. Exotica 3 . Roehrs Co., Rutherford, N.J.
8 . Kiplinger, D.C., H.K. Tayama and G. Staby, 1973. Tests and observations on Rieger begonias. Ohio Florists Assn. Bull. 520: 2-3.
9. Marynen, T.J.M. 1966. Leaf cuttings of 'elatior' begonias; ecological problems. 17th International Hort. Congress, College Park Maryland. 1:500.
10. Mollgaard, H. 1975. A European view-Rieger begonias. Ohio Florists Assn. Bull. 551: 2-4.
11. Nelson, P.V., D.M. Krauskopf, and N.C. Mingus 1974. Foliar analysis of Rieger elatior begonia: sampling scheme and foliar standards, pps. 343-353 in Plant Analysis and Fertilizer Problems, 7th International Coloq. of Plant Nutrition, Hanover Germany, J, Wehrmann, ed.
12. ______, ,______1977. Visual symptoms of Nutrient deficiencies in Rieger elatior begonia. J. Amer. Soc. Hort. Sci. 101: 65-68. 13. Nelson, P.V., D.M. Krauskopf and N.C. Mingus 1979. Minlnum critical foliar levels of K, Mg, and B in Rieger elatior begonia. J. Amer Soc. Hort. Sci. 104: 793-796.
14. Sandved, G. 1969. Flowering in Begonia x hiemalis Fotsch as affected by daylength and temperature. Acta Horticulturae 14: 61-66.
15. Veitch, H.J. 1906. Hortus Veitcheii. London.
16. White, J.W.,Rieger elatior begonias. History and European Research. Pa. Flower Growers Bull.
17. White, J.W. and E.J. Holcomb, Rieger elatior begonias, research at Penn State, progress report III. Pa. Flower Growers Bull. 2. THE PATHOGEN
Identification
The causal organism of powdery mildew of begonia in Ohio is Oidium begoniae Puttemans (6 , 12). 0. begoniae was first described by A. Puttemans in 1911 in Belgium (11) . The genus name Oidium sensu Saccardo will be used instead of the more correct
Acrosporium Nees ex S.F. Gray because the latter name is not in common usage. Although only the imperfect form is known in the
United States, 0. begoniae can be placed in the class Pyreno- mycetes, order Erysiphales and family Erysiphaceae (15,17). The
Erysiphales are fungi which have "asci arranged as a pallisade within a nonostiolate, dark colored perithecium" (cleistothecium), borne superficially on the host surface. All forms are parasitic on above-ground portions of higher plants, are nourished by haustoria, and are obligate parasites (16). The powdery mildews, as members of the Erysiphaceae are called,have hyaline hyphae and hyaline one-celled ascospores and conidia, the latter borne on unbranched conidiophores (17). Powdery mildew is both the name of the organisms and the name of the diseases (17).
Many species besides 0. begoniae have been reported on begonia. Erysiphe cichoracearum DC ex Herat has been reported on begonia at least three times in North America (7,9). Erysiphe cichoracearum should be very distinct from 0 . begoniae because the former is a conidial chain forming mildew and the latter usually has conidia borne singly (fig. 2). Erysiphe polyphaga Hammarlund was described as having a conidial state like that of Erysiphe .
14 15 cichoracearum (8). Hammarlund choughc that E. polvphaga was the perfect state of 0 . begoniae.but the conidial state of his fungus can not be 0 . begoniae if it resembles E . cichoracearum as stated. Erysiphe polyphaga is also identified by its large host range that includes Begonia spp., Kalanchoe spp., Cyclamen persicum V. Poll in, Sedum spp. , Hyssopus officinalis L . , Verbena hybrida hort., and surprisingly Lycopericum esculentum Mill.,
Nicotiana tobaccum L. and Solanum tuberosum L. (8). Blumer found that 0 . begoniae will not grow on any of these plants except for begonias (2). Blumer also found a fungus that he identified as E. polyphaga on cucurbits,and this fungus was unable to infect begonias (1). Cleistothecia of E . polyphaga were found on Begonia 'Gloire de Lorraine' and contained 10-20 asci (S') .
Another species which has been reported to be found on begonia is Erysiphe polygoni DC (9). Sometimes this species is reported as Erysiphe communis. These reports are probably based on conidial states and are Oidium begoniae and not members of the genus Erysiphe.
A new species. Didium begoniae yar. macrospora De Mendonca and
Sequeira was found in Portugal on Begonia rex-culturum, B. semper- florens and B . sutherlandi in 1962 (4). This fungus has larger conidia (34-72 x 9-22.8 >»m) than 0 . begoniae (20-40 x 13-17 ^ ) .
A perfect state for 0. begoniae var. macrospora was found in
England by Brooks and named Microsphaera begoniae Siyanesan (3,13). 16
The cleistochecia of chis fungus were found in groups, were brown to black in color with 8-28 appendages, and contained 6-10 asci which were 5-8 spored (13) . The conidia were said to be borne in short chains; however,I suspect that they are produced one a day as in other species of Microsphaera.
Oidium begoniae Putt, has been known in Europe since 1911 and first became important in Europe in the mid-1930s (15) . It is found world-wide today (9,15). Distinguishing features seen by me include elliptical conidia (20-40 x 13-17 jum) which lack crystalline fibrosin bodies. The conidia germinated on glass at high percentages within 4 hrs. of inoculation, even at low rela tive humidities (Table 1). 0. begoniae has lobate appressoria
(fig. 3) and spherical haustoria complexes about 13-17 pm in diameter at maturity. This description agrees with pictures and descriptions of 0. begoniae made by Zaracovitis (18,19). If there are no air currents,chains of conidia on conidiophores will form even in low relative humidities (less than 25%). I have observed, however, that conidia are usually borne singly in the greenhouse. Conidio’- phores are made up of a basal cell and a distal cell (fig. 2B).
The latter divides to give rise to the conidia. Conidia are produced one a day (fig. 23 ), These features are descriptive of the group of powdery mildews whose perfect states are E. polygoni.
Microsphaera spp., or Uneinula spp. Eliade described Microsphaera tarnavschii as the perfect state of 0. begoniae found in Rumania (5)
Two morphologically identical races of 0. begoniae were found 17
that could only be distinguished by host range. Sphaerotheca
fuliginea from pumpkin was able to infect Rieger begonias.
Details of these findings are found in the next chapter.
Oidium begoniae: Life Cycle
The life cycle of Oidium begoniae is quite simple due to the
lack of sexual structures. The life cycle is started when a
conidium is blown onto the leaf,usually at about 4-6 h after
sunrise. About 3-6 h after landing, lobate appressoria are laid down at 21 C (Table 1, fig. 3). Penetration of the host by the
fungus occurs about 10-14 h after landing of the conidium on the
leaf. From 14-24 h the haustorial complex is formed and grows
to a diameter of about 14 _pm (Table 17 , fig. 4) . Haustorial
complexes are made up of a central haustorium proper which is walled. Exterior to the wall is an electron transparent region known as the haustorial matrix. The matrix contains lobes of
the haustorium and is bound by a membrane (fig. 4,5) . During
the second day at 21 C, hyphae are sent out, one usually from the
original appressorium and others from the end of the conidium
distal to the original germ tube (fig. 6). The hyphae branch and grow for the next few weeks until leaf senescense begins. On
the third day conidiophore initials form. The initial divides once and then the distal cell divides again to give rise to a conidium
(fig. 23 ). On the fourth day the conidium is delimited, commonly
4-6 h after sunrise. Wind then blows this conidium to a new
infection site, and the life cycle begins anew. One conidium is 18
Figure 2. A. Conidia produced in chains (Sphaerotheca fuligi-
ginea) from Cucurbita mixta") . x 120
3. Conidia produced singly (Oidium begoniae).
X 300
Figure 3. Lobate appressorium formed at end of germ tube arising
from conidium of Oidium begoniae. Stained in lacto-
glycerine Trypan blue. x 450
Figure 4. Whole mount of haustorial complex from one day old
infection at 21C. Note inner dense haustorium is
surrounded by lighter matrix area containing haus
torial lobes. Stained in lacto-glycerine Trypan
blue. X 575
20 Table 1: Percent germination and appressorium formation of
Oidium begoniae conidia on glass slides at 21 C over
Percent with Time after inoculation Percent geminated appressoria Oh 0 d 2 0 c 2h 5 c 0 c 4h 35 b 5 b 6h 41 ab 17 a 8h 49 a 25 a 24h 45 a 26 a
■z) Numbers followed by the same letter do not differ signifi cantly using Duncan's new multiple range test (p=0.05). Figure 5. Transmission electron micrograph of mature haustorium
of Oidium begoniae in epidermal cell of Begonia
X hiemalis. H is haustorium body, W is haustorial
wall, L is a lobe of the haustorium and M is the
haustorial matrix. C is the host cytoplasm.
X 8300
Figure 6 . Conidium of Oidium begoniae after 24 h at 21 C. Two
germ tube stage. Whole mount stained in lacto-glycerine
trypan blue. x 300 24 produced per day per conidiophore.
Knowledge of the life cycle is necessary in designing effective control strategies for 0. begoniae. This knowledge allows us to determine the correct timing of control measures, especially since this organism often develops in a diurnal fashion.
Consideration of the life cycle also makes one aware of the vulnera bilities and strengths of the fungus. For instance, the more exposed conidia and mycelia are probably more vulnerable than haustoria.
Double Petri Plates
Powdery mildews have not been grown in pure culture. Still, growing the fungus in Petri plates has many advantages, including conservation of space in growth chambers, conservation of plant material, and the ability to easily examine the material under a microscope, while it is still alive in the dish. Growth of
Oidium begoniae on Rieger begonias was carried out in double
Petri plates (fiS- 7). Double Petri plates are two plates, one stacked on the other. A hole is made in the two plastic plates with a hot cork borer. An excised leaf is placed in the top plate and its
petiole is stuck in the hole so that it is in contact with water or nutrient solution in the lower plate. Roots will usually form
in the lower plate subsequent to immersion of the petiole.
Botrytis infection is seldom a problem at 21 C and 3000 ft.- candles but was a nuisance at 15 C in the dark. Relative humidities in the plates were estimated to be from 70-95 % when
the lids were kept on. Figure 7. Double Petri plates with water in lower plate and
infected B. x hiemalis leaf in upper plate. Notes to Chapter
1. Blumer, S. 1950. Kenntnis der Eryslphaceen. 2. Mittelung. Phytopath. Z. 18: 101-110.
1967. Echte Mehltaupilze. Fischer Verlag, Jena. 436 pps.
3. Brooks, A.V. 1970. Begonia mildew. J Royal Hort. Soc. 95: 235-236.
4. De Mendonca, A. and M. DeSequeira 1962. Erysiphe lusitaniae I. Agronomica Lusitana 24: 109.
5. Eliade, E ., 1972. Microsphaera tarnavschii sp. nov. on begonias grown in Rumania. Lucr. Grad. hot. Buc. 1970-1971: 391-399.
6 . Ellett, C.W. 1966. Host range of the Erysiphaceae in Ohio. Ohio J. of Science 6 6 : 570-581.
7. Gardner, M.W. and G.E. Yarwood, 1978. Host list of powdery mildews of California. Calif. Plant Pathology 42: 1-9.
8 . Hammarlund, C.,1945. Beitrage zur revision einiger imperfekten Mehltauarten. Erysiphe polyphaga sp. nov. Botaniska Notiser 1945: 101-108.
9. Hirata, K. 1966. Host Range and Geographical Distribution of the Powdery Mildews. Published by the author at Niigata University, Niigata Japan.
10. Powell, C.C. and J.A. Quinn 1978. Preventing powdery mildew on Rieger begonias. Ohio Florists Assn. Bull. 589: 5
11. Puttemans, A., 1911. Nouvelles maladies de Plantes cultivées. Bull, de la Socite Royale de botanique de Belgique 48: 238.
12. Quinn, J.A. and C.C. Powell 1980. Identification and host range of powdery mildew of begonia. Submitted to Plant Disease.
13. Sivanesan, A., 1971. A new Microsphaera species on begonia. Trans. Br. Mycol. Soc. 56: 304-306.
14. Stone, O.M. 1962. Alternate hosts of cucurbit powdery mildew. Ann. appl. Biol. 50: 203-210.
15. Wheeler, B.E.J. 1978. Powdery mildews of ornamentals. In The Powdery Mildews. D.M. Spencer, ed. Academic Press, Loudon New York, San Francisco, pps. 411-445. 27 16. Yarwood, C.E. 1973. Pyrenomycetes: Erysiphales. In The Fungi IV A . G.C. Ainsworth, F.K. Sparrow and A.S. Sussman eds., Academic Press, New York, pps. 71-86.
17. ______1978. History and taxonomy of powdery mildews. In The Powdery Mildews, D.M. Spencer, e d ., pp. 1-37.
18. Zaracovitis, C. 1965. Attempts to identify powdery mildew fungi by conidial characteristics. Trans. Br. mycol. Soc. 48: 553-558.
1966. The germination in vitro of conidia of powdery mildew fungi. In The Fungus Spore, M.F. Madelin éd., Butterworths, London, pps. 273-286. 3. HOST-PARASITE INTERACTIONS
InCergeneric host range scudles
Oidium begoniae Pucc. had noc been found co go to any
species outside the genus Begonia in research done in Europe
(1,2). Obscuring this fact is the claim of Hammarlund for Erysiphe
polyphaga which he identified as the perfect state of 0 . begoniae
and which was described as haying a huge host range (see pages 11-
12)t;8). The following experiment is the first host range study
done on begonia in the new world inyolving species inside and
outside the genus Begonia. Identification of hosts of the pathogen
is important in determining sources of primary inoculum and means of pathogen overseasoning.
Procedure: Powdery mildews and leaves for the interspecific
host range tests were brought in from the outdoors or greenhouses
each time they were needed. These leaves were inoculated, incu bated at 21 C in double Petri plates for two weeks and examined
for powdery mildew infections with a dissecting microscope. Natural
contamination on these leaves was checked for by including unin
oculated leaves in the incubation chamber and by inoculating
back to uninfected source host leaves. All leaves were collected
in the city of Columbus, Ohio except for Acalypha rhomboidea which came from Springfield, Ohio. The source of 0. begoniae was the experimental greenhouse at Mikkelsens Inc., Ashtabula Ohio.
The leaves and fungi were collected the summers of 1978 and 1979.
Results and Discussion: Sphaerotheca fuliginea (Schlect. ex 28 29
Fr. ) Poll, from pumpkin (Cucurbita mixta Pang. 'Spooky') was able to colonize the Rieger begonia 'Schwabenland Red' but grew more slowly than 0. begoniae. Sphaerotheca fuliginea spread from single lesions over entire leaves after 3 months incubation.
Sphaerotheca fuliginea was identified by the conidial characteristics including presence of fibrosin bodies. Fibrosin bodies are easily seen when emersed in 2% KOH solution and observed with a compound microscope. This fungus was also identified by means of formation of conidia, size and shape of conidia (elliptical and about 30 x
15 ;cn), the presence of conidia in long chains, and the host it was found on. It retained these characteristics through several transfers on begonia. Sphaerotheca fuliginea also infects Kalanchoe and a wide variety of other hosts (2,19). However, previous reported attempts to infect begonia with the cucurbit mildew failed
(2,18). In the past S_. fuliginea was probably often misdiagnosed as Erysiphe cichoracearum on cucurbits and begonias. Ervsiphe cichoracearum from zinnia (Zinnia elegans Jacq.) and sunflower
(Helianthus annuus L.) produced local lesions in our study.
These resulted from the rapid cell necrosis that followed pen- tration of the cell by the mildew and formation of undersized haustorial complexes (6-10 p m in diameter). These complexes were encapsulated by wall like depositions (fig. 8). A small amount of secondary hyphae was present in the local lesions. Other results of the intergeneric host range study were negative (Table 2).
Although the search for an intergeneric host could continue. Table 2; Host range of begonia powdery mildew and suscepcibility
of B. K hiemalis 'Schwabenland Red' to powdery mildews
from other hosts.
Source of inoculum Fungus Test host
Begonia x hiemalis 0. begoniae Antirrhinum ma jus negative Chrysanthemum sp. negative Cucumis sativa negative Cucurbita mixta negative Kalanchoe blossfe. negative Lathyrus latifolius negative Petunia hybrida negative Phlox gracilis negative Rosa sp. negative Zinnia elegans negative Acalypha rhomboidea E . cichoracearum B. X hiemalis negative Aesculus glabra Uneinula flexuosa negative Ambrosia trifida E^. cichoracearum negative Cichorium intybus E. cichoracearum negative Cucurbita i S. fuliginea positive Erigeron canadensis E . cichoracearum negative Helianthus annuus E. cichoracearum local lesion Ligustrum vulgare Microsphaera pen. negative Parthenocissus sp. U. necator negative Plantago major E^. cichoracearum negative Polygonum sp. E . polygoni negative Quercus robur M. pencillata negative Rosa sp. S_. pannosa negative Syringa chinensis M. pencillata negative Taraxacum officianale S . humuli negative Trifolium pratense E. polygoni negative Viburnum sp. M. pencillata negative Vitus sp. U. necator negative Zinnia elegans E . cichoracearum local lesion z) Explanation of results: positive = mildew from source host was able to infect and complete its life cycle on the test host; local lesion = fungus penetration triggered rapid host response killing both the invaded host cell and the mildew resulting in minute necrotic lesions; negative - fungus was unable to infect test host. it is most likely that the source of inoculum for new mildew outbreaks is infected begonias. Incidentally,the reports of
E. cichoracearum on Erigeron canadensis L . and Cichorium intybus
L. are new reports for Ohio.
Intrageneric host range and resistance of begonias to 0. begoniae
Blumer stated that Lorraine begonia, B. tuber-hybrida.
B. hybrida multiflora. B. elatior (B. x hiemalis) and B. bertini were very susceptible to 0. begoniae (2). Crops of Rieger elatior begonias and tuberous begonias can be ruined if mildew is allowed to establish itself. Strider (20) and Powell and Quinn
(12, 14)noticed a change in the intrageneric host range of 0 . begoniae on elatior begonias. The changes noted were the loss of immunity of the Aphrodite cultivars of Rieger begonia (B. x hiemalis) and a loss in effectiveness of benomyl in controlling the disease.
The change in host range suggested that new races or species of the pathogen were present. In previous investigations only one race was present, so no detailed comparisons were possible.
In this study we procurred two morphologically identical races and have investigated the identity and morphology of the pathogens and the susceptibility of several varieties and species of begonias, using both races in simultaneous inoculation and incubation experi-
Procedure: Begonia stock plants were grown in the Genetics
Department greenhouse at The Ohio State University .where begonia mildew had not ever been present. Leaves from these plants were 32 brought to the lab, inoculated with one of the races, placed in
double Petri plates (see page 24) , incubated at 21 C for two weeks and rated on a 0-3 scale (0 - mildew stops growth at least
48h after inoculation with no sporulation; 1 - a few slow-spreading
lesions present; 2 = mildew easily observed but not present in
quantity to kill leaf; 3 - mildew spreads and kills leaf in two to
three weeks.)* Contamination was checked for by leaving
control leaves among the experimental leaves. Of course all double
Petri plates had their lids on to prevent contamination. 'Stock
cultures' of the two races were maintained on begonia leaves in
double Petri plates. Both races were obtained from an experimental
greenhouse owned by Mikkelsens Inc. of Ashtabula, Ohio.
A study of resistance to our race 2 was made in 1980, with
the 0-3 rating scale replaced by measurements of hyphal growth. The
incubation time was decreased from 2 weeks to 1 week. Growth of
hyphae at temperatures other than 21 C was also investigated. When
resistant reactions were noted epidermal peels were made and
stained in lacto-glycerine trypan blue and then examined under a
compound microscope for changes in haustorial morphology (page 87).
Results and Discussion: Results of the intergeneric host
range studies indicate that sources of Oidium begoniae outside
of the Begoniaceae are unlikely. The intrageneric host range of
the two morphologically identical races confirms previously
published differences in susceptibility between the Rieger begonias
derived from either the Schwabenland or Aphrodite cultivars 33 (Table 3) (12,21). Isolates of 0. begoniae unable to infect
Aphrodite cultivars will be referred to as race 1 in this disser
tation. Those isolates able to infect Aphrodite cultivars will
be referred to as race 2. In my further extension of the intra
generic host range study it can be seen that the parental stock of
these begonias, B. socatrana and unknown tuberous begonias, also
show great susceptibility to both races studied. Some tuberous
begonias are resistant to race 1. Thus, resistance of the Aphro
dite varieties to race 1 may be derived from its tuberous parent.
This resistance is characterized by lack of haustorium formation.
In the compatible reaction well-formed, turgid, unencapsulated
haustoria form (figs. 4-5). Papillae do not form in Rieger begonias
at sites of penetration and it is unknown if they form in more
resistant species.
The rhizomatous B. rex-culturum and the fibrous rooted B .
semperflorens are moderately susceptible to both races. Penetra
tion of the host may be the same as in B. x hiemalis,but growth of mycelium is reduced (Table 4). Encapsulated, shriveled haustoria (fig. 8 , see also pages 111-113) are mixed with normal
turgid conidia one week after inoculation (Table 5). Sporulation occurs as soon as in the susceptible cultivars (in 4 days at
21 C ) , but is reduced in amount, due probably to the reduced growth, reduced number of penetration sites and consequent reduced amounts of nutrients available to the fungus. B. rex-culturum has highly variable amounts of resistance. This 'species' is actually the result of crossing B. rex with many other rhizomatous begonias. The foliage is a mixture of red, dark green and whitish Table 3. Intraspecific host range of Oidium begoniae and general information on begonia
Begonia root sys tem place of Disease Index^ Groijth of origin after 1 we B. X hiemalis 'Aphrodite cherry red' f ibrous Peru X Arabia 0 3 B. X hiemalis 'Aphrodite ioy' 0 3 1356 a^ B. X hiemalis 'Aphrodite pink' 3 3 B. X hiemalis 'Ballaleika' 3 3 B. X hiemalis 'Fantasy' 0 3 X hiemalis 'Flambeau' 0 3 B. X hiemalis 'Krefeld orange' 3 3 B. X hiemalis 'Nixe' 3 3 B. X hiemalis 'Renaissance' 3 3 B. X hiemalis 'Schwabenland pink' 3 3 B. X hiemalis 'Schwabenland red' 3 3 1401 a B. Catalina fibrous Venezuela 0 1 B. hispida cucullifera variegata fibrous 2 2 749 d B.imperial is rhizome Mexico 0 1 1061 c B. manicata crispa rhizome Mexico 1-2 1-2 473 e massoniana 'Iron cross' rhizome 2 2 778 d oxyphylla fibrous 1 1 934 c B. paralis fibrous 0 0 B. pustulate argentea rhizome Mexico 0 0 B. rex-culturum resistant cultiver rhizome Assam 1 I 631 de rex-culturum susceptible rhizome Assam 2 2 1380 a B. schmidtiana fibrous 0 0 B. semperflorens 'pink camellia' fibrous 2 2 1212 b B. semperflorens 'Linda' fibrous 2 2 B. semperflorens 'charm' fibrous 2 2 747 d B. 'Sophie X cecille' rliizome 0 0 B. serratipetala fibrous New Guinea B. socatrana fibrous Socatra 3 3 Table 3 continued
Begonia root system place of Disease Index^ Growth of hypliae 1 week ” B. subvillosa x B. semperflorens fibrous 2 2 737 d^ B. 'silver curl' rhizome hybrid 2 2 1025 B. Thurstonii ^ fibrous 1 1 234 fg B. veitchi carmine y tuberous 0 532 B. vellosiana Y fibrous 1 1 476 B. venosa fibrous Brazil 0 B. vitifolia fibrous Brazil 0 1 B. X argenteo-guttata fibrous Brazil 0 B. X richmondensis Y fibrous 0 114 B. X tuberhybrida tuberous 0-3^ 3 B. curly zip Y rhizome Mexico 1 1 453 B. roulette y 0 0 222 fg B. tiger kitten Y * 0 0 267 f
v) 0=death of mildew within 48 h of inoculation without any sporulation; 1- minute, slow growing colonies; 2=small, visible colonies that do not cause deatli of host; 3- fast spreading colonies at 21 C that lead to death of host, w) length in micrometers at 21 C. x) variability due to diference in susceptibility of varieties of tuberous begonias, y) species has hypodermal layer subtending upper epidermis. z) numbers followed by same letter do not differ significantly at the 0.05 level using Duncan's new multiple range test. 36 Table 4: Penetracion by Oïdium begoniae on selected species
of begonia.
21 C hyphal Begonia 7,penetration(48h) length_____
B. X hiemalis 'Schwabenland Red' 40 1401 ay B . X hiemalis 'Aphrodite joy' 39 1356 B. rex-culturum, resistant variety 43 683 B. silver curl 21 1025 b massoniana 'Iron cross' 1 778 B. old black Joe 13 564 d B. curly zip 13 453 B. tiger kitten 7 267 f B. Thurstonii 15 234 fg B. roulette 11 222 fg B. X richmondensis 13 114 g y) numbers followed by same letter do not differ significantly using Duncan's new multiple range test (p= 0.05). z) length after 1 week in micrometers. Figure 8 . Shriveled, encapsulated haustorium in Begonia x
richmondensis incubated at 21 C. ^ 600.
Figure 9. Longitudinal section of leaf of Begonia 'curly zip'
which has a hypodermal layer (arrow), x 125. 38
Table 5: Shriveling and encapsulation of haustoria of Oidium
begoniae associated with selected cultivars of begonia
after one week and compared with hyphal length.
%haustoria 7,haustoria hyphal Begonia Temp.( C) encapsulated shriveled length^
B. X hiemalis 'Schwabenland R e d ' 21 0 4 1401 a^ B. X hiemalis 'Aphrodite Joy' 21 2 12 1356 a B. rex-culturum 21 24 61 683 b B. semperflorens 21 29 78 747 b B . curly zip 21 100 100 453 cd B. Thurstonii 21 98 100 281 ef B. tiger kitten 21 70 100 267 ef B. X richmondensis 21 95 100 114 g B. X hiemalis 'Schwabenland Re d ' 15 0 0 730 b 'Schwabenland R e d ' 24 13 50 545 c 'Schwabenland R e d ' 28 75 100 208 fg B. X richmondensis 15 61 69 355 de y) numbers followed by same letter do not differ significantly using Duncan's new multiple range test (p- 0.05). z) length after one week in micrometers. 39
green colors. The darker colors are found in depressed areas along veins and ac the margins. The darker areas are more resistant
than lighter areas (Table 6). Furthermore, a cultivar of B.
rex-culturum that contained more red was more resistant than
other cultivars.
Some species, such as B. x richmondensis and B. Thurstonii are very resistant to both races. In this reaction penetration is
reduced (Table 4). Both races of mildew are able to penetrate, and some hyphal formation takes place, but no sporulation occurs.
Haustoria are shriveled and become encapsulated and no growth of
hyphae is seen after 48h at 21 C. These highly resistant species were found to have a hypodermal layer subtending the upper epidermis.
All of the more susceptible species tested lacked this layer of
cells. The epidermal layer contained hexagonal cells that were much smaller than the more spherical hypodermal cells(fig. 9).
Representatives with multiple epidermal layers,such as these, may be found among the Moraceae (e.g. Ficus sp.), Fittosporaceae,
Piperaceae,Malvaceae and some orchids, palms and ferns (7). In
the Begoniaceae the hypodermal layer is considered to have a water
storing function. This layer was not present on the lower leaf
surface. There was no correllation between the resistant reaction
and cuticle thickness.
In a test of representative cultivars and species, the
relative degree of susceptibility of the begonias was found to
change with temperature (Table 6). Species considered highly
resistant to both races at 21 C, such as B. x richmondensis and 40
B. thurstonii, are capable of sustaining growth past 48h and supporting sporulation of 0 . begoniae at lower temperatures.
On moderately susceptible hosts that we tested, such as B. semperflorens, the mildew has a temperature optimum of 18 C.
On tested highly susceptible Rieger begonias the growth optimum is 21 C (Table 6). It would appear that the more resistant the begonia the lower the optimum temperature for mildew growth (fig.10).
Such a situation emphasizes the need to control temperature when evaluating cultivar or species resistance towards a pathogen of this sort. Furthermore, test conditions should be similar to those normally occurring during production of the crop (18-21 C ) .
A cross between resistant B. 'Roulette'(hyphal growth of
222 ^ in 7 days at 21 C ) , a species with a hypodermal layer, and moderately susceptible B. imperial is (1061 ;im), a species without a hypodermal layer, resulted in two moderately susceptible offspring, silver curl and silver sheen (1025 jim ) which lacked a hypodermal layer. In this case the non-race specific resistance is apparently conveyed by a single recessive gene. This type of resistance resembles Van der Plank's horizontal resistance in that it is characterized by a lowered infection period, longer period from penetration to sporulation and lowered amount of spore production (11,15,21). Attempts to incorporate the non-race specific resistance to powdery mildew into B. x hiemalis will be difficult considering the low amount of such resistance found in the parents. Table 6 : Growth of Oidium begoniae hypliae in micrometers after 7 days on selected species of begonia at various temperatures.
Species or cultivar 12 C 15 C 18 C 21 C 24 C 28 C
B. X hiemalis 'Schwabenland Red' 730 c 1401 a 547 d 208 hiy 'Aphrodite Joy' 531 de^ 765 c 1001 b 1357 a 827 c 165 hiy B. semperflorens 424 ef 834 c 1073 b 747 c 280 gi»y 184 hiy B. Thurstonii 377 fg 364 fg 281 gl/ 188 hiy B. X richmondensis 212 hi^ 355 fg 291 gh^ 114 lyz ill lyz 71 lyz B. rex-culturum resistant area” 855 c 823 952 be 284 h 122 susceptible area” 950 be 1117 b 1380 a B . roulette 324 fghV -- 222 hiy 254 hy 140 ly^ B. tiger kitten 398 fgy 356 fgy 267 hiy w) resistant darker and susceptible lighter-colored areas found on same leaf. x) numbers followed by the same letter do not differ significant!y using Duncan's new multiple range test (p - 0.05). y) no sporulation observed, z) growth stopped within 48h. /• \ B. X liiemal is
A. semperflorens \ \ \
Ay/ A, Thurstonii ^ ^ ""'"AA \ 4 8 12 16 20 24 28 Degrees Centigrade
10. Hyphal lengths after 7 days of 3 species of begonia. 43
The race specific immunity is probably an example of Van der
Plank's vertical resistance. This type of resistance is usually
conveyed by one or a few dominant genes (6,22). The race specific
resistance is relatively easy to breed into cultivars of 3.
X hiemalis considering that it is present in many cultivars of
B. X tuberhybrida. Unfortunately, race 2 seems to be the prevalent
race in the United States on B. x hiemalis. However, O'Riordain(lO)
found that 0. begoniae was still unable to infect the Aphrodite
type begonias in Ireland. This is interesting in view of the
suspicion of some Ohio growers that the new race (race 2) recently was imported to the U.S. from Europe. Perhaps resistance to
race 2 will be found in B. tuber-hybrida in the future. Factors affecting race formation in powderv mildews.
The amount of genetic variability in a given species of powdery mildew is affected by many factors. The larger the number of conidia produced, the more likely a virulent mutant will find the host and spread itself. In barley 1 cm^ of
posibility for mutants virulent against resistance genes of the host can be predicted over an entire field (24). Number of host plants and distance of those plants from the primary source of the mutant are also important in insuring that the conidium finds a suitable host. Crop rotation, for this reason, helps keep the number of successful mutants down.
The primary propelling forces for formation of new pathogen mutants are variability in the host such as that which formed race
2 of 0 . begoniae, and variability in the environment, such as that which may have formed benomyl tolerant strains of 0. begoniae. In cereal powdery mildews the varieties of host attacked by races of the pathogen widely varies (variants in host ranges of the pathogen between host species are known as form species). In the gene-for-gene hypothesis the number of potential resistance genes of the host is matched by the number of genes for virulence in the pathogen. However, in horizontal (or quantitative or general) resis tance several combinations of virulence genes may operate against a resistance gene giving a pleiotrophic effect (24). If n viru lence genes are present then 2" races of the pathogen are possible. The appearance of resistance genes, changing environmental
conditions or application of fungicides can create a niche for
an otherwise less competitive strain of the fungus. Powdery mildews
in which sexual reproduction is rare or absent, such as 0 . begoniae,
may not be able to maintain the new mutant when conditions change
once more.
In greenhouse grown flower crops, such as Rieger begonias,
the number of successful fungal races should be smaller than in
cereal crops. The acreage planted is much smaller, and so fewer
mutant conidia will be found. The enclosed nature of the green
house and the distance between greenhouses will act as a strong
barrier to the spread of the pathogen to uninfected crops by
air, especially in the absence of wild hosts. New races are
primarily spread by man transporting infected plants. This
replaces spread of primary inoculum and spread of new races by
wind, and aids in disseminating mutants to new environments where
they may better survive. Selection by fungicides in greenhouses is
probably intense compared to field crop situations. Many fungi
are easily able to form resistant strains to the new systemic
fungicides which often have a monogenic mode of action. Probably
adequate variability in 0 . begoniae is present to overcome any
monogenic form of resistance or fungicide which works against
only one gene of the pathogen. This explains the probable appearance of benomyl tolerant strains of 0 . begoniae. 46
Other host-patho^en inceractions
The development of powdery mildews is also affected by host dependent factors such as age and density of planting.
Weinhold (23) found peach fruits only susceptible when young.
Younger leaves are generally more susceptible to powdery mildew
than old ones (25). This last effect is possibly a result of
thickening of cuticle with time (3). The fact that both young, healthy tissue and wounded tissue are susceptible concurs with a structural defense mechanism. Corner (3) felt that host
toxins were the cause of this resistance, especially in resistant varieties.
A high density of planting would increase the accessiblity of plants to primary and secondary inoculum and thus help
spread the disease at a more rapid rate. This would also increase relative humidity which is known to slightly increase the disease.
High planting density also produces tall, "leggy" plants which are
less attractive.
Experiments were carried out to determine the susceptibility
of young versus old leaves and of the upper versus lower epi dermis. Such information can be used to indicate when the host is most susceptible and where to look for primary infections.
Procedure; Fifty B. x hiemalis 'Schwabenland R ed' were grown
on a shaded greenhouse bench in Columbus Ohio. Infection was by
conidia windborne from plants infected with 0 . begoniae race 1
on an adjacent bench. The percent of leaves infected was deter mined on February 24, 1978, about 1 month after the first mildew was observed on the bench. It was noted whether the infections were on young (less than 1" long), medium (1-4" long) or old 'elephant eared' leaves (greater than 4" long) or were on flowers. It was also noted whether colonies were on the upper or lower epidermis.
Results and discussion: Large older leaves tend to be first
infected by windblown inoculum (Table 7). These leaves are found in
the canopy and so their upper epidermis is the most accessible surface for conidia. As seen in Table 7 and as observed on numerous plants during the course of epidemics,next the medium sized and then the lower, smaller leaves become infected. Accessibility is apparently a more important factor than differences in leaf susceptibility caused by leaf age in timing of infections. Flowers may be somewhat more resistant than leaves. Some of the lower percentage of infection of small leaves and of flowers may be due
to smaller size,too. Obviously a conidium would be less apt to land on an object with 1 inch^ leaf surface than one with 16
inch-. Flowers are shorter lived too and this would cut down on the number of visible infections accumulated over time. Stems are also susceptible.
Later experiments showed that the fungus survives fungicide
treatments in buds and protected places,mainly inside the canopy
(see pages 108 to 128). Survival for 3-4 weeks at 28 C occurred on
leaves that were just breaking bud at the beginning of the heat
treatment. Older leaves were eradicated of 0. begoniae by this
treatment. Interestingly, this may mean that treatment of the 48 Table 7 : Interactions between some host dependent factors and in
fection of Rieger begonias by Oidium begoniae.
Host Factor Newly infected Plants infected for plants one month or more. Percent leaves infected 12 90
Percent of infections on 30 12 older leaves
Percent of infections on 54 64 medium sized leaves
Percent of infections on 17 25 younger leaves
Percent of infected leaves 87 95 with infections on the upper epidermis
Percent of infected leaves 24 89 with infections on the lower epidermis
Percent of flowers infected. 10 62
Total number of leaves 1299 2014 counted. 49
mature leaves used to propagate the 'Schwabenland' varieties of
Rieger begonias may effectively eradicate the fungus. Buds
infected but not opened had large brown lesions, indicating that
infection had taken place, but had no active mildew colonies
after 28 days of 28 C. Ligules on stem internodes were also
suspected of being a place of survival. When returned to favorable
conditions leaves recovering from these treatments will often
show infections that occur first on younger and medium sized
leaves within the canopy, unlike that found in windborne spread.
On B. massoniana 'Iron cross', B. imperial is, B. pustullata
argentea and other species with stout leaf hairs it was noted that
most infections were associated with the leaf hairs. Other noted within leaf differences included light colored areas more
susceptible than dark colored areas on B . rex-culturum (Table 5).
Leaf margins were more susceptible (i.e. hyphal length 749 micro
meters after one week at 21 C) than interior of leaf (about 200
micrometers) in B. hispida cucullifera. This last effect was
associated with leaf hairs and the production of plantlets at the
side of the leaves. These plantlets were at least as susceptible as
the leaf margins. Notes to Chapter three.
1. Blumer, S. 1952. Beitrage zur specialisation der Erysiphaceae. Berichte Schweizischen Bot. Gesell. 62: 383-401.
1967. Echte Mehltaupilze. Fischer Verlag, Jena 436 pps.
3. Corner,E.J.H. 1935. Observations on resistance to powdery mildews. New Phytologist 34: 180-200.
4. DeMendonca, A. and M. DeSequeira. 1962. Erysiphaceae lusitaniae I. Agronomica Lusitana 24: 109.
5. Eliade, E. 1972. Microsphaera tamavschii sp. nov. on begonias grown in Romania. Lucr. Grad. bot. Buc. 1970-1971: 391-399.
6 . Ellingboe, A.H. 1978. A genetic analysis of host-parasite interactions. In The Powdery Mildews. D.M. Spencer ed., Academic Press, New York and London .
7. Esau, K. 1953. Plant Anatomy, J. Wiley and Sons, New York.
8 . Hammarlund, C. 1945. Beitrage zur revision einiger imperfekten mehltauarten. Erysiphe polyphaga nov. sp. Botaniska Notiser 1945: 101-108.
9. Hirata, K. 1966. Host Range and Geographical Distribution of the Powdery Mildews. Published by the author at Niigata Univ., Niigata, Japan.
10. O' Riordain, F. 1979. Powdery mildew caused by Oidium begoniae of elatior begonia- fungicide control and cultivar reaction. Plant Dis. Reprtr. 63: 919-922. ,
11. Parlevliet, J.E. 1979. Components of resistance that reduce the rate of epidemic development. Ann. Rev. of Phytopath. 17: 203- 2 2 2 .
12. Powell, C.C. and J.A. Quinn 1978. Preventing powdery mildew on Rieger begonias. Ohio Florist's Assn. Bull 589: 5
13. Price, T.V. 1970. Epidemiology and control of powdery mildew (Sphaerotheca pannosa)on rose. Ann. appl. Biol. 65: 231-248.
14. Quinn, J.A. and C.C. Powell Jr. 1980. Identification and host range of powdery mildew of begonia. Accepted for publi cation in Plant Disease.
15. Robinson, R.A. 1969. Disease resistance terminology. Rev. of appl. Mycol. 48: 593-606. 16. Rogers, M.N. 1959. Some effects of moisture and host plant susceptibility on the development of powdery mildew of roses, caused by Sphaerotheca pannosa var. rosae. Cornell Univ. Agr. Expt. Sta. Mem. 363.
17. Sivanesan, A. 1971. A new Microsphaera species on begonia. Tr. Br. mycol. Soc. 56: 304-306.
18. Stone, O.M. 1962. Alternate hosts of cucurbit powdery mildew. Ann. appl. Biol. 50: 203-210.
19. Strider, D.L. 1976. Resistance of Kalanchoe to powdery mildew and efficacy of fungicides for control of disease. PI. Dis. Reprtr. 60: 45-47.
1978. Reaction of recently released Rieger elatior begonia cultivars to powdery mildew and bacterial blight. PI. Dis. Reprtr. 62: 22-23.
21. Van der Plank, J.E. 1963. Plant Diseases: Epidemics and Control. Academic Press, New York.
22. ______1968. Disease Resistance in Plants. Academic Press, New York.
23. Weinhold, A.R. 1961. The orchard development of peach powdery mildew. Phytopathology 51: 699-703.
24. Wolfe, M.S. and E. Schwarzbach 1978. Patterns of race changes in powdery mildews. Ann. Rev. of Phytopath. 16: 159-180.
25. Butt, D.J. 1978. Epidemiology of powdery mildews. In The Powdery Mildews, D.M. Spencer, ed.. Academic Press, New York and London, pps. 51-81. 4. EFFECTS OF THE ENVIRONMENT
Introduction
The environment can be more easily controlled in green
houses than in the field. This makes control of disease
with environmental factors more likely with greenhouse
grown crops than field crops. Investigations into effects
of the environment on powdery mildew of begonia, then, is
doubly important. First, the conditions which facilitate
epidemics should be known so that they can be avoided.
Secondly, the conditions that inhibit and eradicate the fungus
should be discovered and their practicality as control measures
should be determined. The following studies of mine were
conducted with these goals in mind.
A review of the literature concerning effects of the envi
ronment on powdery mildews follows. After this my experiments
on effects of the environment on 0. begoniae are detailed.
Temperature: Literature review
Temperature is the most important environmental factor
in development of powdery mildew epidemics (68). Epidemics will only occur within a range of about 7-8 C, although
infection will take place in a range of about 20 C (Table 8).
Since most mildews grow best at temperatures which are best
for host growth, temperature had not proved to be very
useful for control of powdery mildews, including powdery mildew of rose in the greenhouse (38). However, feasibility
52 of concrol of 0 . begoniae by heat treatments was still an
open question at the start of my studies.
Numerous studies have been carried out on the effects of
temperature on germination of conidia. Several authors
observed germination at various temperatures on glass slides
( 3,18,40,70)- Such studies show effects of the physical
environment but do not show host-parasite interactions. More
realistic determinations of germination-1emperature interactions were made using excised leaves floating on nutrient solution
and on whole plants in growth chambers (16,32,38,55,64)-
Such determinations largely parallel those found on glass,
showing that germination is affected more by the environ ment than by genetic interactions between host and parasite.
Typically, at moderate temperatures (17-27 C) there is rapid
germination of a relatively large number of conidia. The mean optimum temperature for gemination is 21 C (56).
At temperatures of 5-16 C there is initially less germination.
However, at these low temperatures conidia may be longer
lived and ultimately have germination percentages like
those found at higher temperatures. At temperatures above
27 C the conidia usually become shriveled and have a low
percentage of germination (18,32,38,40,48,55). Typically no
germination occurs above 31-34 C. (Table 8).
In some cases the optimum temperature for germination may be changed by humidity (56). At high humidities the 54 maximum percent germination of Erysiphe graminis conidia was obtained at 20 C but at low humidity it was 10 C (4Q)•
At 20 C and low relative humidities these conidia dessi- cated before germinating. Lower temperatures protected the conidia from dessication until germination had occurred.
These results can be expected of conidia which germinate poorly at low relative humidities such as Podosphaera leucotricha and Sphaerotheca pannosa. Humidity effects are minimal for species which do not require high humidity for germination, such as Erysiphe polygoni and Uneinula necator.
Infection takes place in a narrower range of temperatures than does germination (5 6). Longree found that development of haustoria was inhibited by 30 C in Sphaerotheca pannosa on rose, while germ tubes and appressoria formed up to 33 C (38).
Often near the upper limit for infection succesful pene tration takes place but interaction between the haustorium and host cause no further development of colonies to take place. In using heat to prevent infection there may be a critical period for application of the heat after inocu lation (43). Mount and Ellingboe found, using E. graminis tritici, that susceptibility of the pathogen varied with the time that heat was applied after inoculation. This time corresponded with penetration and early haustorial formation.
Optimum growth of mycelia was greatest in much the same or at slightly higher temperatures compared to germination. However, Schnathorst (55) found the optimum temperature for infection and growth of Erysiphe cichoracearum on lettuce was 27 C, 9 degrees higher than the optimum for germination.
Stavely and Hanson (59) showed that the time after inoculation at which readings are taken may effect conclusions. Ery siphe polygoni on clover had similar growth of hyphae at
24 and 28 C after 48h but hyphal growth at 24 C had surpassed that at 28 C by a considerable amount after 96h (59). The range of temperatures for hyphal growth is largely dependent on the range of temperatures for haustorial development.
Temperature and light are the most important factors in the quantity of conidia produced. The cardinal temperature for sporulation, penetration and haustorial formation are usually the same. Sporulation has been found to be a good measure of resistance of hosts of fungi (76). In powdery mildews resistance results in impaired haustorial function which in turn results in impaired ability to sporulate. Thus amount of sporulation is a reflection of number and health of haustoria. Tempera ture can drastically change the amount of sporulation. Sporu lation of E. graminis per unit area at 25 C was 25% that at
20 C and the colonies were ten times larger at 20 C so that total sporulation at 25 C was 2.4% that at 20 C ( g).
Powdery mildews survive unfavorable temperatures as cleistothecia and as dormant mycelium. The importance of cleistothecia may be as a source of genetic variation more than as a source of overseasoning inoculum. Many species 56
form few or no cleistothecia and other species produce cleisto
thecia which are not fertile ( 8 ). In tropical areas, espe
cially, cleitothecia are seldom found. Studies of survival
of mildew at temperatures above those at which penetration
occurs are few and survival in lands where such temperatures
are present for long periods of time is largely a mystery.
Usually survival in hot lands is assumed to be by active mycelium but results of this study cast doubts that this is
so in all cases. E. graminis is known to oversummer in the
Middle East as cleistothecia (26,33). Overwintering in temperate
lands is usually by dormant mycelium in buds and protected
places (51,68).
Temperature and rainfall decide the geographical range
of powdery mildews (7,68). Areas of moderate temperature and light
rainfall will be favorable for these fungi (7 ). However,
these fungi have in many cases adapted to less moderate
temperatures (56). Cardinal CemperaCiires for some powdery mildews.
Powdery mildew______Germination______Penetration______Minimum Optimum Maximum Minimum Optimum Maximum
E. cichoracearum 5 18 33 (lettuce, 531 E . cichoracearum 9 22 34 (70) E. graminis (yg) 3 17 31 E. graminis 2 20 30 E. graminis (45) E . polygoni (59) 24 E. polygoni (70) 8 23 32 Microsphaera al- phitoides (29) 20-23 Oidium mangiferae 9 22 30-32 (47) Podosphaera leuco tricha (3) 10 22 28 P. leucotricha (^6)) 4 21 32 P. leucotricha (7g),10 16 30 S. fuliginea (72) 22?! 28 31 S. macularis (32) 3 20 38 S . macularis (4g) 2 20 35 S_. mors-uvae 18 S. pannosa (70) 5 24 34 S. pannosa (peach, 4 21-27 36 64) 2 " pannosa (rose, 3 21 33 38) Uncinula necator 5 27 36 (18) U. necator (70) 3 26 36
Range for all; 2-10 16-27 28-38 2-11 18-27 27-35 58
Moisture: Literature review
The effects of moisture on powdery mildews are impor tant but controversial. Partly this is due to different responses to moisture by the three groups of mildews.
These groups are 1) those that germinate only at high relative humidity (e.g. S_. pannosa and P. leucotricha)
2) those that germinate much better at high humidities but have some conidia capable of germinating at low humidi ties (e.g. E. cichoracearum and E. graminis) and 3) those that germinate well at all humidities such as E . polygoni,
Uncinula spp. and Microsphaera spp. (5 6). Most laboratory studies have shown that high humidity, above 95%, is the optimum for germination of these fungi ( 3 , 32.38,40,55) (Table 9).
The results may depend on the method used. Th . relative humidity on a glass slide may closely approximate the humidity of the air while relative humidity on a leaf surface may be raised by microclimatic influences of the leaf. However, the latter effect did not occur on greenhouse roses (52).
High relative humidities preserve germinability by decreasing conidial dessication rates (48). Growth of mycelia of powdery mildews was greater at lower relative humidities
(12,59).
Given the experimental evidence for affinity of powdery mildews for high humidities it is somewhat surprising that these fungi are associated with arid climates ( 7,6 8) (fig. 11). Cardinal relative humidities for germination of
conidia of powdery mildews.______
Powdery mildew Minimum Optimum Maximum
Erysiphe cichoracearum 0.1 95.6-98.2 100 (lettuce, 55) E. graminis (40) 100 100 M. alohitoides (29) 76-96 P. leucotricha (3) 90 100 S. pannosa (peach, 54) 100 100 S. pannosa (rose,38 ) 97-99 100 S. pannosa (rose, 52) 20 97-99 100 Uncinula necator (18) Little effect 98 S. macularis (32) 8 100 100
Total range 0.1-90 95-100 98-100 H liilliuiia increasing rainfall -
powdery mildews
Inches rain/year FiR.ll Effects of rain on relative numbers of powdery mildews (after downy mildews 61
This is Likely due Co other factors such as lack of free water inhibition and lack of competitors. 0. be^oniae which morphologically has much in common with fungi in the most xerophytic of the three mildew groups can be expected to be among those mildews least effected by relative humidity.
The poor growth and germination of powdery mildews when exposed to free water is a well-known phenomenon (56,68).
This is due both to inhibition of germination (conidia explode) and abnormality of growth. As hyphae mature they become more resistant to inhibition by free water (52). Inhibition by free water is one reason why powdery mildews are associated with drier areas (fig. 11). Elimination of powdery mildews by spraying mist on greenhouse roses for 5 sec per min, 2h a day is one application of inhibition by free water (52).
Still, Weinhold (64) suggested that free water may in small quantities be necessary for germination of S_. pannosa on peach. The point of Weinhold's study was that while submerged conidia were killed, conidia in contact with water but not submerged,germinated at much higher percentages than other conidia.
Yarwood (68) referred to a number of references which gave favorable effects of rain, dew and sprinkler irrigation, but felt that such findings were results of misinterpretation. The group of mildews that require high relative humidity for germina tion (e.g. S. pannosa) may require contact of free water for germination but most mildews do better when it is not present. 62
Drought stress favors development of powdery mildews (6 ,68).
This may explain why E. graminis may be more severe in dry,
sandy soils (39). Powdery mildew on oak increased transpiration
and decreased turgidity and water potential of oak leaves (30).
Presumably rate of mildew infection increases when turgidity
decreases and turgidity decreases due to water loss when mildew
infection occurs (30).
To prevent epidemics of powdery mildews the following should
be taken into account. Although powdery mildews are favored by
high humidity, humidity control will probably not eradicate or totally prevent mildews. Mist might be applied to check or
eradicate these fungi. Plants should be kept watered in soil
that does not rapidly dry. 63
Light: Literature review
Light enhances sporulation and initiates a diurnal cycle
of production and release of conidia that has been often observed in powdery mildews (73). Powdery mildews fall into three groups in response to light: 1) the xerophytic group (e.g. E. polygoni,
Microsphaera spp., Uncinula spp.) produces but one conidium per day
in a strictly diurnal fashion and releases its conidium 2-5h after onset of the photoperiod, given wind to blow it off; 2) The moisture loving species (e.g. S_. pannosa, P. leucotricha) and E. cichora cearum release conidia throughout the day but not at night ; 3)
E. graminis forms and releases conidia both day and night (68,73).
Formation of appressoria, generative cells and conidial germination also usually occur mostly in the light portion of the day in group 1 (68). Childs ( 9) suggested that powdery mildews which produced conidia in chains (e.g. E. cichf-race.irum, S. pannosa) had two periods of conidial development and release, one from 6-8 am and one from 2-4 pm. He found that E. polygoni on clover which produced conidia singly had but one period of release from 10 am to 2 pm. Packham (46) found only one afternoon period of release in S_. pannosa which bears conidia in chains. Pady, Kramer, and
Clary also found but a single period of conidial release in both
E. cichoracearum and E. polygoni. However they found that E. cichoracearum released conidia throughout the day, while E. polygoni released spores around noon(73)(Table 10). Schnathorst
(56) implied that light played little part in diurnal cycles of spore maturation and release in powdery mildews. Instead he 64 attributed these cycles to drying of conidlophores during the day coupled with increased wind. However,studies in growth chambers where these conitions were controlled show that onset of the light is indeed responsible for regulation of diurnal periodicity.(fig. 12).
While light initiates formation of conidia and stimulates germination, light has been shown to inhibit growth of powdery mildews and shading is often favorable to the disease (56).
Light causes the formation of fewer appressoria and longer incubation periods. Mount and Ellingboe (43) reported greatest growth of hyphae of E. graminis in the dark. These effects are caused by the deleterious effects of ultraviolet light and by increases in leaf and air temperatures by direct sunlight.
Direct sunlight commonly causes leaf temperatures to be 10
C or more higher than air temperatures (23). Obviously other factors such as transpiration, reradiation of heat, leaf color and angle are important in this effect (13,22,23). A precon ditioning effect of day length on host resistance has also been claimed. In that study plants exposed to light for 16h had upper and sometimes lower leaves more resistant to E. graminis than plants exposed for 8h (36).
Several studies have been made on the effects of ultraviolet radiation on powdery mildew development. Hey and Carter (31) found that irradiating varieties of wheat for 1 min once a day with rather intense ultraviolet light greatly reduced powdery mildew.
They could eradicate the fungus by this means,but at some expense â
9 midnight
Fig. 12. Diurnal periodicity of conidium release by three powdery mildews (after 73).
Erysiphe graminis — — - E. cichoracearum — E. polygoni ------66 to the host. They found that better control was attained when the plants were Irradiated twice at short intervals rather than once at an interval equal to the short intervals. Mount and
Ellingboe (44) showed that the irradiation had to be applied within 14h of inoculation to inhibit growth of the mildew (i.e. elongation of secondary hyphae), and no pustules formed on leaves
irradiated 14h or less after inoculation. Moseman and Greeley (42), using E. graminis hordei.found that 50-80 sec of intense
irradiation caused germination of conidia to fall from 75 to
24% on water agar. On whole plants the number of pustules fell
from 89% of control to 19% when irradiated from 50-80 sec just after inoculation, and suggested that those pustules that did form were in protected positions. They did not find a critical 14h
period but noted that pustules could not be eradicated 120h after
inoculation. After 120h the fungus could still be made to change
color and grow more slowly. Use of plastics that admit ultra violet light could help control powdery mildews in greenhouses.
Growing plants in sunny locations where possible is suggested toe.
These measures will probably not eradicate all vegetative mycelia
in protected places.
Other light effects include positive photopropism of germ
tubes for green light, and negative phototropism for white light.
This probably helps germ tubes grow towards the leaf surface (68). 67 Nutrition: Literature review
The effects of host nutrition on development of powdery mildews
has been less investigated than temperature and moisture, particu
larly in recent years. In general Yarwood (68) considered high
soil fertility favorable to development of these parasites. This
effect seems primarily due to promotion of disease by high nitrogen.
Spinks (58) found that Erysiphe graminis tritici was favored by high nitrogen levels and low potash and phosphate and that
high potash and phosphate did not counteract the negative effects of high nitrogen. Trelease and Trelease (61) confirmed this working with wheat seedlings in .02M salt solutions of differing propor
tions of calcium nitrate, magnesium sulfate and potassium phos phate (Table 10). In this study the compound containing the sulfate had neither a beneficial or a negative effect on the disease.
Bainbridge ( 2 ) confirmed the stimulatory effect of high nitrogen on E. graminis and showed that this effecL was due to greater hyphal growth, and not to an increase in germination. An increase in ammonia gas has been noticed in infected host cells (53). Inter estingly, free amino nitrogen was correlated with disease resis
tance in tobacco (11).
Studies with tobacco also linked potassium deficiency
to resistance, contrary to the results of Trelease and Trelease on wheat (10,11) . Potassium deficiency led to more free amino nitrogen (H). Potassium deficient leaves had thinner cells walls
too (11). In tobacco effects of phosphorous were found to be minimal. Table 10: Effects of proportions of three salts on development
of E. graminis on wheat seedlings (Trelease and
Trelease 1928, reference 61).
Molecular Proprtions Relative top yield Relative top yield KHoPO/, Ca(NO-^)? MgSO/, uninfected seedlings infected seedlings
5 5 90 71 72 5 47.5 47.5 100 69 5 90 47.5 78 62 33.3 33.3 33.3 96 84 47.5 47.5 5 75 66 47.5 5 47.5 83 83 50 18 32 100 100 90 5 5 81 114 69
The use of sulfur containing compounds as mildicides has been known since ancient times. The sulfur containing amino acid methionine was found to inhibit the formation of powdery mildew on excised cucumber leaves by E. cichoracearum (17) and of P. leuco
tricha on apple (15). Folic acid was found to reverse the inhibitive effects of methionine. Methionine may inhibit the production of folic acid in nature (17). Dekker (17) attempted to inhibit powdery mildew by drenching roots with methionine and had limited success.
Less successful was direct application of methionine to the foliage.
Other amino acids diu not have an inhibitory effect. Using autoradiography, Wieneke, Covey and Benson (15) found that with powdery mildew of apple, sulfur accumulated at young infection sites. No such accumulation occurred with calcium.
Eaton (21) found that mildew was inhibited by boron placed in sand in which he grew wheat and barley in the summer but was unable to duplicate the results in winter. Yarwood (67) found boron deficient sunflowers more susceptible but boric acid in excess did not affect mildew of several plants. Boron may play a role in sucrose transport (25), hormone transport (41), par titioning metabolism between the glycolytic and pentose shunt pathway (37), and complexing polyhydrols (24). Accumulation of polyphenols has been noted in boron deficient sunflowers
(62). Considering that enhanced levels of phenols are often 70 associated with resistance and that boron deficient plants are more susceptible,this is a surprising result.
Bolle-Jones and Hilton (4) observed much mildew of rubber trees in which they had induced zinc deficiency. Low silicon, lithium, cadmium, and manganese may also enhance growth of powdery mildews (68). Few clear effects of these nutrients were observed when they were added to the soil (68). The haustorial sheath membrane is known to firm with calcium and become unstable in
the presence of lithium. A firmed thickened membrane is needed to keep the fungus and host alive (5). 71 Other environmental factors: Literature review
Another environmental factor which can be important is wind.
Wind helps determine rate and distance of spread of conidia. By
checking prevailing winds,prediction of where powdery mildews will next break out in a field can be made (54). Although ventilation is usually recommended to control powdery mildew in
greenhouses by helping lower relative humidity,it may be harm
ful by helping spread conidia (38).
The effects of air pollution, oxygen and carbon dioxide
levels on powdeiry mildews has been little explored. Predisposition
to S. fuliginea in normally resistant pinto beans by wounding
or rust infection was shown by Yarwood (69), 72
Effects of temperature on 0. begoniae
Temperature can affect any or all of the components of
the pathogen life cycle. Temperature can inhibit germination and haustorium formation and thus stop or delay an epidemic.
Alternately an epidemic can be slowed down by decreasing the speed at which germination, penetration, hyphal growth or sporulation take place. In the last chapter a temperature dependent non-race specific resistance was observed. This resistance was invoked at 15-18 C in resistant species and at
24-28 C in B. X hiemalis. This type of resistance was in most cases associated with slowing epidemics, not stopping or delaying onset of them. This temperature dependent non-race specific resistance also was associated with an observable host reaction in which haustoria became encapsulated and shriveled and melanin-like compounds were found in cells containing haustoria. The knowledge of this temperature induced resistance is the first step in controlling the pathogen with heat.
If absolute control of the pathogen is to be obtained using temperature,certain facts must be known. First what is the temperature at which epidemics occur and conversely at what temperature can the pathogen be killed before it completes its life cycle?
Secondly, the various components of the pathogen life cycle may have their own temperature optima and maxima. We have already observed that no sporulation takes place at 28 C 73
after 7 days (see page 41 ). However, germination and haustorium
formation did take place at this temperature. The inhibition
of growth and sporulation is caused by a host reaction. The
temperature at which germination is inhibited will be more
likely to be independent of effects of the host genome. The
cardinal temperatures of the components of the life cycle are
of importance in predicting as well as preventing epidemics.
Thirdly, for purposes of eradication of the fungus, it
should be known how long the inhibitive temperatures should be applied to kill the fungus. Investigation of means of survival
of the fungus during periods of high temperature is naturally
part of such a study. After this third step is completed we will know at what temperature the pathogen is killed, what the most vulnerable stage in its life cycle is, and how long the
temperature must be applied to kill. The final test is to de
termine the practicality of the treatment options. 74
Effect of temperature on visible number of 0. beaoniae colonies on B. X hiemalis
Purpose: To find the cardinal temperatures for visible colonies of 0. begoniae to form on Rieger begonias. Such information will be useful in predicting occurrence of epidemics and will also suggest temperatures at which the disease can be stopped. As such, this experiment lays the groundwork for future experiments.
Procedure: Excised leaves of B. x hiemalis 'Schwabenland
Red' were brought to R.A. Spott's lab at OARDC, Wooster Ohio in March-May 1978. There they were dusted with 0. begoniae race
1 conidia from leaves infected in the greenhouse. The newly inoculated leaves were then placed in dark incubators at constant temperaures of 10, 15, 20, 25 and 28 C. They were examined for number of visible colonies 5, 7, 9, and 14 days after inoculation.
Each treatment contained three leaves and the experiment was repeated 6 times.
Results and Discussion: Visible colonies formed most quickly at 20 C (fig. 13). At 10, 15, and 20 C the number of visible colonies increased with time, but at 25 C colony formation stopped after 1 week. At 28 C no visible colonies formed, although some local lesion formation was noted.
Apparently epidemics can be expected from about 15-23 C.
Temperatures somewhat lower than 10 C and those as high or higher than 28 C will inhibit colony formation. 200
150
S ï
g 50 5
Time, in days
Fig. 13. Number of visible colonies over time at selected temperatures.
10 C ------; 15 C ------; 20 C ------; 25 C ------; 28 C ------76
Effects of temperature on germination and appressorium formation on glass over time
Purpose: To determine the effects of temperature on germina tion in vitro. Such information will show the effects of the physical environment on detached conidia, will test the vulnerability of such conidia to heat treatments,and will indicate at what temperatures the pathogen will most rapidly germinate with consequent rapid onset of epidemics.
Procedure: Oidium begoniae race 2 was grown on detached leaves (B. x hiemalis 'Schwabenland Red') for 1 week at 21 C and 12h light per day in double Petri plates. The leaves were pressed lightly to glass slides. The slides, now with conidia on them, were incubated for 0,2,4,6,8 and 24h at 4,10,15,18,21,24,28,30 or 32 C. There were 4 slides in each treatment and 500 conidia were randomly selected and evaluated for percent germination and appressorium formation. The experiment was repeated 3 times.
Incubation was in the light and at 80-95% relative humidity.
Results and Discussion: The optimum temperature for rapid germination and appressorium formation is about 24 C (Tables n and 12 ). The optimum temperatures for ultimate (after 48h) percent germination are from about 15-24 C, with 21 C being slightly best. The minimum temperature for germination and appressorium formation is below 4 C. The maximum temperature for germination and appressorium formation is 32 C. Table 11: Germination of conidia of 0. begoniae on glass
slides at different temperatures over time.
Percent. Hours after inoculation moerature 2 4 6 8 24 48
4 C Oj^ Oj Oj Oj Oj 8hi 10 Oj 2i 13gh 32cde 45abc 15 Oj 3i 20fg 42abc 50a 18 Oj 17gh 24def 44ab 21 5i 35bc 41abc 49a 45ab 50a 25 lOhi 37bc 45ab 47a 47a 28 0.5ij 27cde 34cd 32cde 31cde 30 li 32 - Oj Oj Oj Oj Oj z) all numbers followed by the same letter do not differ signifi cantly using Duncan's new multiple range test (p-0.05). Numbers are the means of three experiments which were pooled for the statistical test.
Table 12: Appressorium formation of conidia of 0. begoniae on
glass slides at different temperatures over time.
Percent, Hours after inoculation Temperature 2 4 6 8 24 48
4C Ofz Of Of Of Of 4e 15C Of Of O.lef 5e 20a 18C Of 0.4ef 6de 7de 23a 21C Of 5e 17bc 25ab 26ab 29a 25C Of 6de 14c 14c 22ab 24ab 280 Of 0 .2ef 7de 8de 14cd 15bc 300 le 320 Of Of Of Of Of z) all numbers followed by the same letter do not differ signifi- cantly using Duncan's new multiple range test (p-0.05). Numbers are the means of three experiments which were pooled for the statistical test. 78 From 28 C to 32 C germination and appressorium formation are inhibited. It is interesting to note that the optimum for germination on glass is the same as that for growth of hyphae
(see pages 90-91) on B. x hiemalis. This is an indication of the near complete susceptibility of Rieger begonias to the pathogen. From temperatures of 27 C and up the germination is slowed on glass and growth of the pathogen is slowed in vivo.
It was noted in the course of these experiments that it is important to use fresh inoculum. Older conidia are greatly inhibited at 24 C, whereas fresh inoculum has its highest rate of germination at this temperature. For this reason conidia from 7 day old infections produced at 21 C were standardly used
in these experiments. 79
Shriveling of conidia over time on ^lass
Purpose: To determine how long after inoculation shriveling
of conidia occurs. If this moisture stress -induced condition
can be made to occur before penetration of the host,the disease
can be controlled by inducing shriveling.
Procedure: Oidium begoniae race 2 was grown on excised
leaves at 21 C and 12h light per day for 7 days in double Petri
plates. The infected leaves were then appressed to glass slides.
The conidia-bearing slides were incubated for 24h at 10,15,21,24,
or 28 C at 60 or 100% relative humidity. Slides were evaluated
for conidial shriveling at 0,2,4,6 ,8 , and 24h after inoculation.
. Five fields were randomly selected on each slide under a compound
microscope and the 100 conidia uppermost in each field (lOOx) was
counted. There were 2 slides per treatment and the experiment was
repeated a total of 4 times.
Results and Discussion: Dehydration of conidia procédés
gradually over a period of several days. The majority of conidia
probably are killed after penetration and haustorium formation
begins (12-14h after inoculation) at 60% relative humidity
(Table . At 100% relative humidity this dessication is delayed
even more (Table 13). xn any case dehydration of conidia will
not completely control outbreaks of the fungus at any temperature
or relative humidity tested. Lowering the relative humidity to
15-30% will cause some loss of germination due to dehydration of
conidia (Table 15). Table 13 : Percent shriveling of conidia of Oidium begoniae at
various temperatures over time at 100% relative
humidity on glass slides.
Hours ,after inoculation Temperature 2 4 6 8 24
IOC Oe^ Oe 3cd 4cd 18ab 15 Oe Oe 3cd 6cd 20a 21 Oe 3cd lObc 19ab 19ab 24 Oe 2d 16ab 21a 17ab 28 - 3cd 3cd 7cd llabc z) all numbers followed by the same letter do not differ signifi cantly using Duncan's new multiple range test (p-0.05) Numbers are the means of four experiments which were pooled for the statistical test.
Table 14: Percent shriveling of conidia of Oidium begoniae at
various temperatures over time at 60% relative
humidity on glass slides.
Temperature 2 4 6 8 24
IOC Oe^ 0.5e 4d 8cd 22b • 15C Oe 3de Ide 7d 20b 21C Oe Ide Ide 2de 81a 24 Oe 4d 3d 6d 76a 28 2de 4d 6d 16bc 80a z) all numbers followed by the same letter do not differ signifi cantly using Duncan's new multiple range test (p-0.05). Numbers are the means of four experiments which were pooled for the statistical test. 81
Effect of temperature and relative humidity on percent shriveling.
(germination and appressorium formation of conidia on glass and excised leaves.
Purpose: To test the affect of temperature and relative humidity on excised conidia. Such information is necessary in determining when epidemics of 0. begoniae are likely. Also this information could be helpful in setting up a system of control of the pathogen by manipulating the greenhouse environ-
Procedure: (Glass slides) Oldium begoniae race 2 was grown on mature,excised Begonia x hiemalis 'Schwabenland Red' leaves in double Petri plates in a 21 C incubator for 7 days.
The leaves were then appressed to clean glass slides. The conidia were then checked for shriveling. If less than 10% of the conidia were shriveled at time of inoculation the experiment was continued. The slides were placed on platforms in plastic bags above one of several saturated salt solutions.
The salt solutions maintained relative humidities in the bags within limits as follows: Tap water, 100%; NaCl, 80-95%; MgCl*
6 HgO, 49-64%; and CaSO^ (Drierite), 15-30%). The last named salt was not in solution but was placed as solid material in the bottom of the bag. The bags were then placed in incubators at 15, 21, or 29 C (fig. 14). Relative humidity was monitored in the bags with a hydrometer. After 24h the slides were removed and examined under a compound microscope. Five fields (lOOx) were randomly selected on each slide and the 100 conidia uppermost in each Figure 14. Excised begonia leaf in double Petri plate. Double Petri plate is in a plastic bag in which relative humidity is maintained by anhydrous calcium sulfate (white pebble-like material). Temperature and light regime are controlled by placing the plastic bag in a growth chamber. 83
field were counted for number shriveled, germinated and with
appressoria. Conidia obviously damaged by free water were not
counted. There were 2 slides per treatment and the experiment was
repeated 4 times.
(Excised leaves) The top surface of mature, excised leaves were inoculated with the same leaves, at the same time as the
glass slides (see above). They were then incubated in double
Petri plates without lids in the plastic bags described above
concurrently with the glass slides. Conidia were removed from
the leaves on clear Scotch tape 24h after inoculation. The tape was then placed on glass slides and the conidia were examined
under a compound microscope (lOOx) for % shriveling, germination, and appressorium formation. All conidia on the tape were
counted. There were 2 leaves per treatment and the experiment was repeated 4 times.
Results and discussion: Shriveling of conidia was most
effected by humidity, while germination and appressorium formation were most effected by temperature (Tables 15 and 16). Both
temperature and relative humidity were important in determining
all conidial characteristics on glass slides (Table 15).
On leaves the differences between treatments were less pronounced
than on glass, indicating some microclimatic buffering effect.
Shriveling of conidia on glass was correlated with lowering
relative humidity (Tables 15 and 16) . Using Pearson's r test, a
correlation statistic (19), the correlation coefficient for 84
Table 15: Percent conldia germinated, with appressoria or
not shriveled on glass slides at selected tempera
tures and relative humidities after 24h.
Test RH 15 C 21 C 29 C
7oTurgid 1007, 81+18a7Z 65+17b 70+14ab 887, 57+27bc 50+20cd 577, 44+19cd 41+25d 29+23e 237, --- 10+9 f --
^Germination 1007, 44+26bc 52+2lb 34+22cde 88% 67+12a 24+26e 577, 39+20cd 53+21b 25+18e 237, -- 24+16de ---
TAppressoria 1007, 29+3lb 28+17b 4+4 e formed 88% 46+15a 19+2 lb c 577, 15+19cd 27+16b 7+9 de 237, y) All numbers in same test followed by same letter do not differ significantly using Duncan's new multiple range test
(p=0.05).
z) Second number in each column is the standard deviation.
First number is the mean of four experiments which were pooled for the statistical test. Percent conidia germinated, with appressoria or
not shriveled on excised B. x hiemalis leaves at
selected temperatures and relative humidities
after 24h.
Test %RH 15 C 21 C 29 C
% Turgid 100 44+23 a^^ 31+16 a 40+26 a 88 48+28 a 33+18 a 37+29 a 57 41+22 a 14+17 b 9+11 b
% Germinated 100 18+13 b 44+21 a 23+17 b 88 14+14 b 36+25 a 19+10 b 57 16+14 b 38+20 a 17+17 b
7, Appressoria 100 11+10 b 22+12 a 8+9 b formed 88 8+10 b 22+21 a 9+8 b 57 10+11 b 26+19 a 11+14 b
y) Numbers in same test, followed by same letter do not differ significantly using Duncan's new multiple range test (p= 0.05). z) Second number in each column is the standard deviation. First number is the mean of four experiments which were pooled for the statistical test. shriveling and decreasing relative humidity on glass was r™ 0.53
(not significant at 0.1 level) when effects of temperature were not removed. However, within temperatures, the correlation between shriveling and relative humidity on glass was much better.
At 28 C this correlation was r= 0.941 (significant at 0.1 level) and at 21 C r= 0.993 (significant at 0.001 level). On leaves the correlation between shriveling and relative humidity was r= 0.993 (significant at 0.001 level) at 28 C and r- 0.982
(significant at 0.01 level) at 21 C. Temperature and shriveling are less correlated (Tables 15,16). Shriveling was less at 15 C,
100% relative humidity than at other treatments. The cause for this is probably that at high relative humidities and low temperatures the least loss of water from conidia takes place. Shriveling occurs mostly 8h or more after inoculation (Table 14) , except at very low relative humidities,such as 23%. This suggests that powdery mildew of begonia, caused by 0 . begoniae, cannot be controlled by lowering the relative humidity, because effects of low humidity harmful to the fungus occur after haustoria form and therefore after the fungus is able to withstand those effects.
Percent germination and percent appressorium formation are obviously strongly correlated (Tabes 15 and 16). Both have peaks after 24h at 21C. Differences between 15 C and 21 C are small after 24h on glass for both conidial features, but 15 C is much slower in germinating and fotrning appressoria on leaves.
Temperature is more important than relative humidity in determining
percent germination and appressorium formation, especially on leaves
(Table 16). 87
Effects of temperature on haustorium formacion
Purpose: To determine the effect of temperature on
haustoria! formation and growth, and to determine how
long haustoria survive at high temperatures. It was sus
pected that haustoria might be the structure of 0. begoniae
most resistant to high temperature, because of its protected loca
tion. Thus, knowledge of time of survival of haustoria would
be the key to eradicating the fungus with heat.
Procedure: Haustoria were prepared for observation
under the light microscope by placing epidermal peels of
B. X hiemalis 'Schwabenland Red' containing haustoria in a
lacto-glycerine Trypan blue solution in a small glass Petri
plate. The cover was placed on the dish and the dish was auto-
claved at 15 p.s.i. for 2 min. The dish was removed from the
autoclave and the peels were taken out and placed in lacto-
glycerine solution for 2 min. at room temperature. The
peels were then placed on a microscope slide in a drop of
lacto-glycerine solution and examined under the microscope.
Keeping excess water out of the solution during autoclaving is
important to avoid swelling of the haustoria. The lacto-
glycerine solution used was 2 lactic acid: 2 glycerine: 1
distilled water.
Initial haustoria! formation and growth were observed by
preparing and observing epidermal peels as above 10,12,14,16
18,20,22,24,26 and 48h after inoculation of 0 . begoniae race 2 onto excised leaves of B. x hiemalis 'Schwabenland Red'.
These leaves were incubated at 21 C and 12h light per day in double
Petri plates. 20 haustoria were counted per treatment,if possible, and the experiment was repeated 5 times. A similar experiment was carried out once at 15 and 24 C,but readings were only taken 24 and 48h after inoculation.
The maximum temperature for haustorial formation was determined by incubating 0. begoniae race 2 on B. x hiemalis at 24, 28,30 and 32 C and examining the epidermal peels 48h later as detailed
Shriveling and encapsulation of haustoria over time were recorded concurrently with studies on hyphal growth ( see pages 90-93). Mature excised B . x hiemalis 'Schwabenland Red' leaves were inoculated with 0 . begoniae race 2 and placed in double Petri plates. The leaves were incubated at 21,25 or 28 C with 12h light per day. The leaves were periodically removed and epidermal peels were prepared and examined as above.
Results and Discussion: Haustoria of 0. begoniae do not form above 29 C or below 5 C on B . x hiemalis. The maximum temperature for haustoria formation is 2 C lower than the maximum for germination and appressorium formation (compare Tables H and 17 ). This indi cates higher susceptibility of haustoria to high temperatures or the beginning of host resistance mechanisms. No significant difference in haustorial diameters was seen between 21 and 2 4 C at 48 h after inoculation (Table 17) but at 24h the haustoria of 24 C treatments were significantly larger on the average than 21 C haustoria. Table 17 : Growth in micrometers of primary 0. begoniae
haustorial com p l e x e ^ over time.
Hours after Diameter of haustorial complex inoculation 2 1 _C 15_C 2 4 C 30_C
14 Oe= 16 6 .8 d 18 8 .led 20 8 .8 C 22 9.4bc 24 9.7bc 5.8d 13.5a Oe 26 10.4b 48 14.2a 9.7bc 13.5 a Oe y) haustorial complexes= haustorium + haustorial matrix, z) numbers followed by the same letter do not differ significantly using Duncan's new multiple range test (p- 0.05). The numbers are the means of three experiments which were pooled.
Table 18 ; Percent shriveling and encapsulation of haustoria
over time at 21, 24, and 28 C.
Days after 21 C 25 C 28 C inoculation 7, SHRIV^ 7o ENCPy 7,SHRIV %ENCP %SHRIV %ENCP
1 0 0 0 0 0 0 2 0 0 9 0 4 13 0 30 1 7 12 1 44 13 100 75 14 15 4 19 14
y) 7oENCP=percent haustoria encapsulated, z) 7oSHRIV=percent haustoria shriveled. 90 le is likely that haustorial formation and penetration occur earlier at the higher temperature as does germination (Table 11).
At 15 C haustorial growth is slowed (Table 12) . Such a delay indicated a lesser ability to transfer nutrients from the host
to the mycelium. This delay would therefore help slow growth of hyphae and rate of sporulation.
Haustoria lost turgor and died at 28 C before they were able
to support sporulation of the fungus (Compare Tables IB and 22).
After 2 days at 28 C shriveling had begun, but not encapsulation.
Most conidia that had germinated had appressoria but no haustoria.
After 1 week all haustoria were shriveled and 75% were encapsulated
(Table 18) . At 25 C encapsulation and shriveling also took place, but at a slower rate than 28 C. At 21 C shriveling and encapsulation of haustoria had not occurred in the large majority of haustoria
14 days after inoculation. These results are interesting from an epidemiological standpoint because they help us to predict the
survival of the fungus when exposed to these temperatures in
greenhouses. However, 30 C, the temperature at which haustoria are prevented from forming, or 32 C, the temperature at which the
fungus is prevented from germinating, are more promising than 28 C
for purposes of eradicating the fungus (see pages 108-128). Growth of hyphae as effected by temperature and humidity.
Direct temperature effects on hyphal growth of 0. begoniae on numerous species of begonias was discussed on pages 29 -
40 . It was shown that the optimum temperature for growth of hyphae after 1 week was 21 C on susceptible species such as
B. X hiemalis and was 12-18 C on less susceptible species.
The maximum temperature for formation of hyphae of any length was 28 C. At 28 C hyphae stopped growth within 7 days of
inoculation. In this further extension of that study the growth of hyphae at various temperatures over time was inves tigated using B. X hiemalis. The effect of incubating the hyphae at various relative humidities at 21 C was also inves-
Procedure; Mature excised B. x hiemalis 'Schwabenland
Red' leaves were placed in double Petri plates and inoculated with 0. begoniae race 2. The leaves were then placed in incu bators at 15, 21, 24 or 28 C. The leaves were periodically removed at the times indicated in Table 20 and the hyphal lengths were determined under a dissecting microscope equipped with an ocular micrometer. The longest hyphae was measured on each randomly counted colony. Twenty to thirty hyphae were counted per leaf. There were 3 leaves per treatment and the experiment was repeated a total of 3 times.
The effects of different relative humidities on hyphal length was investigated by placing mature excised B. x hiemalis 92 'Aphrodite Joy' leaves in double Petri plates and inoculating them with 0. begoniae race 2. These leaves were placed in plastic bags which were maintained at either 100, 80-95, 49-64 or 15-307, relative humidities by saturated salt solutions as detailed on pages 79-80.
After 7 days of incubation in these bags at 21 C and 12h light per day, the leaves were removed from the bags and hyphal lengths were determined under a dissecting microscope equipped with an ocular micrometer. There were 2 leaves per treatment from which
20 randomly located colonies were selected and 20 hyphae counted per leaf. The experiment was repeated once.
Results and Discussion: Hyphae are grown fastest the first 24h at 24 C but thereafter 21 C is the optimum temperature for growth of hyphae (Table 19) . At 21 C hyphae grow about 220-260 micro meters a day until leaf nutrients and leaf surface area become
limiting. At temperatures above 21 C host resistance mechanisms probably are invoked which slow the rate of hyphal growth (Table
19). At temperatures below 20-21 C slowing of fungal and host metabolism probably account for the slowing in growth. The
hypothesis that the host resistance mechanisms are not involved
in the latter slowdown is supported by the evidence of a similar
slowdown in rate of germination on glass (Table 11). At 28 C
the fungus grows very slowly, is unable to sporulate and stops
growing after 7 days. This temperature is associated both with effects directly inhibitory to the pathogen (i.e. reduced
germination rates on glass. Table 11,15) and with host-pathogen
interactions as evidenced by haustorial encapsulation and a host 93
Table 19 : Growth of hyphae of 0. begoniae on B. x hiemalis
at 15, 21, 24 and 28 C in micrometers as time passes.
Days after inoculation 15 C 21 C 24 C 28 C
1 0 v^ 58 n 98 ran 39 n 2 53 n 228 kl 248 k 86 mn 3 162 kl 500 hi 318 j 4 722 e 421 ij 149 Im 5 424 ij 988 d -- 6 1244 c 445 i 7 742 e 1430 b 547 gh 204 kl 9 601 200 kl 11 -- 1917 a 650 ef 191 kl
z) numbers followed by the same letter do not differ cantly using Duncan's new multiple range test (p= 0.05). Numbers from 2 experiments were pooled.
Table 20 : Effect of relative humidity on hyphal length in
micrometers of 0 . begoniae on B.. H hiemalis at
21 C after 7 days.
Relative humidity Hyphal Length
100 1321 a^ 80-95 1391 a 49-64 1431 a 15-30 1442 a
z) numbers followed by the same letter do not differ significantly using Duncan's new multiple range test (p= 0.05). Numbers from 2 experiments were pooled. 94 melanin-like substance accumulation. The rate of haustorial encapsulation and melanin accumulation, as well as the amount of haustorial shriveling and hyphal growth inhibition at 28 C, is dependent on species of begonia as detailed on pages 31-42.
No significant differences were seen in hyphal lengths at various humidities (Table 20). However, a trend was observed.
The fungus grew slightly faster at the lower relative humidities.
Similar results were obtained in studies of other powdery mildews
(12,59). 95 Effects of temperature and relative humidity on amount: of sporulation
Purpose: To determine the amount of sporulation of 0. begoniae on Rieger begonia as affected by temperature. This information will give an indication of the amount of secondary inoculum that builds up in an epidemic at various temperatures.
Thus, this parameter directly affects the rate at which an epidemic spreads through a crop. Relative humidity effects on sporu lation were incidentally observed concurrent with effects of relative humidity on hyphal length.
Procedure: Excised B. x hiemalis leaves were inoculated with
0. begoniae race 2 and incubated at 4, 15, 21, 25 and 28 C for
9 days. Sporulation was then determined by a method similar to that used by M.O. Garraway to determine amount of sporulation in
Bipolaris maydis (77) . Single colonies and the leaf around them were removed with a cork borer 9mm in diameter. Ten such colonies were removed from a leaf and placed in 20 ml of water. The water and leaf discs were shaken for* 30 sec to remove all mature conidia from the leaf tissue. A drop of water was placed on a glass slide and an ISmm^ cover slip was placed over the drop of water.
The total number of conidia under the cover slips was multi plied by 15 to give the approximate number of conidia per colony.
The mean of six replicates of the treatments is given in Table
21. The amount of sporulation per leaf is then determined
by multiplying this figure by the number of colonies per leaf
determined on page 74-75. The leaves were also examined dally for onset of sporula tion at the various temperatures.
The amount of sporulation at 21 C at different relative humidities was incidentally observed in conjunction with the determinations of hyphal lengths detailed on pages 91-93.
No counts were made.
Results and discussion: The optimum temperature for sporu lation on B. X hiemalis was 21 C (Table 21). On more resistant species such as B. x richmondensis sporulation does not occur at this temperature but will occur at 12-15 C (see pages 33 -
34 ). On B. X hiemalis. conidiophore initials were found 3 days after inoculation and mature conidia were formed in 4 days at 21-25 C. At 15 C mature conidia were formed in 5 days. At
4 C no haustoria were formed and infection did not take place.
At 28 C infection occurred but a resistant host reaction,coupled with impaired fungal vitality,resulted in complete suppression of sporulation.
The amount of sporulation proved a dramatic indicator of the potential harm this fungus can do at 21 C compared With other temperatures. At 21 C over 100 times as many conidia were formed compared with 25 C (Table 21). Seven times more conidia were formed at 21 C as compared to 15 C. Epidemics can be expected to occur rapidly from 17-22 C, the temperatures at which begonias 97
Table 21 : Amount of sporulation per leaf of 0. begoniae
on B. X hiemalis at selected temperatures after
9 days.
Temperature Conidia per Visible Conidia per colony colonies/leaf leaf
4 C 0 d^ 0 0 d^ 15 C 100 b 94 9400 b 21 C 470 a 135 63450 a 25 C 8 c 70 560 c 28 C 0 d 0 0 d
z) numbers followed by the same letter do not differ siginifi- cantly using Duncan's new multiple range test (p- 0.05). are recommended to be grown. Low temperatures (10-16 C) will slow the rate of spread,while high temperatures (27 C or more) will prevent spread of the pathogen. On pages
108-128 I discuss the possibilities of eradicating the fungus using high temperatures.
It was noticed that there seemed to be more sporulation at 25 and 55% relative humidity than at 90 or 100%. 99
Effects of relative humidity on conidial characteristics of conidia still attached to conidiophores.
Purpose: To determine whether relative humidity during sporulation affects the ability of conidia to germinate and form appressoria.
Procedure: Mature,excised B. x hiemalis leaves were inoculated with 0 . begoniae race 2 in the spring of 1979.
Leaves were placed in double Petri plates and grown either at
21 C with 12h light/day or at 15 C in the dark. The double
Petri plates containing the leaves were placed in plastic bags in which relative humidity was maintained by MgCl*5H 20 saturated salt solution (49-64% relative humidity) or by water (100% relative humidity). After one week the leaves were appressed to glass slides on which conidia were there after allowed to germinate for 24h. During the time of germina tion the glass slides were placed in the relative humidity bags in the incubators as indicated in Table 22 . There were 2 leaves per treatment, 4 glass slides per treatment and 50-
200 conidia were observed for percent dessication, germina tion and appressorium formation per slide. The experiment was repeated 5 times.
Results and discusssion: High relative humidity during sporulation helped prevent later dessication of conidia but did not result in higher germination or appressorium formation 100
(Table 2 2 ) . In fact, in this experiment germination was slightly higher when conidia were germinated at 60% relative humidity than when germinated at 100%. This experiment and other observations made in the course of this study suggest that relative humidity may be more important with dessication of older conidia which are found at the end of chains. However, this experiment confirms that little effective control of 0 . begoniae can be expected by lowering relative humidity (gee pages 58-60). Table 22 : Effect of 7 days of selected relative humidities
on conidia intact on conidiophores as shown by
conidial dessication and later conidial ability
to germinate and form appressoria after a 24h
period of detachment.
%RH^while intact 7oRH while detached on conidiophore on glass slides %NS
21 C, 12h 100 100 84a^ 24a 12ab light/day 100 60 57bc 34a 25a 60 100 47c 24a 9b 60 60 29d 29a 20ab
15 C, dark 100 100 89a 26a 10b 100 60 44cd 30a 14ab 60 100 65b 21a 9b 60 60 39cd 27a 11b
v)%NS=percent not shriveled w)%G=percent germinated x)%A=percent with appressoria y) RH=percent relative humidity z)numbers in the same column followed by the same letter do not differ significantly using Duncan's new multiple range test (p= 0.05). Numbers from 5 experiments were pooled for the statistical test. 102
Effect of cemperacure and relative humidity on number of visible
colonies of 0 . begoniae.
Purpose; To determine the affect of relative humidity
and temperature on visible colony formation. Number of visible
colonies is a reflection of the sum of the influences of these environmental parameters on germination, hyphal growth, haus
torial formation and other facets of the pathogen's life
cycle. Thus, the number of visible colonies present at selected
temperatures and relative humidities summarize the influence of
those two factors on disease.
Procedure: Mature excised leaves of B. x hiemalis
'Schwabenland Red' were inoculated with 0. begoniae race 2
conidia derived from week old infections. These leaves were
placed in double Petri plates and incubated at the temperatures
and relative humidities indicated in Table 23 • Relative
humidities were maintained in plastic bags over salt solutions
as previously described (pages 81-82). There was 12h light per
day, but light in the 15 C incubator was only 250-400 f t .-
candles and light in the 21 C and 29 C incubators was 2000-
3000 ft.-candles. After 7 days the number of visible colonies per
leaf was counted. There were 4 leaves per treatment.
Results and discussion: Temperature proved to be more
important than relative humidity in determining number of
infections. Little difference in colony number was seen from 103
50-100% relative humidity (Table 23 ). However, some drop in colony number did occur at 20-35% relative humidity.
Relative humidities that low were also phytotoxic to excised leaves, giving drought stress symptoms such as loss of leaf glossiness and loss of leaf turgor.
Although dropping relative humidity may help control powdery mildew of begonia it can not be considered an effective control measure. High relative humidity does help keep conidial dessication low and thus increase germination, but this is counterbalanced by lower hyphal growth and sporulation rates and probably by increases in antagonistic fungi as well
(68). The temperature optimum for visible colonies, 20-21 C, was expected, since this was the optimum temperature for all stages of the pathogen life cycle. Table 23 : Effect of selected temperatures and relative humi
dities on number of visible colonies of Oidium
begoniae race 2 on B . x hiemalis after 1 week.
Relative humidity 15_C 2LÇ 29 C
100% 62+30ycdZ 100+30 a 0+0 f 80-95% 56+21 de 101+32 a 0+0 f 49-64% 51+18 de 90+42 ab 0+0 f 15-30% 38+21 e 80+21 be OK) f y) Numbers are a percent of visible colonies found at 21 C and 100% relative humidity followed by the standard deviation, z) Numbers followed by the same letter do not differ signifi cantly using Duncan's new multiple range test (p=0.05). Numbers from 5 experiments were pooled for the statistical 105 Prevention of begonia mildew on whole plants by 28 C .
Purpose: To confirm the discovery of inhibition of 0. begoniae at 28 C using whole plants.
Procedure: Five infected begonias were placed at one end of a growth chamber containing 30 uninfected B. x hiemalis
'Schwabenland Red' plants. The temperature was maintained at 28 C and the humidity was near 100%. Humidity was kept high by misting cheesecloth placed in the chamber, thus keeping it constantly wet. Thirty days later when no spread of mildew to uninfected plants was seen and when it was noticed that the mildew on the infected plants seemed dead the plants were removed and placed in a greenhouse in which the temperature was main tained at 21 C. The plants were observed there for 30 days for new appearance of infection.
Results: There was no spread of mildew to uninfected begonias in the 28 C growth chamber. Conidia on diseased leaves and the mycelia thereon appeared dessicated and dead after 30 days. Old, infected leaves on the infected plants dried up and dropped off the plant. New leaves on the infected plants were apparently uninfected. 21 days after removing the plants to a greenhouse with 21 C, powdery mildew colonies were again observed on 1 young leaf of 2 of the 5 previously infected plants. 28 days after removal to the greenhouse a colony appeared on one leaf of one of the previously uninfected plants, apparently inoculated by windborne conidia from the nearby infected plants. 106 Infected plants outplanted in the summer of 1978 also lost all visible colonies of mildew in June after old mildewed
leaves fell off. These plants remained apparently mildew free
until late October when temperatures cooled and the mildew
reappeared.
Discussion: The inhibition of mildew at 28 degrees C is
possibly the main defense of these begonias to powdery mildew.
The male parent of these begonias, B. socatrana, grows on
Socatra Island in the Indian Ocean, where the temperature rises above 28 C most days of the year (Table 24 ). The inhibition of mildew in summer and the high occurrence of the pathogen
in spring and fall can also be explained by high temperatures here in the temperate zone in the summer. The tuberous parents are from Peru, but grow in the mountains. Since the exact cultivar of tuberous begonia that served as the parent
for the Rieger begonias is a trade secret the exact place of origin of the tuberous parent is unknown. Thus I do not know if the 28 C inhibition is a main defense of these begonias against infection too. If temperatures are often in the 27-31 C range in the native habitats of these begonias, then survival as dormant mycelium can be expected (see next section). Table 24 : Monthly mean temperatures on Socotra Island.
January 25 C May 28&C September 26 C February 25 C 27 C October 28 C 26%C July 26 C November 27 C April 27%C August 25 C December 25%C 108 Eradication of Oïdium begoniae using heat: treainnents.
Purpose: High temperature was evaluated for efficacy of eradication of all parts of 0. begoniae (i.e., the conidia mycelia and haustoria) on Rieger begonias. It is hoped that this study will lead to a practical, economical control measure when powdery mildew is present in a greenhouse. Heat treatments may be more efficacious than dinocap or other contact fungicides which cannot eradicate powdery mildew in protected places. Means of survival of the fungus during periods of adverse temperature was also evaluated.
Procedure: Since the diagnosis of a colony as dead or alive by the naked eye may be misleading the effects of the heat treatments on the various stages of the pathogen life cycle were evaluated microscopically. Thus, the fungus was considered eradicated when all of the following criteria were met : conidia 100% dessicated; haustoria 99-100% shriveled; and all hyphal growth stopped and hyphae incapable of resuming growth when placed in favorable conditions. All sporulation must be stopped also but it was found that sporulation was stopped sooner than the above events.
Mature, excised B . x hiemalis 'Schwabenland Red' leaves were inoculated with Oidium begoniae race 2 and incubated thereafter for 1 week at 21 C and 12h light per day in double
Petri plates. At the end of 7 days 40 plates were placed in a hotter incubator (i.e 28, 32 or 40 C). Five plates were left in the control (21 C) incubator. These plates were removed 109 periodically from Che hot incubator and evaluated for conidial turgidity, germ inability and ability to form appressoria and for haustorial shriveling and encapsulation. The control plates were evaluated at the same times for purposes of com parison. The plates from the hot incubators were placed in the 21 C incubator after evaluation to check for recovery of powdery mildew. Haustoria were observed as detailed on page
87. Conidia were germinated on glass slides for 6h at 21 C in the light. The experiment was repeated once at 28 and 40 C. The experiment was also done using a variable temperature of 21 C for 16h a day and 33 C for 8h a day. The higher temperature in this latter experiment was to simulate high temperatures encountered in the afternoon in greenhouses. The lights in the incubator were set so that the 2h preceding the 33 C and the 8h of 33 C and the 2h after the 33 C were in the light portion of the day.
The experiment also was repeated with whole plants in
4 inch pots and 100% relative humidity at 28 and 32 C. This experiment was run twice at 32 C. Whole plants were also placed in the 40 C incubator to check for phytotoxicity.
These experiments were followed up with experiments that used hyphal growth and haustorial characteristics as parameters.
Mature, excised B. x hiemalis 'Schwabenland Red' leaves were inoculated with 0. begoniae race 2. The leaves were then incubated for 2 days at 21 C and 12h light per day. Leaves were removed to 28,30, or 32C incubators except for 5 leaves left at 110
21C as controls. Leaves at 28 or 32 C were periodically removed
(as per Table 27) and examined under a dissecting microscope
equipped with an optical micrometer for hyphal growth.
Epidermal peels for observing haustoria were made as detailed
on page 87. Control leaves were evaluated concurrently with
treated leaves. The treated leaves were placed in the 21 C
incubator after examination and were periodically reevalu
ated for recovery of the fungus' ability to grow.
Epidermal peels from the above experiments were fixed for
electron microscopy in TU glutaraldehyde in O.lM phosphate buffer at pH 6.8 for 2h at room temperature, rinsed with buffer
and stored in buffer in the refrigerator (4 C) until needed.
Then the material was postfixed in 1% osmium tetroxide in
phosphate buffer for Ih at room temperature, dehydrated in an
ethanol-propylene oxide series, and embedded in Spurr's low
viscosity embedding resin. Sections were cut using glass or
diamond knives, mounted on 200 or 400 mesh copper grids and
stained with TU uranyl acetate for 5-10 min and 0.4% lead
citrate for 1-2 min. Electron microscopy was done on a Hitachi
HU-llE transmission electron microscope. Thick sections for light
microscopy prepared from this material were placed on glass
slides and were stained with aqueous 0.25% methylene blue or
left unstained.
Results and Discussion: The most practical temperature
for eradication of 0. begoniae appears to be 32 C. Six days
of this temperature on whole plants in an incubator with Ill 3000 ft.-candles of light seems to kill the pathogen (figs.
15,16,17 and Tables 25,26 and 27). 32 C was found in earlier studies to be the lowest temperature which absolutely stopped germination on glass slides. On excised leaves, in double Petri plates, the fungus was killed in 3 days as evidenced by lack of hyphal growth when the fungus was transferred back to favorable temperatures (21 C) (fig. 17 and Table 27). It is not known for certain why the fungus survives longer on leaves intact on potted plants than on excised leaves (figs. 15,16,17 and Tables
25,26 and 27). Some possibilities are discussed later. At
32 C all growth of hyphae, haustorial formation and sporulation of 2 day old colonies is absolutely inhibited (fig. 17, Table
27).
Thirty two C is preferable to 28 and 30 C, because the fungus is killed much more rapidly at 28 C. It took 2-3 weeks to kill the fungus on excised leaves (figs. 15,16,17 and Tables 25,26,27 and 28) and it took 4-5 weeks to eradicate the fungus on whole plants (Tables 25,26,27). Some hyphal growth occurs, haustoria are laid down, and in some cases conidiophore initials form at 28 C. After 7 days of exposure hyphal growth stops, but the colonies are still capable of reuming growth if placed at 21 C.
Death of Oidium begoniae occurs when haustoria shrivel and become encapsulated. Invaded cells often are browned by melanin like pigments (fig. 19 ). Electron micrographs and thick sections show phenol-like bodies associated with haustoria
(figs. 19,21). Encapsulations are layered. The first layer 100
\\ °L conidia turgid
°L conidia germinable 75- §
I 50-
s
Days of heat treatment^ (excised leaves). Fig. 15 . Conidial characteristics of Q. begoniae over time at 28 and 32 C.
z) colonies grown for 7 days at 21C before treatment. Conidia exposed to eradicative temperature while still attached to conidiophore. 100
75 .2 / 28C haustoria walled I / 32C haustoria turgid I S 28C 25 32C
days exposed
Figure 16. Haustorial turgidity and encapsulation after exposure to high temperatures,
following 7 days of 21 C . 1000
800 21C
600 .5
400
S 32C
0 2 4 6 8 10 12 14 days exposed to treatment^
Fig. 17 . Growth of mycelium at eradicative temperatures.
y) point at which mycelium will not recover when placed at 21 C. z) All treatments grown for 2 days at 21 C before placement at eradicative temperature. Table 25: Effect of eradicative heat treatments on conidial
characteristics of attached conidia of Oidium begoniae
on Begonia x hiemalis.
7oNot %Germi- %with Treatment • Shriveled* nable^ Appressoria^ 1. Excised Leaves 21C,7 days 90aby 38ab 12bc 21C,8 days 9 lab 38ab 12bc 21C,9 days 94ab 34ab 12b c 210,11 days 93ab 38ab 12bc 210,14 days 96a 33abc 8cd 210,21 days 80bc 29b c 18ab 210,28 days 47hii 4fg 3de 210,7days+280,Iday 80abcd 17de 3de 210,7days+280,2days 73cde 4fg Ide 210,7days+280,3days 52ghij Oi Of 210,7days+280,4days Oi Of 210,7days+280,7days 6pq Oi Of 210,7days+280,lOdays O.lqr Oi Of 210,7days+280,14days Or Oi Of 210,7days+280,2days+2l0 ,Sdays 40ab 19ab 210,7days+280,lday+210,14days 71cdef 21cd 6cd 210,7days+280,2days+2l0,14days 38jklm 9efg 3de 210,7days+280,4days+2lO,14days 37jklm 23cd llbc 210,7days+280,7days+210,14days 38jkl 28bc 21a 210,7davs+280,14days+2l0,14days 2gh Ide 210,7days+320,Iday 73cdef 12ef 6cd 210,7days+320,2days 251mn Oi Of 2l0,7days+320,3days iq Oi Of 2l0,7days+320,4days Or Oi Of 210,7days+320,2days+2lO,Sdays 66defg 38ab 17ab 210,7days+320,2days+21C,14days 83abc 46a 25a 210.7davs+320.4davs+2l0.14davs Or Oi Of 210,7days+400,Iday O.Sqr Oi Of 210,7days+400,2days O.lqr Oi Of 210,7days+400,3days Or Oi Of 210,7days+400,2days+2l0,Iday Or Oi Of
2. Whole Plants 2lO,7days 86abc 44a 19ab 210,14days 96a 33ab 6cd 2lO,21days 77bcd 18de 4cd 210,28days 62efgh 16de 9cd 210,7days+280,4days,older leaves 42ijk 3gh O.Sef 2l0,7days+280,7days, " " 19nop 2gh 0.3ef 210, 7days4-280, lldays, " " llnopq 0.4hi Of 2lC,7days+280,14days, " " 6pq 2gh O.Sef Table 25 continued %Not %Germin- %With Treatment Shriveled'^ able^ Appressoria Whole Plants,older leaves contd. 21C,7days+28C,16days 2 pqy 2 hi 0 f 21C,7days+28C,28days 0 r 0 i 0 f 21C,7days+28C,14days+21C,14days 59 fghi 30 be 8 cd 21C,7days+28C,28days+2lC,14days 0 r 0 i 0 f
Whole Plants, younger leaves 21C,7days+28C,4days 59 fgh 5 f 2 de 21C,7days+28C,7days 49 hij 2 g 0.3 ef 21C,7days+28C,lldays 7 pq 0.4 hi 0.2 ef 21C,7days+28C,14days 5 pq 0.6 hi 0.1 ef 21C,7days+28C,16days 11 nopq 2 g 1 de 21C,7days+28C,18days 2 pq 0.8 hi 0.1 ef 21C,7days+28C,21days 0.3 qr 0.1 hi 0.1 ef 21C,7days+28C,23days 0.2 qr 0.1 hi 0 f 2lC,7days+28C,28days 0 r 0 i 0 f 21C,7days+28C,14days+2lC,14days 80 abed 38 ab 11 be 21C,7days+28C,28days+2lC,14days 0 r 0 i 0 f 21C,7days+32C,3days 25 Imn 0 i 0 f 21C,7days+32C,6days 0 r 0 i 0 f 21C,7days+32C,6days+21C,14days 0 r 0 i 0 f 21C,7days+32C,6days+21C,21days 0 r 0 i 0 f
Excised Leaves,treated with 16h,2lC+8h,33C daily 2lC,7days+(21+33),lday 76 bed 14 de 7 cd 2lC,7days+(21+33),2days 51 ghij 23 cd 10 be 2lC,7days+(21+33),3days 47 ghij 17 de 7 cd 21C,7days+(21+33),4days 13 nopq 2 g 1 de 2lC,7days+(21+33),7days 4 pq 1 g 0.1 ef w) conidia appressed to glass slides and examined immediately thereafter. x) conidia attached to conidiophores during treatment appressed to glass slides and incubated for 6h at 21 C and 80-95% relative humidity, y) numbers followed by the same letter in the same column do not differ significantly using Duncan's new multiple range test (p= 0.05). z) minute amount, less than 0.005%. Table 26: Effect of eradicative heat treatments on haustorial
shriveling and encapsulation of Oidium begoniae in
Begonia x hiemalis
Treatment ^Shriveled ^Encapsulated^
1. Excised Leaves 2lC,7days 5 3 21C,Sdays 6 2 21C,9days 10 12 21C,lldays 10 13 2lC,14days 19 5 2lC,21days 6 8 21C.28davs 25 8 2lC,7days+28C,lday 1 4 21C,7days+28C,2days 35 2 21C,7days+28C,4days 39 3 21C,7days+28C,7days 67 36 21C,7days+28C,9days 82 36 21C,7days+28C,14days 95 90 2lC,7days+28C,2liays 99 96 2 1 c ,7days+28C,7days+2lC,7days 61 29 2lr , 7dqvs+28n , 1 i.dflvs+21 r j 7d3ys 99 96 21C,2days432C,2days 99 63 21C,2days+32C,Sdays 98 32 21C, 7days+3 2C,Idays 9 1 21C,7days+32C,2days 31 24 21C,7days+32C,4days 88 76 21C,7days+32C,7days 99 75 21C,7days+32C,7days+2lC,7days 98 58 21C,7days+40C,Iday 41 24 21C,7days+40C,2days 52 12 21C,7days+40C,3days 70 16 Excised Leaves, treated with 16h,21C+8h,33C daily 2lC,7days+(21+33),lday 13 2 2lC,7days+(21+33),2days 37 43 2lC,7days+(21+33),3days 23 44 2lC,7days+(21+33),4days 72 79 21C,7days+(21+33),7days 44 50
2. Whole Plants 2lC,7days 10 5 2lC,14days 12 8 2lC,21days 32 1 Whole Plants,older leaves 210,7days+28C,14days 44 39 210,7days+280,2Sdays 96 92 Table 26 continued
Treatment ^Shriveled T^ncapsulated^ Whole Plants, younger leaves 21C,7days+28C,14days 78 16 2lC,7days+28C,28days______79______54______21C,/days+32C,3days 92 37 21C,7days+32C,6days 99 80 21C,7days+32C,6days+2lC,21days 99 92 y) Of the tests for eradication, haustorial characteristics are the least reliable, due to the subjective appraisal of what constitutes a shriveled haustorium. Encapsulation varied widely from leaf to leaf or even in areas of the leaf. z ) Encapsulations are wall-like depositions that surround the haustorium (fig. 16-19). Table 27. Effects of eradicative heat treatments on hyphal length
of Oidium begoniae on excised Begonia x hiemalis leaves.
Treatment Hyphal length Cum)
21C,lday SI 2lC,2days 162 j 21C,3days 48Shi 2lC,4days 686 g 21C,Sdays 961 f 21C,7days 1401 cd 21C, lldays 1917 b 2lC,37days 4688 a 4C,9days+2lC,5d 914 21C,2days+28C,2days S91 g 21C,2days+28C,Sdays 603 g 21C,2days+28C,7days 608 g 21C,2dayS+28C,9days 616 g 2lC,2days+28c,14days 648 g 21C,2days+28C,9days+21C,Sdays 1138 21C,2days+28C,14days+2lC,Sdays S94 g 2lC,2days+30C,lday 410 2lC,2days+30C,2days 429 hi 21C,2days+30C,4days 461 hi 21C,2days+30C,Sdays 481 hi 21C,2days+30C,7days 47S hi 21C,2dayS+30C,12days S06 h 21C,2days+30C,14days 488 hi 21C,2days+30C,7days+2lC,7days 1370 d 21C,2days+30C,9days+21C,Sdays 1101 21C,2days+30C,12days+21C,7days 931 f 21C,2days+30C,14days+21C,7days 491 hi 21C,2days+32C,3days 232 j 21C,2days+32C,Sdays 226 j 21C,2days+32C,Sdays+21C,7days 193 j z) numbers followed by the same letter do not differ signifi cantly using Duncan's new multiple range test (p- 0.05). Numbers from 2 experiments were pooled for the statistical Table 28; Effect of 28C on sporulation of Oidiutr. beg;oniae
on excised Begonia x hiemalis leaves
Treatment ^spores/colony^
21C,9days 465 a 21C,2dayS+28C,lday+21C,6days 150 b 21C,2dayS+28C,2days+21C,Sdays 60 be 21C,2days+28C,3days+2lC,4days 9 cd 21C,2days+28C,5days+21C,2days 0 d 21C,2days+28C,7days 0 d y) Method used to determine amount of sporulation is given on pages 95-96. z) numbers followed by the same letter do not differ signifi cantly at the 0.05 level using Duncan's new multiple range Figure 18. Encapsulation of haustoria in progress in whole mount stained in Trypan blue. Note encapsulation extends from host wall and partly surrounds haustorium. x 600. Figure 19. Thick section of encapsulated Oidium begoniae
haustorium in Begonia x hiemalis and treated
with 2lC for 7 days followed by 7 days of
28 C. X 1000. Figure 20. Transmission eleccron micrograph of early stage
of encapsulation, showing vessicular network
(N) between the haustorial matrix and host
cytoplasm. Note that encapsulation procédés
from the host cell wall towards the interior
of the cell. Note chloroplast (C) 'emitting'
electron dense bodies, x 8800. Figure 4 is
an electron micrograph of a control haustorium
that can be compared to this one. ï Figure 21. Transmission electron micrograph of encapsulated
and shriveled haustorium. Note the compressed
lobes of the haustorium (L) and the shrunken
matrix area (M). The outer layer appears to be
remnants of the host cytoplasm, x 14,300.
Figure 22. Transmission electron micrograph of encapsulated
penetration peg in tangential section, x 21 ,200.
127 develops as a vesicular network in between the haustorial matrix membrane and the host cytoplasm (fig. 20). This layer becomes electron dense and homogenious as time progresses and haustoria shrivel (figs. 21,22). Another layer is sometimes observed external to the first layer and is more electron dense and thinner. This outer layer appears to be derived from compression of the host cytoplasm (fig.22). Exterior to this layer are often found phenol-like globules (figs. 19,21).
Haustoria] encasement has been viewed as an extension of papillum or wound plug formation in other studies (1,5'.. Such structures can form in response to heat as well as wounding by biotic or abiotic agents (1). The process of encasement begins with an alignment of the host cytoplasm towards the wound. The encasement then begins as a vesiculate electron dense layer between the host cytoplasm and the haustorial matrix (fig. 20). The encapsulation begins at the host cell wall and is 'built' towards the interior of the cell until it surrounds the haustorium (figs. 18,20). Such encasements are known in Oomycetes, rusts and powdery mildew of rose (5,
28,49). The encasements appear to be largely composed of callose, but later in development, polysaccharides and lignins are found too (1,5). The function of encasements in other organisms has been postulated to be: 1) to limit fungal growth. Callose is known to prevent the transport of small molecules; 2) to retard the hypersensitive reaction in incompatible hosts. This gives greater host tolerance to toxic effects of the haustoria, 128 especially if the sheath membrane should be breached during haustorial senescence (5). Evidence from other studies suggest that encapsulations are of host origin (1,5).
The means of survival at 28 C after 1 week when hyphal growth had stopped was sought for. It was conjectured that encapsulated haustoria could serve as means of pathogen survival during periods of high temperature. This idea dates from observations by Peglion in 1905(74) who was working with powdery mildew of Euonymus sp. His suggestion of over summering of this fungus as haustoria was based on appearance of the encapsulated haustoria and not on evidence such as germination of the haustoria after other parts of the fungus had died, I was unable to demonstrate this either. In all cases encapsulation of haustoria was accompanied by evidence of fungal senescence, such as phenol-like bodies when examined under the electron uiicroscope. Furthermore encapsulation and fungal haustorial shriveling were inversely proportional
(Table 26, fig. 16), indicating that they occurred at about the same time. However, some encapsulated haustoria remain alive afor at least 2-4 days since hyphae arising from them will resume normal growth when placed at 21 C (fig. 17).
However survival by this means for longer periods of time is doubtful.
Survival as dormant mycelium was observed and is considered the primary means of survival during periods of 27-30 C. This mycelium was observed not to grow after 1 week of 28 G (Table 129
27). After 2-3 weeks exposure, though, a low percentage of the colonies on excised leaves remained alive and were capable of resuming growth when placed in 21 C (Table 27). On whole plants it was demonstrated that mycelia could survive for up to 4 weeks, primarily on young leaves (Taoles 25,26). These holdover mycelia gave rise to sporulating colonies within 21 days when returned to
21 C.
Forty C seems to be only slightly more effecacious than 32 C
(Tables 25,26) and so the lower temperature appears to be more economical. Forty C is phytotoxic to the host causing leaf brow ning, wilting, and defoliation within 24 h. At 50 C these symptoms are observed in one half hour. Economics of heat eradication of 0. begoniae
I have demonstrated the feasibility of preventing and eradi
cating Oidium begoniae with 32 C heat treatments. Two questions
remain unanswered. They are : 1) will heat work when used in
a commercial greenhouse and 2 ) what is the cost in comparison to
alternative control measures. I believe that the answer to question
one will be yes. Some adjustments to higher temperatures such
as increasing shading to prevent sunscorch will be necessary. Some
fungicides and insecticides are more phytotoxic at higher tempera
tures and extra caution must be made when using them during heat
treatments. If stock plants are treated within one month of
use for propagation, the beneficial effects of short day treatments may be annulled (see Chapter 1). In a preventative, long-term
treatment program these cultural problems would be more serious
than in a short term eradicative program.
The costs of continuous preventative heat treatments
is prohibitive. However, the cost of eradicative heat
treatments are competitive with fungicides. Heat treatments are,in addition,more effective than eradicative contact
fungicides such as dinocap. Furthermore,since benomyl cannot be depended upon and registration of new systemic fungicides may be slow in coming»the use of heat during flowering may be
especially useful. Dinocap, the most effecive fungicide
available,does burn flowers, especially at high temperatures.
The cost of fungicide application is determined by cost of materials added with cost of labor. Several applications will be necessary with dinocap or benomyl. The cost of heat eradi cation is determined by the price of fuel, the outside tempera ture, the amount of insulation in the greenhouse and the time of application. This is observed in the formula worked out by Gray (27) H=UA(t]^-to) where H=heat required in BTU's per hour,
U= a constant relating to insulation, A= exposed surface area in square feet, t%= inside temperature in degrees F and tg = outside temperature in degrees F.
Some examples will demonstrate the costs of fungicides versus heat treatments.
1. Fungicides costs:
Price of materials $2-$20/ acre
Cost of labor= 2 men x 4 hrs x $5/ hr. per acre = $40
If three applications (weekly) are needed,then
total cost = ($42 to $60) x 3= $126-$180/acre
2. Heat treatment costs:
Price of fuel : Coal 520-25/ ton Natural gas $3.30/ 1000 ft Fuel oil $1.00 or more/ gal.
Heat produced by fuels : Coal 2.4-3.2 x lO' BTU/ ton Natural gas 1.1 x 10^ BTU/1000 ft Fuel oil 1.8 X 105
Amount of fuel used in BTUs: In this calculation the normal amount of fuel used must be subtracted from the total amount used to give the added amount used in heat treatments like so:
H (in BTU s)= (UA (t3 -ti))-(UA(t2-t^)) 132
where C3 is 32 C and C2 is the normal temperature heated to.
Some problems are worked out below:
a)doubIe plastic over glass, one acre treated, outside temperature averages 40 F, 90 F (32 C) for 6 days used to eradicate 0. begoniae. Normal greenhouse temperature is 70 F.
H= (0.38 X 53500 (90-40))-(0.38 x 53500(70-40)) = 406600 Btu/hr.
6 days = 144 hrs. 406600 BTU/hr. x 144 hrs. = 58,550,400 BTU
Cost with coal=58550400 BTU ^ $23/ton 2.8 X 10^ BTU/ton =$48
Cost with natural gas = 58,550,400 BTU $3.30/ 1000 ft^ 1.1 X 10*3/1000 ft-^ = $176
Cost with fuel oil = 58,550.400 BTU $1.00/gal 1.8 X 10^ BTU/ gal. ^ =$325
b) single glass layer, gas used as fuel, one acre treated, 40 F outside, 70 F inside normally, treatment is 90 F for 6 days.
H = (UA (to-t,))-(UA (t2-C]_) = (1.2 X 53,500(90-40))-(1.2 x 53,500(70-40)) = 1,284,000 BTU/hr. for 144 hrs=1.85 x 10^ BTU
cost = 1.85 X 10^ BTU______$3.30/ 1000 ft^ 1.1 X 10*3 BTU/ 1000 ft 3 = $555
Very few greenhouses will have an acre of infected begonias.
Treating smaller areas than above will certainly lower the cost of the treatments and make them more competitive with fungi cides. Insulation, as seen in the problems above certainly helps make this a more attractive control measure! 133
Perhaps a good time to apply this treatment would be just prior to shipping by the propagator. A small, well-insulated room or chamber could be heated to this temperature. This would add assurrance to the wholesaler that he has received disease-free plants. This treatment would not interfere with short day treatments as plants are normally given long days at this time. It could be done under lights instead of under glass, thus reducing possibilities of sunscalding at high temperatures.
Treatment of the young plants is yet to be extensively tested, though. Effects of Light on 0. begonlae
Purpose: To determine if photoperiodic effects on sporulation
and release of conidia occur in 0 . begoniae,as reported in ocher
species of powdery mildews. Such effects could lead to diurnal
synchrony in the life cycle of the pathogen, Then the correct
timing of heat or chemical treatment can be precisely determined
by onset of light or dark periods.
Light is also a form of energy that can be absorbed by the
host. A certain portion of this energy is converted to heat which on a sunny day could raise the temperature of leaves.
Investigations were made to determine if this effect could help
eradicate powdery mildew with heat.
Procedure: Determination of photoperiodic affects: Oidiura begoniae race 2 was inoculated onto excised B. x hiemalis *Schwaben- land R e d ' and incubated for 5 days in 12h light per day with onset of the light period at 7 am. The light was 2000 ft.-candles
in intensity. The excised leaves were then folded and taped so
that conidiophores on the edge of the fold could be continuously observed with a compound microscope. The leaf was left at the
same position on the microscope stage so that development of conidiophores could be observed over a period of several days.
The microscope was inside the 21 C growth chamber during the
period of observation. The light period was changed periodically
so that onset of day occurred at different times and the effects
of this change on conidiophore and conidium development was 135 observed.
Excised B. x hiemalis leaves were also inoculaced wich 0. begoniae race 2 and grown under conditions of 24h light and 12h light/ day. Incubation under these conditions was for 5 days and was also under conditions of 21 C and 100% relative humidity.
Light was 2000 ft.-candles. At the end of 5 days hyphal length and synchrony of conidiophore development was observed. Amount of sporulation was qualitatively noticed.
Diurnal periodicity of conidium release on whole plants in
the greenhouse was determined using a Burkart spore trap in
March, 1978. Cellophane strips were lubricated with Vaseline petroleum jelly to which spores were stuck after being sucked into contact with the cellophane strip. Suction of the trap was set at the rate of 10 Liters/ minute. Strips were cut into 1 day
long strips, mounted on glass slides and examined under a compound microscope (lOOx) for conidia. Glass slides were marked so that one hourh time on the tape could be determined.
Determination of light-leaf surface temperature interactions:
The air, leaf and soil temperatures of 10 plants under shade was determined at lpm-2pm on March 3, 1980. The plants were placed
in full "sun for 30-50min. after readings were taken and the
readings were taken again using the same leaves as before. Inten
sity of light on leaf surfaces was also determined. Leaf tem
peratures were determined by placing a normal mercury thermometer
on the leaf surface, rolling the leaf around the thermometer. 136
removing the plant to a shady location and then reading the temperature after 60 sec. It did not appear that temperatures were arti
ficially raised inside the rolled up leaf, if it was removed to a
shady location between the rolling and reading.
Results and Discusssion: Development of conidia of 0. begoniae was in a diurnal fashion that was regulated by onset of the light
portion of the day. The production of conidia occurred with much the same timing as it did in Erysiphe polvgoni on clover (73).
Division of the distal cell of the conidiophore is completed by late afternoon, about 10-15 h after onset of photoperiod, giving rise to a conidial initial (fig. 23)(Table 29). The conidial initial enlarges
for 22-24 h and then begins to swell and assume conidial shape.
Forty eight h after delimitation (5-7 h after sunrise) the conidium is
cut off from the conidiophore. Thus, it takes about 48 h from the
initial mitotic division to conidial release. Conidiophores out
of synchrony in Table 29 were often found to be not producing conidia at all or were just forming. Twenty four hours after the first mitotic division, another one takes place. Thus, 2 conidia in different stages of development will be maturing on the same conidio
phore and 1 conidium is released per day. Changing the time of
onset of the light period changed the timing of development of conidia
in a corresponding manner (Table 31). Thus, if lights were changed
so that they came on in a growth chamber 4 h earlier than the previous
day, within 48 h conidia also were being formed 4 h earlier.
Continuous light and dark had little effect on germination
percentages on glass or hyphal lengths on excised leaves (Table 30). Table 29 : Percent of conidiophores and conidia of Oidium
begoniae on Begonia x hiemalis with indicated stage
of development by hour of day.
Stage Hours after onset of light period 0-2 2-4 5-6 7-10 IL-12 57, 67, 47, 127, 107, 2 07, 117, 27, 07, 227, 3 47, 137, 07, 37, 37, 2+1 47, 77, 47, 97, 327, 3+1 807, 56% 317, 117, 297, 3+ld(=3) 57, 6% 58% 667, 37,
z) number is number of calls in conidiophore plus number of conidia. d=detached conidia still on conidiophore, but ready to be blown away, (see fig. 23). Percents derived from watching the development of about 150 conidia.
Table 30 ; Effect of continuous light and dark on hyphal length
of Oidium begoniae after 5 days growth on Begonia
X hiemalis.
Treatment Hyphal length in micrometers
I2h light, 12h dark/day 975 a^ 24h light/day 943 a 24h dark/day 917 a
z) numbers followed by the same letter do not differ signifi cantly using Duncan's new multiple range test (p-0.05). Numbers from 2 experiments were pooled for the statistical A
[_3 [=-] czb
0 0 ft 0 0 f t
r l dlb c (z:.n
sunrise 3h 5h 9-12h 17U
3+1 3+ld 3+1 3+1
Fig. 23 . Diurnal development: o£ conidiophores and conidia of Oidium beRoniae. y) numbers in this row refer to number of conidiophore cells plus conidia. z) numbers in this row refer to hours after sunrise. 139
Table 31. Effect of changing onset of light on percent stage of
development of conidiophores and conidia.
______Time of onset of lights Time of reading Stage^ 1 am 5 am 8 am
1 8 % 8 % 5 7= 2 8 10 0 3 4 12 4 2+1 4 2 4 3+1 25 68 80 3+ld 50 0 5
11 am 1 4 4 6 2 9 8 11 3 3 8 13 2+1 40 4 7 3+1 45 40 56 3+ld 0 36 6
4 pm 1 9 12 2 5 0 3 6 3 2+1 42 9 3+1 30 11 3+ld 6 66
y) Number of cells in conidiophore plus conidium. d“ detached conidium. See fig. 23 for further illustration. z) Percent derived from watching the development of 40-65 conidia per treatment. 140 However, reduced sporulation was noticed with these treatments.
In continuous dark, synchrony of conidium production was lessened somewhat and a greater percentage of conidiophores were in early stages of development (Table 32).
Conidia trapped in the greenhouse, in a Burkart spore trap, showed that release occurred mostly from 11 am to 1 pm (Table 33),
This is close to the 5-6 h after sunrise that we would have expected from watching conidia develop in the lab (fig. 23, Table 29,31).
Reading the cellophane strips from the Burkart spore trap was not exact and an error of 1 h in reading them is possible.
Elevation in leaf temperature over air temperature by direct sunlight was minimal in the test reported in Table 34. In a slightly less controlled experiment leaf surface elevations of 5 C were found. Elevation of air temperatures in sun versus the shade was significant and is likely to be the main cause for greater amounts of powdery mildews in the shade. Relative humidity was close to equal, both outside and inside the canopy of the plants, except when the plants had just been watered or when large temperature differences were noted. In these cases relative humidity was greater under the canopy. The fact that canopy differences in relative humidity plays only a small part in increasing begonia mildew in the shade. With plants such as rose or zinnia, which have less dense canopies, relative humidity can be expected to play an even smaller part in shade increased mildew. Table 32 : Effect of continuous dark on synchrony of develop
ment of conidiophores and conidia of 0 . begoniae
after 5 days, 3h after onset of light period on
fifth day.
Stage^ 12h light, 12h dark/day 24h dark/day
1 6 .6 % 8 .8 % 2 12.3 21.4 3 12.9 25.5 2+1 4.9 9.9 3+1 63.2 32.7 3+ld 0 1.6
z) number is number of cells in conidiophore plus number of conidia. d=decached conidia still on conidiophore, but ready to be biown away. Table 33 : Conidia of Oidium begoniae collected by a Burkart
spore trap set amongst infected B. x hiemalis
plants in a greenhouse in March, 1978 at different
hours of the day.
Time U 1 3Z6 3/7 3/27 3/28 3/29 3/30 Total l-7am 0 0 2 1 0 3 0 0 0 0 0 0 9-lOam 0 0 0 0 0 0 0 0 10-llam 1 0 0 0 0 0 0 1 llam-noon 17 1 45 32 2 0 15 112 . noon-lpm 1 4 5 198 0 0 6 214 l-2pm 0 0 0 4 0 0 1 5 2-3pm 0 0 0 0 0 0 0 0 3 -4pm 0 0 0 0 0 0 0 0 4-5p 0 0 0 1 0 0 0 1 5-6 pm 0 0 0 1 0 0 0 1 6-7pm 0 0 0 0 0 0 0 0 7-8pm 0 0 0 0 0 0 0 0 8-9pm 0 0 0 0 0 0 0 0 9-lOpm 0 0 0 0 0 0 0 0 10-llpm 0 0 0 1 0 2 0 3 ll-12pm 0 0 0 0 0 0 0 0 12pm-lam 0 0 0 0 0 1 0 1
Table 34; Temperature of Begonia x hiemalis 'Schwabenland
R e d ’ leaves in sun or under shade; readings taken
March 3, 1980.
Treatment Lux of light Temperatures ( C) air leaf Sun 75,500 25.4 26.4 27.8 Shade 8,450 24.5 21.6 21.5 143
Use of light induced synchronous morphogenesis of Oidium begoniae and of heat to prevent: powdery mildew infections.
Purpose: To determine whether applying 29 C at certain times of day will prevent 0. begoniae infections. If so, prevention by heat treatments would become more economical.
Since production and release of conidia tends to be diurnal and synchronous, germination, penetration and development of new colo nies is also fairly synchronous. This means that after the most vulnerable stage is found,only this stage need be treated.
Furthermore,use of lights can change the diurnal cycle of the fungus and place vulnerable stages in the noonday sun. The sun would then supply the eradicative or preventative heat without additional cost to the grower.
Procedure: Mature, excised B. x hiemalis leaves were inocu lated with Oidium begoniae race 2. The leaves were then placed in a 29C or 21C incubator for the hours indicated in Table 35.
After 24h all leaves were incubated in double Petri plates at 21 C and 12h light per day. Seven days after inoculation, the number of visible colonies per leaf was determined. There were
5 leaves per treatment and the experiment was repeated 3 times.
Results and Discussion: The greatest inhibition occurred when 29 C was applied 8-16h after inoculation (Table 35), This corresponds with penetration and the initial stages of haus- torial formation. This is not surprising in light of previous Table 35 ; Effects of 29 C given at various periods before
or after inoculation on number of visible Oidium
begoniae colonies on B. x hiemalis after 7 days at 2lC.
Hours after inocu• Hours after onset Developmental Colonies lation at 29 of light at 29 C stage at 29 C
Constant 21 C never at 29 C no stages 172 d%z Constant 29 C for seven days. always at 29 C all stages • 0 a -24 to 0 19-5h before onset preinoculation 138 cd 0 to 24 5 to 29 all stages 21 b 0 to 4 5 to 9 germination 126 c 4 to 8 9 to 13 germination and 131 cd appressorium forms 8 to 16 13 to 21 penetration and 25 b haustoria form 16 to 24 21 to 29 haustoria en 98 c large w) At 21 C all other times and for last 6 days, x) Numbers followed by same letter do not differ significantly at 0.05 level using Duncans new multiple range test, y) This column tells how many hours after sunrise the heat was applied. Note that in normal circumstance the hottest part of the day occurs 4-7h after sunrise which corresponds approximately with the fourth treatment. At this time inoculation and germination occurs in 'natural' infections. 2) Numbers are the means of three experiments which were pooled for the statistical test. 145 findings of effects of temperature (e.g.^ pages 76-89). These previous experiments showed that germination and appressorium formation occurred at somewhat reduced rates (50%+ of control), that haustorium formation was much reduced (less than 5% of control) and that haustoria soon died at 28 C. It is interesting to note that so short an exposure (8h) can cause these events to
Unfortunately, total control did not occur (Table 35) and surviving colonies had normal amounts of sporulation. Since no powdery mildew can be tolerated in greenhouses on susceptible begoniasjthis treatment can not be recommended. The expense of the heat would not justify it.
Use of supplemental lighting to produce long days that increase vegetative growth can also be used to change the onset of the light period. Applying this light well before sunset may move the timing of the diurnal cycle of the fungus back (see last section) and expose the more vulnerable stages of the fungus to the heat of midday. To properly treat the fungus using lights, turn the lights on at about 10 pm and leave them on till sunrise.
Then allow the greenhouse to heat up to 29 C (86F) in the afternoon.
Shade greenhouse benches to prevent sunscorch. The practicality of
this method of prevention is untested in commercial greenhouses. 146 Effects of free water on Oidium begoniae conidia.
Purpose; To determine the effects of free water on conidia
of 0. begoniae. The inhibitory nature of free water on powdery mildews is well known (68). In greenhouses rose powdery mildew was controlled by misting the crop (52). Inoculation of powdery mildew of begonia by spraying conidia suspended in water has been tried (75). It was not known how long the conidia can
survive in such suspensions. Germination of conidia of Sphaero-
theca pannosa on peach that were in contact with water but not
submerged germinated at a higher rate than conidia not in contact with water(64). An investigation was made wich conidia of 0. begoniae to see if this phenomenon occurs on begonias too.
Morphology of germ tubes and appressorial formation was observed
Procedure: Conidia of 0 idium begoniae race 2, at least 957, nonshriveled from 7 day old infections, were mixed into distilled water for 0,10,30,60 or 120 min. and then sprayed onto excised B. x
hiemalis leaves. Colonies/leaf were counted after 7 days
incubation at 21 C. There were 3-5 leaves per treatment and the
experiment was repeated 5 times.
Conidia of 0. begoniae race 2 were either inoculated onto
drops of water or inoculated onto parafilm. They were then
incubated at 21C, 100% relative humidity, and in the light.
After 7h the parafilm and water drops were placed under a
compound microscope and conidia were evaluated for shriveling,
germination, and appressorial formation. 600 conidia were 147 counted per treatment and the experiment was done twice.
Results and Discussion: Conidia lost their ability to cause disease when suspended in water for 30-60 minutes (Table
36). Conidia were often observed on leaves during the course of these studies which had not germinated and were in the midst of water condensation marks (fig. 24). Since free water injures the ability of conidia to germinate in 10-60 min. care must be taken when inoculating plants by this method as it is a source of variability and error. Free water may be a possible control measure but this needs further testing.
Although submersion in water is probably universally harmful to powdery mildew conidia, condensation, dews and relative humidities causing condensation are linked to epidemics of powdery mildew of apples and roses (3,52). These mildews germinate at very low rates on dry glass slides (i.e. 2% at 99% relative humidity) but germinate much better when floated on drops of water (64). Conidia of begonia mildew floated on water drops also germinated at higher percentages in 7h (Table 37). However, it is not known whether this was due to more rapid germination or whether ultimately a higher percentage of conidia would have germinated. Germ tubes were noted to grow towards dry surfaces and away from water. Germ tubes on water were much longer than on dry surfaces. Free water may have slowed dessi cation of conidia which seems only natural. Appressorial formation was much inhibited on water drops (Table 37). Table 36 : Number of visible colonies formed from conidia
suspended in water for various periods of time and
afterwards sprayed onto B. x hiemalis leaves.
Minutes conidia emersed Trial 1 _2_ __5_ 1 19 13 3 TT 11 13 10 31 -- 2 18 0 13 30 0 1 2 18 0 4 60 0 0 0 0 120 0 0 0 1 0 0.2
Table 37. Percent dessication, germination appressorium
formation of conidia floating on water or on a dry surfac.
Treatment %dessication ^germination %appressoria _ k_ 2 1 2 1 2 on parafilm 3.2 13.0 37.9 38.5 11.8 20.5 on water drop 0.5 2.5 52.1 77.2 1.6 0.1 Figure 24. Ungerminaced conidia included inside water
condensation mark 24h after inoculation
X 450. 150 Effects of drought stress on 0. beejoniae infections.
Purpose: To verify reports that begonia powdery mildew
is more serious when plants are under-watered ( 6 ). If so,
this imformation would reinforce the need for proper watering.
Soil amendments that retain water such as Viterra-hydrogel could also be useful in keeping plants watered and disease
Procedure; Twenty five plants (B. x hiemalis 'Schwabenland
Red') were placed on greenhouse benches at Wooster, Ohio to the south of a group of begonias infected with 0. begoniae race 1, These plants were watered once per week. To the west of the infected plants were set 20 more plants which were watered 3-5 times per week. Both groups of plants were periodically checked for number of mildewed leaves per plant. Both groups had 5 rows of plants with the nearest row within 6 inches of the infected
Twenty plants were grown in 4 inch pots in which 0.25 lbs. of
Viterra 2 hydrogel (Union Carbide Corp.) was added per cubic foot of Jiffy mix. These plants were randomly intermixed with
20 plants planted in Jiffy mix alone. All plants in this experiment were watered 3 times per week. The number of visible colonies per leaf was counted after serious infection had occurred.
In both experiments care was taken to keep water off of leaves, thus minimizing inhibitory effects of free water . 151 Results and discussion: Watering plants once per week caused the
soil to often appear dry, but caused no wilting. Plants grown
in such conditions did lose some of their leaf glossiness though, and appeared a paler green. Such plants had 70% of leaves with visible infections 1 week after the first visible colony appeared when plants watered 3 times per week had 22% of leaves visibly
infected.
In a separate experiment,addition of Viterra 2 hydrogel,
a soil additive which increases soil moisture holding capacity,
had 53.2 visible infections per plant while untreated plants had
120.5. This result was significant,using a student's t test,at
the 0.05 level.
These results point out the importance of keeping plants watered. Viterra 2 hydrogel can aid the grower in this pursuit. 152
Effects o£ nutrition on number of colonies of Oidium begoniae.
Purpose: To observe the affect of nutrients on 0. begoniae.
Such information can be used to better understand onset of epidemics and to control the disease.
Procedure: On April 13,18, 20, 25 and 27, 1978 ,100 ml of boric acid solution (after 67') at 0,25,75 and 225 ppm were poured into pots containing B. x hiemalis 'Schwabenland R e d ' plants.
There were 20 plants in each treatment. There were 16 rows of plants (5 to a row) set on a greenhouse bench in Wooster Ohio.
Complete randomization of the treatments was amended so that at least one plant of every treatment was found in every row. Plants
infected with 0 . begoniae race 1 were placed in a row 6 inches
from the westernmost row of experimental plants and mildew
spread from these plants through the experimental ploP^
The effects of nitrogen, phosphorus, potassium and magne
sium sulfate in double Petri plates on the disease was inves
tigated in the spring of 1979. Nitrogen (NH4NO3 ), phosphorous
(NaH2P04) , potassium (KCl), phosphorus + potassium, nitrogen + phosphous + potassium or magnesium sulfate (MgSO^) were mixed
in distilled water as 0.001,.0.002, 0.004. or 0.008 molar
solutions. There were either 3 plates per treatment repeated
4 times or 5 plates per treatment repeated 3 times. Mature, excisedB. x hiemalis 'Schwabenland Red' leaves were placed in the
plates and allowed to grow in the nutrient solution for 2 weeks 153 in the 21 C Incubator with 12h light per day (2200-3000 ft.-
candles) . The leaves were then inoculated with 0. begoniae
race 2 and incubated for 7 days more. The number of colonies
per leaf was determined.
Results and Discussion; Boric acid delayed the appearance
of infections on B. x hiemalis leaves but did not slow the rate
of spread of the disease,once plants were visibly infected (Table
38). The delay was longest at high concentrations of boric acid.
Loss of protectant ability may have been due to leaching of boric acid from the soil. If boric acid had been added during the period
of infection the disease may have been prevented. Severe
phytotoxicity was noticed at 75 and 225 ppm on May 27 after four weekly treatments with 100 ml of these solutions. It was
suspected that both root uptake and solution in contact with
foliage was responsible for necrotic spots and marginal burn seen
on the leaves. At 225ppm plants were also severely stunted and
had greatly reduced number of shoots. Follow up experiments in double Petri plates indicate that boric acid may inhibit mildew
only at concentrations that harm the host.
Boric acid may be involved in partitioning metabolism
between the glycolytic and pentose shunt pathway (37) . At
high concentrations this may lead to production of phenols and
phytoalexin type compounds which in other plants have been
found to have the intermediaries of the pentose shunt pathway as
precursors. The high concentration of these compounds produced when 225ppm boric acid is added weekly may be harmful to host 154
Table 38 : Effect of boric acid root drenches on percent of
Begonia x hiemalis 'Schwabenland Red' plants infected
with Oidium begoniae race 1.
Distance from Date of reading^ Treatment^ source of inoculum^ izi. 5/9 5/16 i m control(Oppm) 0-34 cm 10 .0 = 34.2 70.0 92.8 8.5 39.0 58.1 92.8 75 ppm 1.2 2.5 38.6 71.4 225ppm 0.0 0.0 20.7 53.4 control 34-68 cm 0.0 11.3 35.1 79.8 25ppm 0.0 2.0 14.0 73.5 75ppm 0.0 0.0 10.5 34.7 225ppm 0.0 0.0 5.6 21.4 control 68-102 cm 0.0 5.8 26.0 69.5 0.0 0.0 12.0 35.6 75ppm 0.0 1.6 17.7 33.8 225ppm 0.0 0.0 2.8 16.9 control 102-136 ctn 0.0 0.0 27.0 62.9 0.0 0.0 13.6 46.4 75ppm 0.0 0.0 7.1 28.6 225ppm 0.0 0.0 2.4 15.9
w) Boric acid of these concentrations pured onto potted plants on 4/13,4/18,4/20,4/25 and 4/27, 100ml per treatment. On 4/27 severe phytotoxicity was noted on the 75 and 225 ppm treatments, x) The plants were first exposed to infected plants on April 13. The first infections were noted on plants adjacent to these infected plants on April 27. y) One row of infected plants was set on one side of the plot to act as the source of inoculum. Naturally uninfected plants closest to this row were infected first by windborne conidia, and the disease tended to spread row by row through the plot, z) Numbers are percent of leaves with visible infections. 155
and fungus alike. These compounds may be produced in response
to wounding by boric acid too, and their presence would coinci
dentally inhibit powdery mildew. This inhibition was not due
to lower soil pH which was measured at 6.25 after the fifth
225ppm treatment. Control pots also had a pH of 6.25.
In double Petri plates phosphorus (NaH2P0 /^) alone appeared
to be the best treatment for suppressing powdery mildew of
begonia (Table 39). Unfortunately this beneficial effect was
not found when potassium chloride was added. Nitrogen (NH4NO4 )
at very high levels may have slightly decreased mildew but the
difference was not statistically signigicant from the control.
Potassium (KCl) and magnesium sulfate appeared to have little
effect on powdery mildew colony numbers. These results must be
interpreted with care because of the artificial nature of the
environment in double Petri plates.
From the literature it was suspected that boron and sulfate would have been inhibitory to mildew and that nitrogen would
have been stimulatory. The stimulatory effect of nitrogen is
associated with the rapid production of young, susceptible
leaves in most plants,but no such leaves were produced in the
3 weeks the excised leaves were in double Petri plates. So this
effect of nitrogen was not present. A literature review of the
effects of nutrition was presented on pages 67-70 . 156
Table 39 : Effects of various nutrient regimes on number of
visible colonies of Oidium begoniae per Begonia
X hiemalis 'Schwabenland R e d ' leaf in double Petri plate.
Nutrient regime .OOlM .002M .004M .008M control (no nutrients) 100 bc^z 100 be 100 be 100 be NH4 NO 3 95abc 90abc 71ab 7 lab NaH2 P04 Slabc 64a 75ab 74ab KCl 89abc 109 c 78abc 96abc NaH2P04 + KCl* 77abc 97abc 92abc 98abc NH4NO 3 + NaH2P04 + KCl Slabc 83abc 85abc 77abc MgS04 109 be 99abc SOabc 87abc x) each salt added at indicated molarity y) numbers followed by the same letters do not differ signifi cantly at the 0.05 level using Duncan's new multiple range
z) numbers are number of visible colonies expressed as a percent of the control. Numbers are the means of three or four experiments which were pooled for the statistical test. General Summary: Prevention and eradication of Oidium begoniae
using environmental manipulations.
In summary,0. begoniae has an optimum temperature of 21 C
for germination, appressorial formation, haustorial formation,
growth of hyphae, sporulation and formation of visible colonies. *
Germination and hyphal growth may be more rapid at 24 during
the first 4 and 24h respectively, though. The minimum temperature
for germination is less than 4 C,but haustorial formation will not take place below 6-8 C. The maximum temperature for germination, haustorial formation and hyphal growth are 32, 30 and 28 C respec tively. Prevention of the disease by growing begonias above these
temperatures is not economically or culturally desirable.
Eradication of 0. begoniae with 32 C for 6 days may be a useful control measure. This treatment could be used to kill mildew in protected places that a contact fungicide could not reach. It could also be used in place of benomyl during time of flowering. Proper shading of benches is necessary to prevent sunbuming of leaves and flowers when high temperatures are present. Eradication with temperatures below 32 C take too long
to be economically feasible. Temperatures above 32 C are not much more efficacious or are phytotoxic.
High relative humidities decrease dessication of conidia and
thus slightly increase germination. However, high relative humidity may slightly decrease hyphal growth and sporulation. 158
The overall effect of high relative humidity was to slightly
increase the number of visible colonies. However, lowering the relative humidity is not considered an effective control measure of this fungus.
The onset of the day synchronizes a diurnal production of conidia. Changing the time of onset of light changes the time of release of conidia and thus affects the timing of germination,
penetration and other stages of the pathogen life cycle. Intense
light raises air temperature and additionally increases leaf
temperatures,causing inhibition of the fungus in sunny locations.
Submersion in free water is inhibitoiry to conidial germination.
Free water on leaves may be the main cause of lower germination rates on leaves compared to glass slides. However, conidia
floating and not submerged in free water had enhanced germination
rates but rarely formed appressoria.
Powdery mildew was more abundant on drought stressed plants.
Addition of Viterra 2 hydrogel to the soil, which would reduce drought stress, reduced disease also. Keeping plants adequately watered will help keep disease low.
High boron and phosphorus helped keep disease low. However,
the boron was effective only at phytotoxic levels and the effects
of phosphorus were negated by addition of potassium. Nitrogen,
potassium and magnesium sulfate appeared to have only slight
effects, if any, on amount of disease when compared to no fertili
zation at all in double Petri plates, using excised leaves. Noces CO Chapter four.
1. Aisc, J.R. 1976. Papillae and related wound plugs of plant cells. Ann. Rev. of Phytopathology 14: 145-163
2. Bainbridge, A. 1974. Effect of nitrogen nutrition of host on barley powdery mildew. PI. Path. 23: 160-161.
3. Berwith, C.E. 1936. Apple powdery mildew. Phytopathology 26: 1071-1073.
4. Bolle-Jones, E.W. and R.N. Hilton 1956. Zinc deficiency of Hevea brésiliens is as a predisposing factor to Oidium infection. Nature 177: 619-620.
5. Bracker, C.E. and L.J. Littlefield 1973. Structural concepts of host-parasite interfaces. In Fungal Pathogenicity and the Plant's Response. R.J.W. Byrde and C.V. Cuttings eds., Academic Press, London, New York, pps 159-317.
6 . Brooks, A.V. 1970. Begonia mildew. J. Royal Hort. Soc. 95: 234-236.
7. Boughey, A.S 1949. The ecology of fungi which cause economic plant diseases. Trans. Br. Mycol. Soc. 32: 179-189.
8 . Butt, D.J. 1978. Epidemiology of powdery mildews. In The Powdery Mildews. D.M.Spencer ed., Academic Press, London, New York. pps. 51-81.
9. Childs, J.F.L. 1940. Diurnal cycle of spore maturation in certain powdery mildews. Phytopathology 30: 65-73.
10. Cole, J.S., 1964. Powdery mildew of tobacco (Erysiphe cichoracearum). I. Effect of nitrogen, phosphorous and potassium, growth, and chemical composition of infected and healthy tobacco grown in water culture. Ann. appl. Biol. 54:291-302.
11. ______, 1966. Powdery mildew of tobacco (Erysiphe cichoracearum). V. Susceptibility of proximal and distal parts of leaves from different stalk position on intact and topped field.plants in relation to free amino nitrogen and carbohydrate content. Ann. appl. Biol. 58: 61-69.
1966. Powdery mildew of tobacco (Erysiphe cichoracearum). VIII. Some effects of methods of inoculation and air humidity on germination of conidia and growth of hyphae on leaves. Ann. appl. Biol. 58: 401-408. 13. Cook, G.D., J.R. Dixon and A.C. Leopold, 1964. Transpiration: its effects on plant leaf temperature. Science 144; 546-547.
14. Comer,E.J.H. 1935. Observations on resistance to powdery mildews. New Phytologist 34: 180-200.
15. Covey, R.P. 1971. The effect of methionine on the develop ment of apple powdery mildew. Phytopathology 61: 346.
16. Coyier, D.L. 1968. Effect of temperature on germination of Podosphaera leucotricha conidia. Phytopathology 58: 1047-1048.
17. Dekker, J. 1969. 2-methionine induced inhibition of powdery mildew and its reversal by folic acid. Netherlands J. PI. Path. 75: 182-185.
18. Delp, C.J. 1954. Effect of temperature and humidity on the grape powdery mildew fungus. Phytopathology 44: 615-624.
19. Downie, N.M. and R.W. Heath 1970. Basic Statistical Methods. 3rd edition. Harper and Row, New York.
20. Drandarevski, C.A. 1969. Untersuchung uber den echten Ruben- mehltau Erysiphe betae (Vanha) Weltzien. II. Biologie und K1imaabhangigkeit des Pilzes. Phytopath.Z. 65: 124-154.
21. Eaton, P.M. 1930. The effect of boron on powdery mildew and spot blotch of barley. Phytopathology 20: 967-972.
22. Gates, D.M. 1965. Heat transfer in plants. Scientific American 213: 76-84.
23. Gates, D.M., R. Alderfer and E. Taylor 1966. Leaf temperature of desert plants. Science 159: 994-995.
24. Gauch, H.G. 1972. Inorganic Plant Nutrition. Dowden Hutchi son and Ross Inc., Stroudsburg, Pa.
25. Gauch, H.G. and W.M. Dugger 1953. The role of boron in the translocation of sucrose. Plant Physiol. 28: 457-467.
26. Eshed, N. and I. Wahl 1975. Role of wild grasses in epidemics of powdery mildews on small grains in Israel. Phytopathology 65: 57-63.
27.Grey, H.E., 1954. Greenhouse heating. Cornell Univ. Agr. Expt. Sta. Bull. #906. 33 pps. 28. Heath, M.C. 1971. Haustorial sheath formation in cowpea leaves immune to rust infection. Phytopathology 61: 383-388.
29. Hewitt, H.G. 1974. Conidial germination in Microsphaera alphitoides. Trans. Br. Mycol. Soc. 63; 587-628.
30. Hewitt, H.G. and P.G. Ayres 1975. Changes in CO2 and water vapor exchange rates in leaves of Quercus robur infected by Microsphaera alphitoides (powdery mildew). Physiol. PI. Path. 7: 127-137.
31. Hey, G.L. and J.E. Carter 1931. The effect of ultraviolet light radiations on the vegetative growth of wheat seedlings and their infection by Erysiphe graminis. Phytopathology 21: 695-699.
32. Jhooty, J.S. and W.E. McKeen 1965. Studies on powdery mildew of strawberry caused by Sphaerotheca macularis. Phytopathology 55: 281-285.
33. Koltin Y. and F. Kenneth 1970. Ann. appl. Biol. 65: 263-268.
34. Last, F.T. 1953. Some effects of temperature and nitrogen supply on wheat powdery mildew. Ann. appl. Biol. 40: 312-322.
35. Last, F.T. 1962. Effects of nutrition on the incidence of barley powdery mildew. Plant Pathology 11: 133-135.
36. Jones, I.T. 1975. The preconditioning effect of day-length and light intensity on adult plant resistance to powdery mildew of oats. Ann. appl. Biol. 80: 301-309.
37. Lee, S. and S. Aronoff 1967. Boron in plants: a biochemical role. Science 158: 798-799.
38. Longree, K. 1939. The effect of temperature and relative humidity on powdery mildew of roses. Cornell U. Agr. Expt. Sta. Memoir # 223.
39. Mans3on, T. 1955. The grass mildew Erysiphe graminis, on wheat. Rev. of appl. Mycol. 35: 175-176.
40. Manners, J.G. and S.M.M. Hossain 1963. Effects of tempera ture and humidity on conidial germination of Etrysiphe graminis. Trans. Br. Mycol. Soc. 46: 225-234.
41. Mitchell-, J.W., W. Dugger, and H. Gauch 1953. Increased trans location of plant-growth modifying substance due to applica tion of boron. Science 118: 354-355. 42. Moseman, J.G. and L.W. Greeley 1966. EffecC of ultra violet light on Erysiphe graminis f. sp. hordei. Phyto pathology 56:1357-1360.
43. Mount, M.S. and A.H. Ellingboe 1966. Effects of environment on formation of secondary hyphae of Erysiphe graminis f. sp. tritici on wheat. Phytopathology 56: 891.
44. Mount, M.S. and A.H. Ellingboe 1968. Effects of ultra violet radiation on the establishment of Ervsiphe graminis f. sp. tritici on wheat. Phytopathology 58: 1171-1175.
45. Nair, R.K.S. and A.H. Ellingboe 1965. Germination of conidia of Erysiphe graminis f. sp. tritici. Phytopathology 55: 365- 368.
46. Packham, J. McC. 1969. Studies on the epidemiology of greenhouse roses. Phytopathology 59: 1043.
47. Palti, J. 1974. Powdery mildew of mango. Plant Dis. Reptr. 58: 45-49.
48. Peries, O.S. Studies on strawberry mildew, caused by Sphaero theca macularis (Wall ex Fries) Jaczewski. Ann appl. Biol. 50: 211-224.
49. Perera, R. and J.L. Gay 1976. The ultrastructure of haustoria of Sphaerotheca pannosa (Wall ex Fr.) Leveille and changes in infected and associated cells. Physiol. PI. Path. 9: 57-65.
50. Perera, R. and B.E.J. Wheeler 1975. Effect of water droplets on development of Sphaerotheca pannosa on rose leaves. Trans. Br. Mycol. Soc. 64: 313-319.
51. Price, T.V. 1970. Epidemiology and control of powdery mildew (Sphaerotheca pannosa) on roses. Ann. appl. Biol. 65: 231-248.
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56.______, 1965. Environmental relationships in the powdery mildews. Ann. Rev. of Phytopathology 3: 343-366.
57. Smith, H.G. and I.D. Blair 1950. Wheat powdery mildew investigations. Ann. appl. Biol. 37: 570-583.
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Late Additions
72. Bitterly, R. 1978. Powdery mildew of cucurbits. In The Powdery Mildews. D.M. Spencer ed.. Academic Press, London, New York.
73. Pady, S.M., C.L. Kramer and R. Clary 1969. Sporulation in some species of Erysiphe. Phytopathology 59: 844-848.
74. Peglion,V. 1905. I n t e m o cella nebbia o mal bianco dell ' Euonymus japonicus. Atti Accad. Lincei, Roma. V. 14: 232- 234.
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77. Garraway, M.O. 1973. Sporulation in Helminthosporium mayd is : inhibition by thiamine. Phytopathology 63: 900-902. 165
5. CHEMICAL CONTROL
Chemical control of powdery mildews, history and Importance
In this century the growth of the use of fungicides has been rapid. The ease, economy and efficacy of use of fungicides has in fact caused may growers to ignore cultural and environmental factors which promote or prevent disease.
The earliest mention of pest averting sulfur was in 1000 B.C.
The earliest record of the use of sulfur to treat powdery mildew was the treatise of Forsyth, written in 1802 (133). Sulfur and soap was found to be effective against powdery mildew of peach by J. Robertson in 1824 ( 93). After Tucker added lime to the sulfur to treat powdery mildew of grape in England in 1845 (110), sulfur became the most widely used fungicide in the world. About
1850 application of sulfur by dusting instead of spraying was discovered in France on grapes (133). Soon lime and sulfur was being boiled together by Grison (133).
The effective treatment of powdery mildews with organic fungicides began with the discovery of the specificity of dinocap for these diseases (224). Since the discovery (51,61) of benorayl, more and more systemic organic fungicides are being discovered and introduced. Systemics are likely to be the fungicides of the future.
Sales of fungicides to combat powdery mildews account for about 18% of the total amount of fungicides sold worldwide ( 23).
Barley, apples and grapes are the major crops on which these
fungicides are used. 166 Summary of names, formulas, use and efficacy of fungicides used against powdery mildews.
Inorganic chemicals
1 .Sulfur No tolerances set for sulfur Sulfur is registered by the U.S. Environmental Protection Ag e n c y (21) for use versus powdery mildews on the following crops: alfalfa, alder, apple, aster, azalea, beans, blackberry, blueberry, boysenberry, buttonbush, calendula, carnation, catalpa, carrots, cedars, cherry, cherry laurel, chrysanthemum, clover, cole crops, cosmos, crepe myrtle, cucurbits, currants, dahlia, delphinium, dewberry, dogwood, eggplant, Euonymous, flax, garlic, gladiolus, golden fleece, gooseberry, grape, hibiscus, holly, hollyhock, honeysuckle, horsechestnut, hydrangea, huckleberry, ivy, juniper, lady's mantle, lettuce Ligustrum, lilac, linden, loganberry, mango, natrimony vine, nectarine, onion, peach, peanut pias, peppers, phlox, plum, poplar, prune, quince, raspberry, rhodo dendron, rose, silver vine, Smilax, spinach, spirea, spruce, staghorn sumac, strawberry, sunflower, sweet pea, sycamore, trumpet vine, verbena, violet, willow, witch hazel, zinnia, sugar
Beware of phytotoxicity problems, especially at high temperatures Formulations : 1. Flowers of sulfur 2 . Micronized sulfur, used as a dust or volatilized. Trade names: Cosan, Kumulus S (BASF), Sofril Efficacy: peas-excellent (131). 3. Wettable sulfur, applied in a spray mix. Trade names: Hexasul, Sulfex Efficacy: apples-mediocre ( 75) to excellent (201); rose-mediocre ( 46) to excellent ( 53); zinnia excellent ( 35). 4. Micronized wettable sulfur Trade names : Cosan, Elosal (Hoecht) Kumulus S (BASF), Thiolux (Sandoz), Thiovit (Sandoz). Efficacy: peas-excellent (131). 5. Colloidal sulfur Trade name: Sulkol 6 . Flowable sulfur, applied in a spray mix. Trade names: Magnetic 6 (Stauffer), Super 6 (Griffin), THAT Efficacy: apples-excellent ( 86,199,227); grape-excellent ( 28); potato-mediocre ( 60); sugar beet-excellent (102,178).
2. Lime Sulfur (Calcium polysulfides) No tolerances set by E.P.A. Registered vs. powdery mildews on: almonds, alfalfa, apples, begonia (tuberous), blackberry, cherry, clover (red), colum bine, crepe myrtle, dahlia, delphinium, Euonymous, fruit trees 167 (nonbearing), gooseberry, grape, Lilac, marigold, peach, pear, plum, prune, raspberry, sweet pea, rose, and all shrubs and shade trees. Beware of phytotoxicity problems, especially at high temperatures. Formulated as liquid concentrates. Ortho (Chevron) a main formulator in the U.S. Efficacy: apples-excellent (226, 227).
3 .Coppers. Although not usually thought of as powdery mildew fungicides, copper fungicides are somewhat effective. No tolerances set. 1. Copper ammonium carbonate. Trade names: Copper-count N, Oxycop 8L registered vs. powdery mildews on cucurbits. 2. Copper-ammonium complex. registered vs. powdery mildews on cucurbits. 3. Copper-Bordeaux mixture. registered vs. powdery mildews on roses. 4. Copper hydroxide. Trade name: Kocide registered vs. powdery mildews on blackeyed peas, pumpkin, squash and in New York on grape. Efficacy: cucumber-mediocre (114); sugar beet-excellent (178); Viburnum-excellent (218); wheat-poor ( 31). 5. Copper oleate registered vs. powdery mildews on apples, pears, blackberries, raspberry, grape, nectarine, peach, plum, prune, straw berry, beans, cole crops, cucumbers, squash, lettuce, melons, tomato, all flowers including rose, all shrubs and shade
Some phytotoxicity problems on flowers. Efficacy: zinnia-fair (167) to excellent (197). 6 . Copper oxide (Cu20 and CuO) Trade names : Copper-Sandoz, Fungi-Rhap, Oleocuivre, Perecot, Perenex, Triangle, Yellow Cuprocide. registered vs. powdery mildews on grapes. 7. Copper oxychloride (CuCl2 3Cu(OH)2) Trade names: Chempar, Cobox, Colloidox, Coprantol, Coxysan, Cu-56, Cupramar, Cupravit, Cuprokylt, Cuprosana, Cupro- vinol, Cuprox, Fytolan, Kauritil, Recop, Rhodiacuivre, Vivicuivre, Vitigran. Registered vs. powdery mildews on apples, grapes, nectarine, peach, pear, plum, prune, strawberry. 8 . Copper oxychloride sulfate. Trade names: COCS, Copro 50 & 53. Registered vs. powdery mildews on grapes, strawberry and in western U.S. on cucurbits. 9. Copper 8-quinolinace (oxine-copper) Trade names: Bioquin, Cunilate 2472, Dokirin, Fruitdo. Registered vs. powdery mildews on greenhouse carnations, chrysan themums and roses. Apply with dinocap. 10. Copper sulfate, basic (Tribasic copper sulfate) Trade names: Kilcop 53, Kobasic, Phyto-Bordeaux, TNCS 53, Triangle. Registered vs. powdery mildews on apples, grapes, beans, cucurbits, roses, zinnias. 11. Copper sulfate-penCahydrate (common bluestone) registered vs. powdery mildews on rose. 12. Copper-zinc chromate complex Trade name: Miller 658 (discontinued). Registered vs. powdery mildews on oaks, roses. 13. Copper salts of fatty acids Trade name: Citcop Not registered vs. powdery mildews. Efficacy: Cucumber-excellent (113,114); rose-excellent
4.Other Inorganic Chemicals. The trade literature has claims for control of powdery mildews by sodium bicarbonate, epsom salts, and many other materials. Experiments with controls are usually lacking although sodium bicarbonate was recently tested in Japan ( 92) on cucumber.
Organic Fungicides (nonsystemic)
Dinitrophenols 1.Dinocap (2,6-diniro-4-octylphenylcrotonate) (DNOPC) Trade names: Crotothane, Karathane, Mildex (Dikar=Karathane + Dithane 0=C-CH=CH-CH] M-45). Introduced 1949 (175). N02- % ^ C H ( C H 2)-CgH^ Formulated as a wettable powder and as a liquid concentrate, oral LDgg male rat 980 mg/kg Registered vs. powdery mildews on all ornamental shrubs and trees, African violets, apples, apricots, asters, begonias, calen dula, cherry, chrysanthemums, cineria, gerbera, grass lawns, gardenia, grape, hydrangea, larkspur, lilac, melon, pear, pumpkin, rose, snapdragon, squash, zinnia. Slight flower phytotoxicity problems. Efficacy: apples- mediocre (201) to excellent ( 17, 39, 40» 43, 44, 70, 78, 88,120,122,182,189,200); begonia-excellent (143,170, 194); cherry-excellent (33); chrysanthemum-excellent (143), cantalope-excellent (135); cucumber-excellent (169); grape-fair (150); rose-poor (127) to mediocre (125) to excellent (124,143); squash-excellent (230); Kalanchoe- poor (195); wheat-excellent (231).
2. Binapacryl (2-sec-butyl-4,6-dinitrophenyl-3-methyl-2- butenoate) Trade names: Acricid, Ambox, Dapa- 0=C-CH=C(0113)2 cryl, Endosan, HOE 2784, Morocide, 6 Morrocid (Hoechst) N02-|r"''^-ÇH-C2H5 Formulated as a wettable powder, CH3 liquid concentrate and dust. NÔo Introduced 1959 (232) Oral LD50 male rat 161-421 mg/kg No registered use. Used in Europe on fruits and ornamentals. Efficacy: mediocre (201) to excellent ( 23, 77, 206)on apples; cantalope-poor (135).
3. Dinobuton (2-(1-methyl-2-propyl)-4,6-dinitrophenyl iso- propylcarbonate) Trade names : Acrex, Dessin, Dinofen, Drawinol, MC 1053, TaIan, UC 19786. Basic producer Keno Gard (Sweden) Formulated as an emulsifiable concentrate. 0 CH3 Oral LD5Q male rat 2500 mg/kg O-C-O-CH-CH3 No registered use. Used in O 2N-^c='-irCH-CH2-CH3 Europe on glasshouse crops, CH3 apples, cucumbers and hops. NO2
Quinomethionate (6-methyl-1,3-dithiolo(4,5-6)quinoxaIin-2- one). Other common names: oxythioquinox, chinomethionat. Trade name: Morestan, Bay 36205, 85 2074, Forstan. Formulated as wettable powder, dust, smoke generator. ^ /N S Introduced 1962. CH3-/ Y V \ = q LD^Q male rat 2500-3000 mg/kg. / Registered vs. powdery mildews ^ N S on apples. Used in Europe on fruits, vegetables, field and forage crops, ornamentals. Some phytotoxicity problems. Efficacy: apples-excellent (226), melons-excellent (135); peas-excellent ( 131); rose-mediocre(180) ; snapdragon-excellent (144). Drazoxolon (3-methyl-4,5-isoxazoledione-4-(2-chlorophenyl) hydrazone) Trade names: Ganocide, Mil-col, PP781, Saisan, Spracol 781, (basic producer ICI). Formulated as an aqueous suspen- Cl Sion in grease. /&=\ Introduced 1967 LD50 oral male rat 125 mg/kg No registered uses in U.S. Used in Europe on apple, black currants and rose.(23 ). Low phytotoxicity
Ditalimfos (0,0-diethyl phthloimido-phosphonothioate) Trade names: Dowco 199, Plondrel, Laptran, Millie. Formulated as wettable powder and 0 emulsif iable concentrate. S OCHgCH] LDggoral male rat 4930-5600 mg/kg ^ "T Causes skin irritation in some OCHgCU^ workers. 0 Not registered in U.S. Sold only in England by Dow. Used there on apples, cereals, ornamentals. Efficacy: rose-excellent ( 99,100,148).
Piperalin (3-(2-methyl piperidine)propyl 3 ,4-dichlorobenzoate)
/~^N-CH2-CH2-CH2-0-C-^^CI
Trade name: Pipron (Elanco) Formulation: Liquid concentrate LDjg oral, male rat: 2500 mg/kg Registered for use on catalpa, chrysanthemum, dahlias, lilacs, phlox, roses, zinnia. Efficacy: mediocre to excellent on rose ( 32, 45, 99,100,161,164) zinnia-excellent (155). Chlorothalonil (tetrachloroisophthalonitrile) Trade names: Bravo, Daconil 2787, Exotherm Termil. Formulations: wettable powder, flowable, exothermic dust. ÇN LD5Q oral male rat 10,000 mg/kg Cl'Y^'S— Cl Registered vs. powdery mildews on Cl-L/^CN cantalope, melons, cucumbers, zinnia. Cl Efficacy: cantalope-excellent (119); cucumber-excellent (22); lilac-excellent (163); melons- poor (135) to excellent ( 55), cucumber-excellent(112,114) rose-poor (126) to excellent (220); sugar beet-excellent (102); squash-excellent ( I4 ) ; zinnia- poor (216). Now registered on Viburnum too. 171 Folpet ( N-(Trichloromethylchio)phthalimide Trade names : Phalcan (Chevron-Ortho), thiophal, Folpan (Stauffer) 0 Formulations: wettable powder R and dusts. NSCC13 LDggOral male rat lOOOOmg/kg Originated 1952. u Registered vs. powdery mildews on blueberries, huckleberries, grapes, cucumbers, melons, pumpkins, squash, asters, chrysanthemums, lilacs, phlox, rose, snapdragons. Efficacy: phlox-excellent (215); rose-poor (180) to fair (233); Viburnum-excellent (218); zinnia-poor (151); Kalanchoe good(130). Thiram (Bis(dismethylthio-carbamoyl) disulfide or tetramethyl thiuramdisulfide). Trade names: Aatack, Arasan, Aules, Chipco Thiram 75, Fermide 850, Fernasan (ICI), Hexathin, Mercuram, Nomersam (ICI), Polyram-ultra (BASF), Pomarsol forte, Spotrete, Tersan 75, Thimer, Thioknock, Thiotex, Thiramad, Thirasan, Thiuramin, CH.I3 Thylate, Tirampa, TMTDS, Trameton, 'N-C-S-S-C-N Tripomol, Tuads, Vaneide, Tm-95, c h,3; ' S S ^CH3 Vancide-TM flowable. Formulations: wettable powder, water suspension, dust. Origin, 1931, DuPont. LD50 oral male rat 780-865 mg/kg Not registered vs powdery mildews : the U.S. but has good activity against them (23 ). Fluotrimazole
Used in Europe on cereals and apples (23 ). <>
(^■CF3
Halicrinate
Used in Europe on cereals and apples ( 23 ).
OCOCH-CH NItrotaI-isopropyl
Used in Europe on cereals and apples (23).
° 2 Î '- V 'S ’ °
I n [3 CH3
ChIorquinox(5,6.7 ,8-tetrachloro- quinoxaline) Trade name: Lucex (discontinued) Used in Europe on barley (23).
Other organic contact fungicides
Maneb, zineb, captan and captafol are ineffective against powdery mildews (23,57,77,98,117,136,160,180). Even so zineb is registered against powdery mildews on asters, carnations, dahlias geraniums, hydrangea, rose, euonymus, hawthorn and lilac. There are some reports of success of these fungicides in fighting powdery mildews too: Banrot-excellent on zinnia (198); captan-fair on apple (81); Dithane-good on apples (149), excellent on rose (142). Cyprex is not effective against powdery mildews either (142).
Systemic Organic Fungicides
N - (b-cyanoe thyl)monochloroace t imide Trade name: Udonkor (discontinued by Nippon Soda Co.) CI-CH2 -Ç-NH-CH2 -CH2 -CH3 Very specific against powdery Ô mildews (61).
Benzimidazoles
1. Benomyl (methyl 1-(butylcarbamoyl)-2-benzimidazolecarbamate) Trade names: Benlate, Tersan 1991 (Dupont) Formulated as a wettable powder LDgg oral male rat: 10000+ mg/kg Originated late 1950s I-CH2-CH2-CH2-CH3 Registered on apples, peaches, apricots, cherries, plums, prunes, "C-NH-COOCH3 blackberries, boysenberries, dew- berries, loganberries, raspberries, grapes, pears, pecans, strawberries, cucurbits, flowers, orna- mentals and shade trees. Efficacy: apples-excellent ( 1 , 13 , 39 , 74 , 79 , 88 ,121, 138 201) to mediocre (142); cherrles-poor ( 67) to excellent ( 33); grapes mediocre (202) to excellent ( 16, 28); straw berries -excel lent (101,117); cucurblts-excellent ( 4, 5, 36,135,140, 234) ; mango-excellent (136) ; barley- excellent ( 6 ); oats-excellent ( 7 ); wheat- poor ( 96) to excellent (8); sugar beets-poor (105); azalea-excel lent (235); calendula-excellent (171); crabapple-excellent (162); crepe myrtle-excellent (106); kalanchoe-excellent (130* 195); lilac-excellent (153); pecan-excellent (209); phlox-excellent (215); hydrangea-excellent (152); rose- poor ( 147) to excellent ( 53, 62, 99, 180, 219, 221) to mediocre ( 148, 210) : Viburnum-excellent (218); zinnia- excellent (154, 167)* Also excellent as a seed treatment on pea; orchardgrass-excellent (111). Beware of resistant strains of the fungus which commonly appear after continuous applications of benomyl alone. (see page 177),
2. Carbendazim ( 2-(methoxycarbonylamino)-bendimidazole ) Trade names : HOE 17411 (Hoechst), Bavistin (BASF), Derosal. Formulation: wettable powder. yN LD50 oral, male rat 15000+mg/kg I \\ | 0 Not registered in U.S. This L H C-NH-C-OCH- compound is the active ingredi- ent (breakdown product) of beno- H rayl and should work against the same mildews as benomyl. Efficacy: apples-excellent (201); rose-fair (125) to excel lent (155); zinnia-excellent (156).
3. Cypendazole (1-(5-cyanopentylcarbamoy1-2-(methoxycarbonylamino) -benzimidazole) Trade name Folcidin, DAM 18654 0=C-NH-CgH]^Q-CN (discontinued by Bayer AG) 1 LD^q oral, male rat 2500+ mg/kg 0 ■NH-C-O-CH,
4. Thiophanate methyl (dimethyl(1,2-phenylene)bis-(imino carbonothioyl))bis(carbamate) y. S 0 Trade names: Cercobin-M, Fungitox, /^NH-C-NH-^OCH^ habilite, Mildothane, Tops in M. U ^NH-C-NH-pOCH^ Formulations: wettable powder, paste, ^ S 6 ULV. Now registered as Zyban for roses and zinnias. Should be effective against same mildews as benomyl. Converted to carbendazim at a slower rate than benomyl. Efficacy: apple-poor ( 57) to excellent (201); barley-excel lent ( 6 ); cucurblts-excellent ( 22); begonia-good ( 26); grape-poor ( 16) to excellent ( 16); kalanchoe-good (196); rose-excellent (109); snapdragon-excellent (144); straw berry-excellent (117); zinnia-excellent (160,216).
5. Thiophanate (1,2 bis(3-ethoxycarbonyl-2-thioureido) benzene Trade names: Cleary 3336, Cercobin, Topsin E (Nippon Soaa Co.) y. S 0 Formulation: wettable powder ^'^NH-Ü-NH-C-0 C2Hg LD oral, male rat 15000+mg/kg U ^ N H - C NH-(:-0C2H 5 Not registered in U.S. against ^ s Ô powdery mildews. May be used on turf.
6 . Thiabendazole (2-(4-thiazolyl)-benzimidazole Trade names : Apl-Luster, Mertect (Merck), TBZ, Tecto, Thiben- zole, Biogard, Tobaz. Formulations: wettable powder, flowable, smoke generator. -NH LDsooral, rat 3100 mg/kg Not registered vs. powdery mildews in U':S. coVi Efficacy: rose-excellent (125); strawberry-poor (117); wheat-excellent (12 , 15)
7. Fuberidazole (mix of 2(2'-furyl)-benzimidazole and hexa- chlorobenzene) Trade names: Bay 33172, NeoVoronit, Voronit, Voronit special (with PCNB)(Bayer AG). LD^q oral, male rat, 1100 mg/kg Not registered in U.S. Used vs. smuts in Europe.
Hydroxypyrimidines
1. Dimethirimol (5-n-butyl-2-dimethylamino-4-hydroxy-6 methyl- pyrimidine) Trade names: Milcurb, PP 675 (ICI). C4H 9 Formulation: aqueous liquid CH3-fîJ^0H LDgQ oral male rat 2350 mg/kg N^N First of these compounds to be ^ introduced (circa 1968) ( 23). Not registered in U.S. CH3 CH3 Gives 6 weeks of protection when used as a soil drench on greenhouse cucumbers ( 61 ). 175 2 . Ethirimol (5-butiyl-2-ethyIaniino-4-hydroxy-6-methyIpyrimidin) Trade names: PP 149, Milcurb, Milcurb Super, Milgo, Milgo E, Ç4H 9 Milstem, New Milstem (ICI). Formulation: aqueous suspension H 3Y 7 -O' LDgg oral, maie rat 6340 mg/kg No registered uses in U.S. Y Used in Europe on barley where NH it is supposed to be effective C2H 5 (23). Good soil drench.
3.Bupirimate (5-butyl-2-ethylamino-6-methylpyrimidin-4-yl- dimethylsulfamate) Trade names: PP 588, Nimrod (ICI). C^Hg Formulation: Emulsifiable concentrate. I LD50 oral, male rat 4000 mg/kg CH^-yj^-OSOgN(CH3)2 May soon be registered in U.S. for N N use on ornamentals and apples. y Used in Europe on apples, roses, NH cucurbits and other horticultural ^2^5 crops. Not as effective a soil drench as ethirimol. Efficacy: apples-poor (123) to excellent ( 2 , 57, 72, 79, 85,149,193,236); cucurbits-excellent i 5 , 11,112,183); grape-poor (203) to excellent ( 10); rose- poor (148) to excellent (45,47,128,161)crabapple-excel- lent (162); sugar beet-excellent ( 34); zinnia-excellent ( 35).
Morpholines
1. Dodemorph (cyclomorph) (n-cyclododecyl-2,6-dimethylmorph- olinium acetate) Trade name: Meltatox (BASF) p— 7 y— ^CH] Formulation: wettable powder r— ' '— pN D LD cq oral, male rat 4180 mg/kg '— , t— < ^— ^CHo Originated 0 1969 ( 23). '— ' Registered in the U.S. on greenhouse roses (Mallinckrodt, distributor). In Europe used on apples and ornamentals. Mode of action; Effects cell membrane permeability (187). Efficacy: crabapple-mediocre (162); lilac-fair (163); rose- excellent ( 99,100,141,148,210).
2. Tridemorph (n-tridecyl-2,6-dimethylmorpholine) Trade name: Calixin (BASF) /— rCH^ LD50 oral, male rat 1112 mg/kg H3C-C12H24-N 0 No registered uses in U.S. ^CH] Used in Europe on cereals. Mode of action: Protein syntheis inhibition ( 63). Efficacy: barley-excellent ( 66); wheat-poor ( 97). 3. Fenpropemorph (cis isomer of 4-(3-(p-Cert-butylphenyL)-2- methylpropyl)-2 ,6-dimethylmorpholine) Trade names: Corbel, Rol4-3I69, BAS42100F (BASF) LD50 oral, male rat 3515 mg/kg Mode of action: sterol inhibitor. No registered uses. May be good for use vs. powdery mildew on cereals ( 25,116)
Organic Phosphates
1. Pyrazophos (0,0-diethyl-0-(5-methyl-6-ethoxycarbonyl-pyra2olo- (1.5-a)-pyrimid-2-yl)-thiophosphate) Trade names: Afugan, Curamil, HOE 2873. Formulation: Emulsifiable concentrate. 0 S LD50 oral, male rat 460-632 mg/kg. CzHs-OC-i^ N — N No registered uses in U.S. CHg-l ^ j l — 0-P^ Used in Europe on apples, hops, V ^ OC2H 5 cucurbits and ornamentals. Efficacy: apples-excellent ( 2 , 18, 57, 86, 72, 79, 87, 149,190,201,226); cucurbits-excellent ( 5 , 22,112,158); phlox-excellent (215); rose-excellent (124,128,147,154); zinnia-excellent (154,156,214).
2. Triamiphos (p-(5-(amino-3-phenyl-lH-l,2,4 triazol-l-yl)- N,N,N',N'-tetramethyl phosphonic diamide) Trade name: Wepsyn 155.(Phillups- Duphar B.V.) LD5Q oral, male rat 20 mg/kg! Once used on apples and roses in Europe, but of no practical impor tance (23,137).
3. O-O-diethylphthalimidophosphonothioate Efficacy: apples-excellent (201)
Pyrimidines
1. Denmert (s-n-butyl-s'-p-tert-butylbenzyl-N-3-pyridyl-dithio carbonimidate) New experimental fungicide ( 61)•
2. Fenarimol (a-(2-chlorophenyl)-a-(4-chlorophenyl)-5-pyrimidine- methanol) Trade names : Bloc, Rimidin, Rubigon, Cl EL 222 (Elanco) ,— OH Formulations: Emulsifiable concentrate, wettable powder (for turf). LD 50 oral rat 2500 mg/kg f| Used in Europe. Efficacy: apple-excellent ( 1 , 9 , 71 , 77 , 177 80 , 83 , 84 ,142,149,190,199,211,236,87); barley- excellent ( 6 ); crabapple-fair (162); cucumber-excellent ( 5 ,112); grape-excellent ( 10, 16); lilac-excellent (163); wheat-excellent ( 8 , 97); rose-excellent ( 45,100, 128,148,157,161,221,223); zinnia-excellent (151).
J. imazaiii f-- Trade name; Fungaflor Cl-^ ^ C 1 No registered uses N ^ N-CH2 -ÇlP-==/ Used in Europe for common 6CH2CH=CH2 root rot of wheat.
4. Nuarimol Trade names: Trimadol, Trimunol(Elanco) Efficacy: rose-excellent when volatilized ( 45).
5. Parinol (a,a-bis(p-chlorophenyl)-3-pyridine-methano1) Trade name P a m o n (Elanco) Formulation: Emulsifiable concentrate LD 50 oral, rat 5000 mg/kg No registered uses. In Europe used on roses, zinnias, non bearing apples and grapes. Efficacy: apples-mediocre (211); cantalope-excellent (135); crape myrtle-excellent (106); rose- excellent ( 54,180).
6 . Triarimol (a(2,4-dichlorophenyl)-a-phenyl-5-pyrimidinemetnanol) Trade name EL 273 (Elanco, discon- Cl tinued 1972). OH /=\ LD 5Q oral, rat 600 mg/kg Cl-(\zs— C-<(^ h Experimental use only. y L Does not effect germination, but (j | does inhibit haustorial formation. N ^ N Inhibits sterol synthesis (172,173). Efficacy: apples-excellent (39,44); Chinese photinia- excellent (108); crape myrtle-excellent (106); phlox- mediocre (215); snapdragon- excellent (144); squash- excel lent (204); rose-excellent (106,147,159); zinnia-excellent ( 20 ).
7. Triademefon (l-(4-chloro-phenoxy)-3,3-dimethyl-l-(l,2,4- triazol-l-yl)-butan-2-one) Trade names: Bayleton, MEB6447 __ (Mobay, Bayer) C l ^ Ç ^ O - Ç H - C O - C - (CH^) Formulations: wettable powder and emulsifiable concentrate. N LD5Q oral, rat 568 mg/kg N— J/ No registered use vs. powdery mildew in U.S. Mode of action: ergosterol inhibition. and inhibits gibbereLlic acid synthesis in the host. Efficacy: apple-excellent ( 2 , 19, 41, 42, 43, 72, 74, 81, 82, 181, 190, 199); azalea-excellent (164); cereals- excellent (12 , 15, 37,104,115,134,185); cucumber- excellent (112); grape-excellent ( 10, 27,150, 202); rose- poor (147) to excellent (166,223); squash-excellent (5, 237) ; sugar beet-excellent (178); zinnia-excellent (213,238)*
8 . Baycor (B-((1,1'-biphenyl)-4-yloxy)-a-(l,l-dimethylethyl)-IH -1,2,4-triazole-l-ethanol) Trade names: Baycor, Bay KWG 0599------(Mobay, Bayer) / ^OCH-CH-C(CH^)^ Experimental uses only. ---' OH Efficacy: apples-fair (igl) to u N excellent ( 13 , 43 , 77 , 85 , 88 , N— fi 211,226); wheat-mediocre (134 , 185) to excellent ( 15, 37).
9. Sis thane (a-butyl-a-phenyl-lH-imidazole-l-propanenitrile) Trade names: Sisthane, RH 2161 (Rohm and Haas) Formulation: Emulsifiable concentrate. LD^q oral, rat 1590 mg/kg Experimental use only Efficacy: apple-mediocre ( 43) to excellent (41, 42, 59, 85,123,181,226, 88 ); wheat-excellent (15); zinnia-excellent (213); excellent on rose when volatilized ( 45). 10. Triforine (N,N'-(1,4-piperazinediyl bis(2,2,2-trichloroethyli- dene) ) -bis- (formamide) ) Trade names: Triforine, Cela W-524, CME 74770, Funginex, Saprol,(Celamerck, Chevron) Formulation: Emulsifiable concentrate. LDgp oral, rat 16000+ mg/kg Cl3C-CH;NH-CH0 Registered for use on greenhouse '
Mode of action: ergosterol inhibi- [l tion. Breakdown of product occurs rapidly in solution at 21 C.( 48). CI3C-CH-NH-CHO Efficacy: apples-excellent ( 13,30 , 88,211); cucumbers-excellent (114); lilac-excellent (153); Phlox-excellent (215); rose-poor (147,223) to excellent(46,54, 99,124,128,129,154,233); viburnum-excellent (218); zinnia- poor (156,217) to mediocre (167), to excellent (160); Kalanchoe- cxcellent (195). 11. Chloraniformethane (1-(3,4-dichloroanilino)-1-formylamino -2 ,2 ,2-trichloroethane) Trade names: Imugan, Bay 79770, Milfaron (discontinued by Bayer). LD50 oral, rat 2500 mg/kg Cl-^ 'VnHCH-NHCHO Used in Europe on barley ( 23, 6 6 ). Cl^=/ CCI3 Triazoles
1. CGA-64251 (1-(2-(2,4-dichlorophenyl)-4-ethyl-l,3-dioxolan -yl-meChyL)-lH-1,2,4 triazole. (Ciba-Geigy) Formulations: wettable powder and emulsifiable concentrate. LD50 oral, rat 1343 mg/kg Experimental uses only. Efficacy: apples-excellent ( 43, 68, 88,199); grape-excel lent (192); rose-excellent (166).
2. CGA-64250 (1-L(2(2,4-dichlorophenyl)-4-propyl-l,3-dioxolan -2-yl)methyl)l-H-l,2,4-triazole) (Ciba-Geigy) Formulations: Emulsifiable concentrate and granular. LD50 oral, rat 1517 mg/kg Ciba-Geigy claims good vs. powdery mildew of cereals. -rï Experimental use only. '---- Η CHo-CHo-CHn
Other Systemic Fungicides
Procaine An early systemic no longer of importance (23). (C2H5) 2N -CH2CH2 -0 -C NH2
6-aza-uracil An early systemic no longer of importance (23).
Metiram (60%)+ nitrothal-isopropyl (12.5%) (tris(ammine(ethylene- bis(dithiocarbamato)zinc(2+))polymer. 5-nitro-benzene-l,3 di- carboxyic acid bis (1-methylethyl)ester. Trade name: Pallinol (BASF) Formulation: wettable powder. H S H S LD50 10000 mg/kg CHo-N-C-S- ÇH2N-C-S- No registered uses. Used in CH2-N-C-S-Zn(NH3)- CH2 N - C - S - Europe on apples ( 23), H S S H S Efficacy: excellent when and mixed with sulfur on apples 0 ÇH3 (57). y_^C-0-CH-CH_ 02N-( \ '^C-0-CH-CH3 6 CH3 Antibiotics
1. Griseofulvin (7-chLoro-4,6-dimethoxycumaran-3-one-2-spiro-l' -(2‘-methoxy-6 '-methylcycLohex-2'en-4'one). Trade names: Grisetin, Gresfeed, Fulvicin, Fulcin, Grifulvin, Mir- 0CH3 fulvin. (Merck or Murphy ) ..— ■ / OCH3 Formulation: dust CH^O-C y— ^ Low toxicity. C l ^ = / C. V o Originated in England, 1939. Iso- 0 ' lated from Pennicillium griseofulvum. No registered use in U.S. Of no practi cal importance in treating powdery mildews.
2. Cycloheximide (3-(2-(3,5-dimechyl-2-oxocyclohexyl)-2-hydroxy- ethyl)glutarimide) Trade names: Actidione (Upjohn),----- CH3T'-rO pH y— ^0 Actispray, Acti-aid, Hizarocin. / ^C-CH?/ V n h Formulation: wettable powder. — H '— ^0 LDgQ oral, rat 2 mg/kg! Registered for use vs. powdery mildews on begonia (tuberous), cherry, chrysanthemum, crepe myrtle, euonymous, honeysuckle, magnolia, phlox, zinnia, California live oak, turf grass.
Efficacy: Chinese photinia-excellent (107); crape myrtle- excellent (106); sugar beet-excellent (178); rose-excellent (ISO). 181 Resistance to fungicides
The new systemic fungicides are often ineffective after repeated use because of the presence of resistant strains of powdery mildews. This has been seen frequently with benomyl, the most widely used systemic fungicide (Table40) . Systemic fungicides appear often to interfere with the actions of but one fungal gene locus ,thus making resistance easily acquired
( 61 ). To prevent the buildup of resistant strains, foregoing the use of a systemic fungicide and related compounds for a period of time after repeated use is wise. Also,use of an unrelated mildicide in alternation with the systemic fungicide will help prevent buildup of resistant strains. Other possibilities for preventing this buildup include using the highest registered dosage every time the fungicide is used and not spraying all of the crop with the same chemical.
Table 40: Reported resistant strains of powdery mildews to ______mildicides.______
Host Fungus Chemical References barley E. graminis ethirimol (212) begonia 0 . begoniae benomyl (145,197) bluegrass E. graminis benomyl ( 50) cantalope E. cichoracearum benomyl ( 50) E. graminis benzimidazoles (208) cucumber S. fuliginea benomyl ( 50,179) cucumber S. fuliginea dimethirimol ( 24) eggplant E. cichoracearum thiophanate methyl ( 50) gherkins S. fuliginea triforine ( 69) muskmelon S. fuliginea benorayl (140) S . pannosa benomyl ( 94,228) strawberry S. humuli benomyl ( 50) tobacco E. cichoracearum benomyl ( 38) Control of powdery mildews by dormant season sprays of surfactants.
Surfactants are known to have fungicidal properties.
Cationic surfactants are particularly fungitoxic. These
compounds cause loss of differential permeability of fungal
cell membranes( 64). Burchill, while investigating the abilities
of surfactants to act as chemical pruners of diseased apple
buds,found that powdery mildew of apple could be eradicated by
dormant season sprays of surface active agents (E.C. Hislop,
personal communication)• Instead of pruning diseased buds,the
surfactants killed the mildew infecting the buds without killing
the buds. Since then,several studies of this phenomenon have
been made with apples ( 49, 65, 89, 90, 91,191). Use of this
technique may prove valuable in controlling other powdery mildews which overseason in buds.
Chemical control of powdery mildew of begonias.
Introduction:
Currently, a protectant fungicidal spray program is recom mended for control of Oidium begoniae. The only highly effec
tive fungicide registered by the U.S. Environmental Protection
Agency is dinocap. This chemical should be applied every 7-
10 days to susceptible cultivars until flowering. Unfortune-
ately,dinocap burns flowers, especially under high temperatures.
Benomyl is effective sometimes, but resistant strains of the
fungus are probably widespread, having been found in Ohio,
Missourri, North Carolina and Ireland (145,197). Cycloheximide 183 and lime-sulfur are registered on tuberous begonias. The efficacy
of cycloheximide is somewhat questionable, compared to that of
dinocap (Table 41)(205,207). Lime-sulfur is known to be more
phytotoxic than dinocap. Triforine may soon be registered and
alternating this chemical with benomyl may give good systemic
eradication,as well as protectant ability. The development of
effective systemic fungicides would hopefully give us a tool to
eradicate mildew in protected places,such as buds. The development
of effective fungicides that do not b u m flowers is needed. Indeed,
several promising fungicides were tested in the experiments which follow, including triademefon, Baycor, CGA 64251, fenarimol,
and triforine. Table 41 lists results of fungicide trials
on begonia found in the literature. Table 41: Fungicides tested against Oidiun begoniae in
______the literature and their efficacy______
Chemical Efficacy and references
Sulfur dust excellent (194) (some phytotoxicity) Sulfur + Copper hydroxide excellent (194) (some phytotoxicity) Colloidal sulfur excellent (207) Sulfur + Ferbam mediocre (205) Cupric dihydrazinium mediocre (205) Dinocap excellent (143,194,205,207) some phytotoxicity (194,205) mediocre (207) to excellent (207) Chlorothalonil mediocre (194) fair (205)
Systemic organic fungicides
Benomyl poor (145,171) to excellent ( 26, 194,195) slight phytotoxicity (194) Thiophanate methyl mediocre (194) to excellent ( 26) (slight phytotoxicity (194)) Cypendazole excellent (145) Ethirimol poor (171) Bupirimate no phytotoxicity ( C.C. Powell) Dodemorph no phytotoxicity ( C.C. Powell, unpub lished obseryations) Pyrazophos no phytotoxicity ( C.C. Powell , unpub lished observâtions)excellent (143,145) Triamiphos fair (207) to excellent (207) Fenarimol some stunting (C.C. Powell, unpub lished obseryations) mediocre (171,194)(no phytotox icity (194)) Triarimol excellent (171) Triadernefon stunting (C.C. Powell, unpublished observations) Triforine mediocre ( 171) to excellent (145,194) (no phytotoxicity (194)) Chioraniformethane excellent (143)
Miscellaneous
Bio-S (growth hormone) (26) Resimethrin (insecticide) mediocre ( 19Q Cycloheximide (antibiotic) poor (207) to fair (205) to excel lent (207) Procaine hydrochloride mediocre and phytotoxic (207) Prevention o£ powdery mildew of Rieger begonias with new systemic fungicides: 1978.
Purpose: To test the efficacy of several new, unregis tered systemic fungicides. Such fungicides are needed to eradicate the fungus in protected places on the host and for use during flowering,when dinocap would b u m flowers. The new fungicides tested were triademefon and triforine.
Procedure:'Schwabenland Red'and'Aphrodite Cherry Red'
Rieger elatior begonias (Begonia x hiemalis Fotsch) were grown in 4 inch pots on a shaded greenhouse bench. The plants were sprayed with the fungicides listed in Table 42 on November 27,
1978. Rates are listed in Table 42 . Mature leaves from the canopy were removed from the plants on Nov. 29 and Nov. 31.
There were 10 leaves per treatment. On Nov. 29 all 10 were Schwab enland Red but on Nov. 31 5 were Schwabenland Red and 5 were
Aphrodite Cherry Red. Leaves were inoculated in the lab with
Oidium begoniae Putt, race 2 sensu Powell and Quinn (239) on the day that they were detached from the sprayed plants. Leaves detached on Nov. 29 were incubated in double Petri plates (see page 14) in the dark at 15 C. Leaves detached Nov. 31 were incubated in double Petri plates at 20 C with 12h of light per day. Data presented in Table 42 was collected one week after inoculation. Percent infection was determined by examining leaves under a dissecting microscope at 25x. A grid was laid over the leaf and the 23 points where the lines of the grid met were checked for infection. This was done for all the 186 leaves in the treatments and the percent of leaf surface infected was calculated by dividing the number of infections at the intersections by the total number of intersections and multi plying that result by 100. The percent of microscope fields in which infections were present was also noted. For formulas and additional information on these fungicides see pages 156
CO 181. Incidental infection by Botrytis cinerea was also noted. Each leaf was rated on a 1-5 scale for B. cineraa.
Results and discussion: Neither of the two new systemic fungicides, triforine and triademefon, could be recommended from the results of these trials (Table 42). Triforine was not as efficacious as dinocap or benomyl, even though the latter two fungicides were applied without the recommended spreader- sticker. In part, this result may have been unfair to triforine, as mycelium contained in local lesions would have counted as infections. It is likely that this fungicide breaks down more quickly on leaf surfaces than it does inside the leaf (48 ).
T hus,control occurs after penetration of the leaf by the fungus.
Triademefon caused stunting, lack of shoot number and brittleness of foliage at the rates applied. Leaves of triade- mefon-sprayed plants were more susceptible to sunscalding and it may have caused some direct marginal necrosis of foliage. This fungicide may be better applied as a soil drench at much reduced rates ( see pages 196-199). Considering that it has been sug gested that triforine and triademefon have the same mode of action (e.g. ergosterol synthesis inhibition (23))as fungicides Table 42: Preventive ability of systemic fungicides expressed
as percent leaf surface and percent microscope
fields with infections of Oidium begoniae.
Treatment Rate Inoculated Nov. 2 9 z Inoculated Nov (Nov. 27) (oz/100 gal) (incubated 15 C) (incubated1 20 TMT %LS
Control 4 10 1 17 Dinocap 20WD 6 0 1 0 0 Benomyl 50WP 8 0 1 0 0 Triforine 207JEC 12 1 4 2 19 Triforine 20%EC 24 2 22 3 10 Triforine 20%EC 48 0 2 2 18 Triademefon 25WP 2 y 0 0 0 0 Triademefon 25WP 4 y 0 0 0 0 w) %LS=percent leaf surface infected. x) %MF=percent of microscope fields (25x) in which infections were seen, y) severe phytotoxicity at these rates, z) fungicides applied November 27. it is interesting that they have different effects on the pathogen and host. Triademefon acts as a gibberellic acid synthesis inhibitior and this may account for its added side effects ( 23) •
It was odd that benomyl was effective against race 2 here but was not the following spring (see pages 190-192).
Observations of Botrytis cinerea infection were made incidental to this study. B. cinerea often naturally infected leaves in the double Petri plates^when leaves are incubated at 15 C in the dark (Table 42). From the petiole ,the Botirvtis spreads to the leaf as large, brown, watery lesions. Leaves were rated on a 1-5 scale with 5 being the highest Botrytis infection. Amount of B . cinerea on dinocap, benomyl and triade mefon treated leaves did not differ significantly from controls
(Table 43). Future tests of fungicides on Oidium begoniae were carried out at 21 C and 12h light both because this is the optimum temperature for 0. begoniae and because B. cinerea infections can be largely avoided. 189 Table 43: Relative amount of Botrytis cinerea on Leaves treated
with selected fungicides.
Treatment Rate (oz/100 gal) 15 C, dark^ 20 C, 12h lij
Control __ 2.4^ 1.4 Dinocap 20WD 6 2.2 1.0 Benomyl 50WP 8 3.0 1.6 Triforine 20%EC 12 2.8 1.0 Triforine 20%EC 24 4.8 1.4 Triforine 20%EC 48 3.4 1.0 Triademefon 25WP 2 1.6 1.0 Triademefon 25WP 4 2.2 1.2 x) Botrytis rating: l=no Botrytis, 2=1-25% of leaf surface covered, 3=26-75% of leaf surface covered, 4=76-99% of leaf surface covered, 5=all of leaf surface covered, y) Plants sprayed Nov. 27, placed in incubator Nov. 29 and evaluated z) Plants sprayed Nov. 27, placed in incubator Nov. 31 and evaluated 190 Prevencion o£ powdery mildew of Rieger begonias with new systemic
funeiicides, 1979.
Purpose; To further test the efficacy of several new,
unregistered systemic fungicides. Such fungicides are needed to
eradicate the fungus in protected places in the host and for use during flowering when dinocap would burn flowers. The new
fungicides tested were triforine, Baycor, fenarimol and CGA 64251.
Procedure: Excised leaves from'Schwabenland Red'Rieger elatior begonias were dipped in fungicide suspensions at rates
indicated in Table 43. These leaves were incubated for one week at 21C with 12h light per day, then dipped in water to simulate overhead watering. The leaves were then returned to the incubator
for another week. Two weeks after dipping in fungicides, the
leaves were inoculated with Oidium begoniae race 2 by blowing conidia off of stock leaves with 1 week old infections. The number of colonies per leaf was counted one week after inoculation.
There were 5 leaves per treatment and this experiment was repeated
6 times for a total of 30 leaves per treatment. Data were collected on May 7, May 14, May 21, May 28, June 4, and June 11,1979. Phyto
toxicity was checked for on whole plants grown on a shaded green house bench. Rates used were the same as in the double Petri plate
trials and plants were sprayed twice at a 14 day interval and observed 7 days after the second spray.
Results and Discussion: All of the experimental systemic fungicides tested proved efficacious in preventing powdery mildew
infections (Table 44). Unlike dinocap, these fungicides do not Table 44: Fungicide spray trials for control of powdery mildew on
begonias: Spring 1979. Protective ability two weeks
after application on excised Begonia x hiemalis leaves.
Mean number % of leaves Chemical oz/100 gal. Colonies/leaf with local lesions
Dinocap 20WD 6^ 0 c" Fenarimol 12.5% EC 20ppm 0 c 94 Fenarimol 12.5% EC 40ppm 0 c 100 CGA 64251 .8% E-H 8 0 c 88 CGA 64251 .8% E-H 16 0 c 83 Baycor 25WP 16X 0 c 67 Baycor 251VP 32^ 0 c 72 Triforine 20% EC 8 1 be 94 Benlate 50WP 8X 26 a 90 Lysol aerosol spray to run-off^ 7 b 93 Control 53 a 44 x) Exhalt 800 adjuvant added at rate of 8 oz/100 gal. y) Lysol was phytotoxic at this rate. z) Numbers followed by the same letter do not differ significantly at the 0.05 level using Duncan's new multiple range test. Numbers are the means of six experiments which were pooled for the statistical test. 192 prevent germination. Instead penetration and haustorial forma tion occur but growth of mycelium is extremely limited and sporulation is prevented (see pages 207-214). Fenarimol, Baycor, and triforine may have similar modes of action, that is,inhibition of fungal ergosterol synthesis (23), and this occurs too slowly to stop germination. Indeed, germination occurred on leaves dipped in fenarimol 5 minutes after inoculation with 0. begoniae in a separate experiment. Of the experimental fungicides tested, fenarimol and triforine were slightly phytotoxic to flowers and Baycor left an unsightly residue on the flowers. Only CGA 64251 appeared both nonphytotoxic and left no residue. Since these new fungicides appeared promising in these experiments,attempts to register these compounds on begonias should probably be encouraged.
The loss of effectiveness of benomyl was an unpleasant surprise, as this was the most effective fungicide registered that did not badly burn flowers. Why resistant strains of the fungus may be present in apparently high numbers is mysterious ,as leaves on which the fungus was maintained were not treated with benomyl. This loss of effectiveness was noted in North Carolina with a fungus resembling race 2 (194) and in Ireland with a fungus with a host range like that of race 1 (145) too.
Lysol aerosol disinfectant was included in the treatment to verify reports of efficacy versus 0 . begoniae reported to me to be found in the trade literature. It was extremely phytotoxic when sprayed to runoff and later proved to be phytotoxic when sprayed at a lower rate on whole plants too (see pages 197-199). 193
Prevention of powdery mildew o£ Rieger begonias with triade mefon soil drenches, 1979-1980.
Purpose: To verify a claim that triademefon controls
Oidium begoniae for 6 weeks when applied as a soil drench at the rate of 2.5 grams/100 Liters (Durchs, personal communication).
Such a drench could be applied just prior to sale to insure disease
free plants in the home for 6 weeks or longer. Triademefon also could replace dmocap as the commercial protectant fungicide during flowering if triademefon is found not to b u m flowers.
Procedure: Schwabenland Red Rieger begonias (Begonia x hiemalis) were grown in six inch pots on a shaded greenhouse bench. On Nov. 8, 1979 the soil in 11 of these pots was drenched with 100ml of triademefon (Bayleton 25WP, 2.5 grams active ingredient/100 Liters water) suspension. Ten undrenched plants were set between these plants to act as controls. All of these plants were inoculated by blowing air over leaves of stock plants infected with Oidium begoniae, race 2. In an adjacent greenhouse 20 uninoculated plants of the same age were grown as uninoculated controls. Data were collected on
D e c . 26, 1979 and Jan. 16, 1980. Number of powdery mildew colonies per plant, height of plants, number of flowers , number of leaves and phytotoxic symptoms were noted.
Results and Discussion: 50 days after application,control of
the fungus by the triademefon drench was absolute (Table 45).
Side effects were not severe but were noticeable. These Table 45; Effect of triademefon soil drench (100ml of 2.5
gram a.i./lOO L solution) on Oidium begoniae and
on Begonia x hiemalis plants after 50 days.
Controls______Triademefon uninoculated inoculated Drench
Number colonies/plant 0 34 0 Height of plant(cm) 19 15 17 Number of flowers/plant 0.4 1.0 3.5
Table 46: Effect of triademefon soil drench ( 100ml of 2.5
gram a.i./lOO L solution) on Oidium begoniae
and on number of leaves of Begonia x hiemalis
after 70 days.
Inoculated control Triademefon Number of colonies/plant 158 3 Number of leaves/plant 6 14 195 included smaller leaves that were an attractive, darker green than control leaves. Stunting was not observed (Table 45).
Some increase in flower number was noted in the triademefon plants,when compared with the uninoculated controls (Table 45).
This difference could, however, have been due to differing environmental conditions in the two greenhouses. No flower burn ing was noted on any treatment. Higher amounts of triade mefon drench (e.g.,25g/100L) are severely phytotoxic to
Rieger begonias,causing stunting, marginal leaf bum, smallness of foliage and reduced number of shoots. These side effects are presumably linked to gibberellic acid synthesis inhibition
( 23).
Follow up observations noted complete inhibition of powdery mildew on the treated begonias until about Jan. 10, 1980. Data collected on Jan. 16 showed that 70 days after drenching almost all mildew was still inhibited (Table 45). By this time the fungus was causing severe defoliation on control plants (fig. 25).
In a separate set of experiments, Nancy Hardman and C.C. Powell showed that the amount of fungitoxin in leaves remains at nearly the same level for 70 days after drenching with triade mefon ( 76). Indeed, some fungicidal activity was still evident on plants casually observed on May 8, 1980, over 150 days after the drench was applied. By this date all inoculated control plants had been killed. Figure 25. Results of triademefon soil drench. B. x hiemalis 'Schwabenland Red' plant on far left is stunted and has marginal leaf necrosis from overdose of fungicide. Plant second from left is the inoculated control and has been defoliated by 0. begoniae. Plant second from right has been drenched with 100ml of 2.5g/100L triademefon. Note dark green leaves. Plant on far right is an uninoculated control plant. Flowers farthest right belong to the latter plant. 197 Prevention of powdery mildew of Rieger begonias with chemicals
found in the home and with Exhalt 800, an adjuvant, 1979.
Purpose: To verify the efficacy of some home remedy
type chemicals advocated in trade literature as preventing
powdery mildew on begonia (Jim Mikkelsen, personal communication).
The growers advocating such materials usually do not have control
plants, plants sprayed with standard fungicides and random design in
their experiments. These chemicals could, if proved effi
cacious, be used by homeowners. If such materials proved as efficacious as dinocap they could be used to replace
standardly used chemicals at some savings in cost of materials by commercial growers too.
Procedure: Krefeld Orange Rieger elatior begonias
(Begonia x hiemalis) were grown in 4 inch pots on a shaded greenhouse bench. There were 8 plants per treatment, each plant comprising one randomized replication. They were
inoculated by blowing air over leaves of stock plants, in fected with Oidium begoniae race 2, on August 19 and 27, 1979.
Plants were sprayed with a 2-gallon hand pump sprayer (25 p.s.i.) on August 15,22,29 and Sept. 5. Data was collected
September 6. For listing of materials used see Table 46,
Results: All products controlled 0. begoniae to some degree, but none was as effective as dinocap (Karathane)
(Table 47). Lysol aerosol spray was particularly phytotoxic, but other treatments were not obviously harmful to foliage. Table 47: Efficacy of homeowner remedy type chemicals in
controlling Oidium begoniae on Rieger elatior
begonias.
Treatment Number of Infections . per 100.gal. per plant per leaf Control Dinocap 20WD + Exhalt 800 1/2+1/2 tsp. 6+8 oz. 0.0 c 0.0 Exhalt 800 3/4 tsp. 13 oz. 1.9 b 0.1 Baking Soda (Arm 2 tsp. 25 oz. 1.6 b 0.1 and Hammer) Baking soda + 2+3/4 tsp. 25+13 oz. Exhalt 800 Lysol liquid dis 2 1/2 Ts. 128 infectant Lysol aerosol dis- 3 second mist 8 inches infectant from plant Epsom salts 1 tsp. 15 oz. 8.8 b 0.8 Epsom salts + 1 + 3/4 tsp. 15+13 oz. 4.4 b 0.3 Exhalt 800
z) Treatment followed by same letter do not differ significantly using Duncan's new multiple range test (p ^ 0.5) 199 Discussion: Since none of chese chemicals was as effective as dinocap, none can be recommended for use in a commercial greenhouse. Use of such chemicals would result in the presence of some mildew with consequent customer dissatisfaction. However, some use of these materials may aid the homeowner in controlling this disease. Addition of the spreader-sticker (Exhalt 800) did not appreciably enhance the effectiveness of the tested products although the spreader-sticker alone was somewhat effective in preventing infections. Lysol aerosol disinfectant should be avoided, even though some European trade publications have advo cated its use, due to its high phytotoxicity. 200
Eradication of powdery mildew of Rieger begonia wlch new systemic
funi^icides, 1978.
Purpose: To test the eradicative ability of new systemic
fungicides. New fungicides are needed that do not b u m flowers
and v M c h eradicate powdery mildew in protected places on the
Procedure: Excised leaves of Begonia x hiemalis 'Schwabenland
Red' were inoculated with Oidium begoniae race 2 on December 18,
1978. Inoculation was by appression of infected stock leaves with week old infections to the upper surface of the experimental
leaves. These leaves were dipped in fungicides at rates indicated
in Table 47 on December 26, 1978, 8 days, after inoculation. The
percent leaf surface covered was determined as on pages 182-185 on
January 2, 1979 and the percent germination of conidia was observed
on January 3. There were 8 leaves per treatment. The leaves were
incubated between inoculation, fungicide application and data collection in double Petri plates at 21 C and 12h light per day.
Results and Discussion: All fungicides tested except
triforine 29% EC at 12 oz/100 gal were effective (Table 48). Triade mefon has serious side effects,including leaf burn and stunting of
plants at the rates at which it was applied. The two higher rates
of triforine appeared to be efficacious but were observed to slightly burn flowers in test plots at Mikkelsens,Inc. However, the phyto
toxicity exhibited was probably lower than that expected with dinocap sprays. Thus, triforine may be used in the future for
eradication of 0. begoniae. Further tests using lower rates of 201 triademefon could show this compound to be an effective eradicant spray for the future.
Table 48: Effects of selected eradicative fungicides on percent
leaf surface covered and percent germinability of
14 day old colonies, 7 days after fungicide appli-
Treatment Rate 7oLeaf Surface %conidia (oz/lOOgal) covered germinable Control 36 42 Dinocap 20WD 6 87 0 Benomyl 50WP 8 137 0 Triforine 20%EC 12 25 20 24 97 48 9y Triademefon 25WP 2 gy 0 4 - y) This number probably reflects dead mycelium that would have remained after fungicide application, z) Trade names of products used are Karathane (dinocap), Benlate (benomyl), Bayleton (triademefon). For more information on these chemicals see page 159-173. 202
Eradication of powdery mildew of Rieger begonias wich new
systemic fungicides, 1979.
Purpose: To further test the ability of unregistered
systemic fungicides (i.e. Baycor, fenarimol and CGA 64251)
to eradicate Oidium begoniae from Rieger begonia leaves.
A new fungicide is needed that does not b u m begonia flowers.
It is also hoped that a systemic fungicide will eradicate mildew from buds and protected places.
Procedure: Excised leaves of Begonia x hiemalis 'Schwaben
land Red' were inoculated with Didium begoniae race 2 by
appressing infected stock leaves to their upper surface. These
leaves were incubated for one week at 21 C, 12h light a day and
then were dipped in fungicides at the rates noted in Table 49.
The leaves were then returned to the incubator for one week, after which they were appressed to uninfected leaves to test for viable conidia. These newly inoculated leaves were then placed
in the incubator and after one week the number of colonies per
leaf was counted. There were 7 leaves per treatment and the
experiment was repeated once.
Results and discussion: All fungicides tested were very
effective except for benomyl (Table 49). Baycor, fenarimol
and CGA 64251 are not soon to be registered for control of
powdery mildew on minor ornamental crops such as begonia.
For effects of these fungicides on fungal morphology see
pages 204-211. Table 49: Colonies per leaf formed from conidia obtained from
leaves on which eradicative fungicides had been
applied.
Treatment Rate Colonies per leaf (oz/lOOgal) (as a percent of control) Control 100.0 ^ +342 Dinocap 20WD + Exhalt 800 6 + 8 0.9 c ±3 Benomyl 50WP + Exhalt 800 8 + 8 18.4 b ±18 Baycor 25WP + Exhalt 800 32 + 8 0.4 c ±1 Fenarimol 12.57, EC 40 ppm 0.3 c +0.5 CGA 64251 0.87, E-H 16 0.2 c ±0.8 y) numbers followed by same letter do not differ significantly using Duncan's new multiple range test (p - 0.05). z) + numbers are standard deviations of previous numbers. The 'previous numbers' are means derived from data pooled from two separate experiments. 204
Prevention and eradication of powdery mildew of Rieger begonias with volatilized systemic fungicides, 1979.
Purpose: To determine if volatilization of certain systemic fungicides is effective in preventing and curing 0. begoniae
infections. Such treatments may be used in place of sprays. An advantage of volatilized fungicides over sprays would be ease of application, as no mixing would be necessary and tedious spraying of every plant would be eliminated. Although the following procedures were designed by C.C. Powell and myself, the inspiration for these tests was similar work done by Duane Coyier in Oregon on rose powdery mildew (45).
Procedure: Preventive and eradicative trials were carried out simultaneously. In preventive trials excised leaves
(Begonia x hiemalis 'Schwabenland Red') were placed in a fume hood.
In the hood undiluted chemicals (Table 50) were placed in an electric frying pan and the chemicals heated therein. Fenarimol was volatilized at 105 C as in Coyier's experiments (45). Dinocap was heated at 205 C (400F). Rates used were comparable to rates used in spray trials. Inoculations with 0. begoniae by leaf appression were made both 30 min. before or 30 min. after volatilization. There were 5 leaves in each treatment, except for the control in which there were 10 leaves. The experiment was repeated once. Leaves were dipped into dinocap and fenarimol solutions for purposes of comparison and inoculated 30 min after the spray was applied. One week after the volatilization Table 50: Prevention of powdery mildew of begonia using
volatilized fungicides.
Treatment Rate Colonies/leaf ^
Trial 1 Trial 2 Control 150 136 Fenarimol, 12.57<£0, spray 40ppm 0 0 Fenarimol, 12.5%E0,inocu lation 30min before volatilized O.lml/m^ 2 191 1 Fenarimol, 12.5%E0,inocu lation 30min after volatilized O.lml/m^ ^ 15 0 Dinocap 20WD , spray 6 oz/lOOgal 0 0 Dinocap 20WD, inoculation 30 min before volatilization 0.4ml/m^ ^ 0 0 Dinocap 20WD, inoculation 30 min after volatilization 0.4ml/m^ ^ 0 0 y) colonies counted one week after fungicide application, z) fenarimol (EL 222) volatilized at 105C and dinocap (Karathane) volatilized at 205C. Plants and excised leaves exposed for 6h.
Table 51: Eradication of 0.begoniae with volatilized fungi
cides as measured by conidial characteristics one
week after treatment.
Treatment Rate Percent Percent shriveled germinable ^ Control 15 26 Dinocap 20WD,spray 6 oz/lOOgal 100 0 Dinocap 20WD, volatilized ^0.4ml/m^ ». ^ 97 1 Fenarimol 12.5%EC, volatilized O.lml/m.3 z 20 15 y) conidia germinated on glass for 6h at 21C. z) fenarimol (EL 222) volatilized at 1050 and dinocap (Karathane) volatilized at 2050. Plants and excised leaves exposed for 6h to volatilized fungicide one week before conidial character istics were assayed. 206
or spraying the number of colonies per leaf were counted.
Eradicative tests were carried out in an identical fashion to
that above,except that inoculation took place 1 week before
volatilization. Instead of counting the number of colonies,
the percent of shriveled, germinable and appressorium forming
conidia was observed 1 week after volatilization. Conidia were so observed on glass slides after 6h incubation at 21 C,
85% relative humidity and continuous light.
Two whole plants were included in each treatment to check
for possible phytotoxicity of the volatilized chemicals.
Results and Discussion: The results show that spraying and volatilizing dinocap are equally as effective in protectant ability, but that fenarimol is more effective as a spray than
volatilized (Table 50). Unfortunately, dinocap burns flowers
in both types of application. For eradicative use spraying is more effective than volatilization wich both dinocap and fenarimol
(Table 51).
Dinocap inhibits germination of conidia. With fenarimol germination and penetration occurred in both treatments.
However, only on the volatilized leaves was the fungus able to complete its life cycle through to sporulation. Coyier found that effective eradication of rose powdery mildew took
4 weeks of 6-8h nightly volatilizations of fenarimol or nuarimol. He reheated the initial dose nightly for one week and added new fungicide to the volatility container weekly (45). 207 Some variability was found between the first and second trials.
Fenarimol was much more inhibitory the second time as a pro tectant. It is not known why this occurred. 208
Effects of fungicides on fungal morphology
Purpose: To note the effects of the fungicides used in these studies on the various morphological stages of 0. begoniae .
Special attention was given to likelihood of fungal stages being able to survive fungicidal treatments. This includes ability of the fungus to complete its life cycle.
Procedure: In the course of the previous experiments microscopic examinations of germination or haustorial characteris tics were often made. Haustoria were observed in epidermal peels prepared ^as on page 87 . Percent germination of conidia of 0. begoniae race 2 on excised B. x hiemalis leaves was observed on epidermal peels too. Triforine was tested as a protectant fungicide for ability to inhibit fungal growth on excised B. x hiemalis leaves at the rates and with incubation times as detailed in Table 53. Incubation in these experiments was in double Petri plates at 21 C in 12h light/day.
The eradicative efficacy tests of new systemic fungicides detailed on pages 202 to 207 were also observed for effect on dessication of conidia on conidiophores and for effects on haustorial characteristics. Haustoria were observed using the epidermal peel-Trypan blue technique described on pages 86-87,
Results and discussion: Dinocap was by far the most effective inhibitor of germination of 0. begoniae conidia
(Table 52) . Benomyl, when effective, also strongly inhibited Table 52; Effect of protectant fungicides on germination of
conidia of Oidium be:;oniae on Begonia x hiemalis.
Treatment Rate (sprays) Percent germination^
Control 38 a^ Triforine 20%EC 12oz/100gal 37 a CGA 64251 0.8%E-H 16oz/100gal 13 b Dinocap 20WD 6oz/100gal 1 c Triademefon 25WP 2oz/100gal 14 b y) Percent germination was determined by observing epidermal peels stained in trypan blue 48b after inoculation. Leaves had been dipped in fungicides 2 weeks prior to inoculation (see page 183). z) numbers followed by the same letter do not differ significantly using Duncan's new multiple range test (p=0.05).
Table 53: Effect of triforine as a protectant fungicide on
hyphal length of 0. begoniae on B. x hiemalis.
Treatment Time of inocu Hyphal length lation after treatment (in um)^ Control 1430a7 Triforine 6%EC 12oz/100gal 106 b Triforine 67oEC 45oz/100gal 126 b Triforine 6%EC 45oz/100gal 1 week 110 b y) numbers followed by same letter do not differ significantly at 0.05 level using Duncan's new multiple range test, z) Hyphal lengths measured 7 days after inoculation. 210 germination. Lysol inhibited germination strongly the first 24h
and caused distortion of germ tubes of germinated conida. Some
lysol treated conidia germinated on glass with forked germ
tubes like those normally found in Sphaerotheca fuliginea by
Zaracovitis (229). The new systemic fungicides, triforine, triade mefon, fenarimol,and CGA 64251 usually lower germination (Table 52).
However, these ergosterol inhibitors allowed germination, appres-
sorium formation, penetration, haustorial formation and hyphal
growth to occur to some extent. Hyphal growth, though, was limited and local lesions quickly formed before sporulation occurred.
These local lesions resulted from death of some invaded cells.
Haustoria in these cells were often encapsulated and shriveled.
Mycelium arising from these haustoria was severely stunted, with hyphae about 100 micrometers long in triforine treatments (Table
53). These hyphae had club-shaped endings (fig. 26). Triforine was equally effective when used at ); the recommended rate with a
2 week wait until inoculation as it is at the full rate with no wait until inoculation (Table 53). No recovery of these stunted
colonies was found on leaves kept in the incubator at 21 C and
90% relative humidity for 2 months.
Dinocap had the most drastic results on extant 0. begoniae
colonies in eradicative tests. Dinocap shriveled more haustoria and caused more melanin like pigments ,to accumulate than other fungi
cides tested (Table 54). Most haustoria are shriveled and killed Figure 26. Club-shaped hyphal ending of Oidium begoniae one week
after protective treatment with triforine.
X 600.
Figure 27. Oidium begoniae haustorium one week after eradicative
treatment with dinocap. Note shriveling, x 600
Figure 28. Oidium begoniae haustorium one week after eradicative
treatment of triademefon applied as a drench.
X 600
Table 54: Effect of eradicative fungicide treatments i
haustorial characteristics of 0. begoniae oi
B. X hiemalis 7 days after treatment.
Treatment Mte ^Shriveled %Encapsu- 7oWith lated browning^ Control 7 5 0 Dinocap 20WD 6oz/100gal 81 37 52 Fenarimol 12.5%EC 40 ppm 52 48 33 Triademefon 25WP O.Soz/lOOgal^ 30 19 9 Triforine 20%EC 12oz/100gal 40 28 26 y) Browning of host cells and haustorium prabably due to phenols, z) 100ml of this solution applied as a soil drench
Table 55: Effect of eradicative fungicide treatments on dessica
tion of conidia from treated colonies , 7 days after
treatment.
Treatment % not dessicated
Control 96 Dinocap 20WD 60oz/100gal 4Z Triforine 20%EC 12oz/100gal 44Z Triademefon 25WP 0.3 oz/lOOgaiy 52
y) 100 ml of this solution applied as a soil drench z) very few recognizable conidia left, so actual percentage of original number of conidia is possibly much smaller. 214 before encapsulacion with dinocap (fig. 27). Ergosterol inhibitors
fenarimol, triademefon and triforine had not killed most haustoria
7 days after treatment,as evidenced by haustorial shriveling
(fig. 27, Table 54). Colonies treated with triforine had many non-shriveled conidia, of which a small percentage were still
germinable (Table 55). Triademefon drench (100ml of .03 oz a.i./
100 gal.) was more lethal to conidia but appeared to be the most
gentle treatment for haustoria (fig. 28). Other tests indicate that
triademefon slowly but effectively eradicates 0. begoniae in a period
of weeks. Fungitoxin from a single treatment of triademefon probably remains in a plant for several months (see page I9Q).
Dinocap appears to be the most effective fungicide for quickly killing most stages of the fungus. Other fungicides are less effective or work more slowly. However, dinocap is probably not very systemic and burns flowers. Haustoria killed by dinocap are often not encapsulated. Encapsulations may protect the host from toxic fungal breakdown products. Perhaps treatments which result in lowered percentages of encapsulations will prove to be more phytotoxic treatments too.
It is known from the literature that chemicals as well as heat, wounds,and pathogens can cause wall depositions and production of phenols and phytoalexins, and other substances whose synthesis is
invoked by wounding (see also page 112). The similarity of the morphology of the general non-race specific resistance, of the heat
treatments ,and of treatment with the ergosterol inhibiting fungicides is noted. All cases have the following in common: 215 germination reduced about 50%; hyphae grow but sparingly and quit growing within 7 days; haustoria become encapsulated, shrivel and are associated with melanin-like substance; host local lesions form; sporulation is totally inhibited. The club-shaped hyphal endings are probably unique to the fungicide treatments. Notes to Chapter five.
Source of formulas: ( 23,137, company technical information
Source of figures: (137, company technical information sheets).
1. Abdel-Rahman, M. 1977, Fungic. & Nematic. Tests 32: 1
2. ______1977. Fungic. & Nematic. Tests 32: 2
3. ______1977. Fungic. & Nematic. Tests 32: 60
4. ______1977. Fungic. & Nematic. Tests 32: 79.
5. ______1977. Fungic. & Nematic. Tests 32: 100.
6. ______1977. Fungic. & Nematic. Tests 32: 109.
7. ______1977. Fungic. & Nematic. Tests 32: 113.
8. ______1977. Fungic. & Nematic. Tests 32: 126.
9. ______1978. Fungic. & Nematic. Tests 33: 10.
10. ______1978. Fungic. & Nematic. Tests 33: 52.
11. ______1978. Fungic. & Nematic. Tests 33: 69.
12. ______1978. Fungi:. & Nematic. Tests 33: 113.
13. ______1979. Fungic. & Nematic. Tests 34: 1
14. ______1979. Fungic. & Nematic. Tests 34: 77-78.
15. ______1979. Fungic. & Nematic. Tests 34: 104.
16. Abdel-Rahman, M. and S. Baron 1977. Fungic. & Nematic. Tests 32: 60.
17. Albert, J.J. 1974. Fungic. & Nematic. Tests 29: 8.
18. ______1977. Fungic. & Nematic. Tests 32: 4-5.
19. ______1978. Fungic. & Nematic. Tests 33: 11
20. Ahrens, R.W. and G.L. Worf 1971. Fungic. & Nematic. Tests 26: 132. 21. Anonymous, 1979. USDA Compilation of Registered Uses of Fungicides and Netnaticldes. Agricultural Research Pesticide Impact Staff, Science and Education Administration, USDA, Washington D.C.
22. Baldwin, R.E. and J.A. Francis 1979, Fungic. & Nematic. Tests 34: 62-53.
23. Bent, K.J. 1978. Chemical control of powdery mildews. In The Powdery Mildews, D.M. Spencer e d ., Academic Press London, New York, pps. 259-283.
24. Bent, K.J., A.M. Cole, J.A. Turner and M. Woolner 1971. Resistance of cucumber powdery mildew to dimethirimol. Proc. 6th British Insectic. Fungic. Conf. 1971: 274-282.
25. Bohnen, K. 1979. Fenpropemorph (Ro 14-3169, BAS 421 00 F ) , a new systemic fungicide for the control of powdery mildew and rust on cereals. Proc. Intl. Congr. PI. Protection abstract 307.
26. Bosnians, P. Bestrijdingsen bemes tings proeven bij begonia. (Control and fertilization experiments in begonia culture). Meded Fac. Landbouwet Rijksuniv Gent. 36; 1042-1048.
27. Braun, A.J. 1977. Fungic. & Nematic. Tests 32:62.
28. Braun, A.J. and R.C. Pearson 1978. Fungic. & Nematic. Tests 33:54.
29. Brown, I.F. 1970. Proc. 7th Intl. Congr. PI. Protection, abstract # 206.
30. Byrde, R.J.W., C.W. Harper, M.E. Holgate, and Jo Hutcheon 1976. Fungic. & Nematic. Tests 32: 6-7.
31. Carroll, R.B. 1977. Fungic. & Nematic. Tests 32: 126.
32. Christensen, C.D. 1975. Fungic. & Nematic. Tests 30: 139.
33. Cimanowski, J. Anna Masternak and D.F. Millikan 1970. Effectiveness of benomyl for controlling apple powdery mildew and cherry leaf spot in Poland. Plant Dis. Reprtr. 54: 81-83.
34. Claflin, L.E. 1978. Fungic. & Nematic. Tests 33: 64.
35. ______1978. Fungic. & Nematic. Tests 33: 129. 218
36. Cole H., J.S. Boyle and C.B, Smith 1970. Effect of benomyl and certain cucumber viruses on growth, powdery mildew, and element accumulation by cucumber plants in the greenhouse. PI. Dis. Reprtr. 54: 141-145.
37. Cole, H. D.J. Royse, L.V. Gregory and J.E. Ayres 1979. Fungic. & Nematic. Tests 34: 105.
38. Cole, J.S. 1978. Powdery mildew of tobacco. In The Powdery Mildews. D.M. Spencer ed. Academic Press, London, New York, pps. 447-472.
39. Covey, R.P. 1971. Orchard evaluation of ti;o new fungicides for the control of apple powdery mildew. PI. Dis. Reprtr. 55: 514-516.
40. Covey, R.P. 1974. Fungic. & Nematic. Tests 29: 12
41. Covey, R.P. 1977 Fungic. & Nematic. Tests 32: 8
42.______1978. Fungic. & Nematic. Tests 33: 14
43.______1979. Fungic. & Nematic. Tests 34: 3
44. Coyier, D.L. 1971. Control of powdery mildew of apples with various fungicides as influenced by seasonal temperature. PI. Dis.'Reprtr. 55: 253-266.
1979. Chemical and biological control of rose powdery mildew on greenhouse roses. Progress report for Joseph Hill Memorial Foundation Grant 1978-1979.
46. Coyier, D.L. and H.L. Dooley 1975. Fungic. & Nematic. Tests 30: 139.
47. ______and 1978. Fungic. & Nematic. Tests
48. Darda, S., R.L. Darskus, D. Eichler, W. Ost, and M. Wotscho- kowsky 1977. Hydrolysis and photolysis of the fungicide triforine. Pestic. Sci. 8: 183-192.
49. Burchill, R.T. and M.E. Cook, 1975. Control of scab and powdery mildew of apple with a reduced number of sprays. Plant Pathology 24: 194.
50. Dekker, J. 1976. Acquired resistance to fungicides. Ann. Review of Phytopathology 14: 405-428. 51. Delp, C.J, and H.L. Klopping 1968. Performance attributes of a new fungicide and mite ovicide candidate. PI. Dis. Reprtr. 52; 95-99.
52. Dooley, H.L. and J.E. Bennett 1970. Fungic. and Nematic. Tests 25:118.
______1971. Fungic. & Nematic. Tests 26: 129.
54. ______and 1973. Fungic. & Nematic. Tests 28: 109.
55. Dougherty, D.E. 1978. Fungic. & Nematic. Tests 33:75.
56. Drake, C.R. 1974. Fungic. & Nematic. Tests 29: 43-44.
57. ______1977. Fungic. and Nematic. Tests 32: 9.
58. ______1979. Fungic. & Nematic. Tests 34: 9.
59. ______1979. Fungic. & Nematic. Tests 34: 15.
60. Easton, G.D. and M.E. Nagle 1978. Fungic. & Nematic. Tests 33: 81.
61. Edgington, L.V., R.A. Martin, B.C. Bruin and I.M. Parsons 1980. Systemic fungicides : a perspective after 10 years. Plant Disease. 1: 19-23.
62. Engelhard, A.W. 1978. Fungic. & Nematic. Tests 33: 126.
63. Fisher, D.J. 1974. A note on the mode of action of the systemic fungicide tridemorph. Pestic. Sci. 5: 219-224.
64. Forsyth, F.R. 1964. Surfactants as fungicides. Can. J. Bot. 42: 1335-1347.
65. Frick, E.L. and R.T. Burchill 1972. Eradication of apple powdery mildew from infected buds. Pi. Dis. Reprtr. 56: 770.
66. Gilmour, J. 1978. Barley mildew control in Southeast Scotland. Pestic. Sci. 7: 115-134.
67. Gilpatrick, J.D. 1977. Fungic. and Nematic. Tests 32: 37.
1979. Control of apple and stone fruit diseases with CGA 64251. Proc. IX Intl. Congr. PI. Protection YA 312. 69. Gilpatrick, J.D. and R. P r o w i d enci 1973. Proc. 2nd Intl. Congr. PI. Proc., Minneapolis, abstract v 780.
70. Gilpatrick, J.D. and C.A. Smith 1974. Fungic. & Nematic. 29: 17
71. and 1977. Fungic. & Nematic Tests 32: 11.
72. and 1977. Fungic. & Nematic, 32: 13.
73. and 1977. Fungic. & Nematic 32: 14.
74. and 1978. Fungic. & Nematic, 33: 18.
75. and 1978. Fungic. & Nematic. Tests 33: 19.
76. Hardman, Nancy 1980. Senior thesis. Dept, of Plant Pathology, The Ohio State University.
77. Harper, C.W., R.J.W. Byrde, M.E. Holgate, and Jo Hutcheon 1979. Fungic. & Nematic. Tests 34: 6.
78. Hickey, K.D. 1974. Fungic. & Nematic. Tests 29: 21-22.
79. ______1977. Fungic. & Nematic. Tests 32: 15.
80. ______1977. Fungic. & Nematic. Tests 32: 16.
81. ______1977. Fungic. & Nematic. Tests 32: 17.
82. ______1977. Fungic. & Nematic. Tests 32: 20.
83. ______1977. Fungic. & Nematic. Tests 32: 21.
84. ______1978. Fungic. & Nematic. Tests 33: 21.
85. ______1978. Fungic. & Nematic. Tests 33: 22.
86. ______1979. Fungic. & Nematic. Tests 34; 8.
87. ______1979. Fungic. & Nematic. Tests 34: 9.
88. Hickey, K.D., A.E. Davis and J.C. Scalea 1979. Fungic. & Nematic. Tests 34: 6-7. 89. Hislop, E.C., and D.R. Clifford 1974. Eradication of powdery mildew from apple buds. PI. Dis. Reprtr. 58: 949.
90. Hislop, E.G. and D.R. Clifford 1975. Eradication of apple powdery mildew (Podosphaera leucotricha) with dormant season sprays of surface-active agents. Ann. appl. Biol. 82: 557.
91. Hislop, E.C., D.R. Clifford, M.E. Holgate and P. Gendle 1978. Eradication of apple powdery mildew (Podosphaera leucotricha) with dormant season sprays of surface-active agents. Pestic. Sci. 9: 12-21.
92. Homma, Y. 1979. An attempt on the use of sodium bicarbonate as a fungicide. Proc. Intl. Congr. PI. Protection ■> 317.
93. Horsfall, J.G. 1945. Fungicides and their action. Chronica Botanica Co., Waltham Mass.
94. Jarvis, W.R. and K. Slingsby 1975. Tolerance of Botrytis cinerea and rose powdery mildew to benomyl. Can. PI. D. 55: 44.
95. Jbooty, J.S. and D.S. Bebar 1972. Control of powdery mildew of pea and pumpkin by seed treatment with benomyl. Indian J. of Agricultural Sci. 6: 505.
96. Johnston, H.W. 1970. Control of powdery mildew of wheat by soil appliel benomyl. PI. Dis. Reprtr. 54: 91-93.
97. 1978. Fungic. & Nematic. Tests 33: 115.
98. Jones, J.P 1974. Fungicides for control of target leafspot, soil rot, and powdery mildew of cucumber. PI. Dis. Reprtr. 58: 636.
99. Judd, R.W., and G.S. Walton 1974. Fungic. & Nematic. Tests 29: 114.
100. ______and 1975. Fungic. & Nematic. Tests
101. Kavanaugh, T. 1972. Control of powdery mildew (Sphaerotheca macularis) of greenhouse strawberries with systemic fungi cides. PI. Dis. Reprtr. 56: 235.
102. Kerr, E.D. 1977. Fungic. & Nematic. Tests. 32: 77.
103. Kingsland, G.C. 1977. Fungic. & Nematic. Tests 32: 129. 104. Kingsland, G.C. 1979. Bayleton 5ŒVP as a foliage fungicide for the control of powdery mildew of wheat in South Carolina. Proc. IX Intl. Congr. PI. Protection, abstract # 314.
105. Kontaxis, D.G. 1975. Low rates of sulfur against powdery mildew on sugar beets in Imperial Valley, California. Calif. Agriculture 29: 15.
106. Lambe, R.C. 1970. Fungic. & Nematic. Tests 25: 113.
107. 1970. Fungic. & Nematic. Tests 25: 120.
108. 1971. Fungic. & Nematic. Tests 26: 120.
109. 1973. Fungic. & Nematic. Tests 28: 109.
110. Large, E.C. 1940. Advance of the Fungi. H. Holt & Co., New York.
111. Leath, K.T. and C.C. Berg 1978. Longterm control of powdery mildew of orchardgrass with benomyl. PI. Dis. Reprtr. 56; 75-76.
112. Lewis, G.D. 1977. Fungic. & Nematic. Tests 32: 84.
113.______1977. Fungic. & Nematic. Tests 32: 101.
114.______1979. Fungic. & Nematic. Tests 34: 64.
115. Line, R.F. and J. Kosciuk 1979. Fungic. & Nematic. Tests 34: 108-109.
116. Locher, F . , M. Hampel and R. Saur 1979. Results of field trials with fenpropemorph, a new fungicide for mildew and rust control in cereals. Proc. IX Intl. Congr. PI, Protec tion, Washington D.C., abstract # 308.
117. Maas, J.L. 1970. Fungicidal control of Botrytis fruit rot and powdery mildew of strawberries. PI. Dis. Reprtr. 54: 883-886.
118. Mac Nab, A.A., 197/. Fungic. & Nematic. Tests 32: 99.
119. ______1978. Fungic. & Nematic. Tests 33: 69.
120. Mac Swan, I.e. 1974. Fungic. & Nematic. Tests 29: 27
121. ______1977. Fungic. & Nematic. Tests 32: 26.
122. ______1978. Fungic. & Nematic. Tests 33: 28. 123. Mac Swan, I.C., 1979. Fungic. & Nematic. Tests 34:12.
124. Mac Swan, I.C., J.R. Dilworth and B.J. Moore 1972. Fungic. & Nematic. Tests 27: 117
& Nematic,. Tests 28: 110.
126. 1974. Fungic & Nematic,. Tests 29: 114.
127. 1977. Fungic 5c Nematic,. Tests 32: 139.
128. Mac Swan, I.C. and B.J. Moore 1977. Fungic. 5c Nematic Tests 32: 140.
129. Mancl, M.K. and G.L. Worf 1975. Fungic. & Nematic. Tests 30:141.
130. Manning, W.J., P.M. Vardaro and M.D. Connor 1972. Effective ness of several fungicides for control of powdery mildew of Kalanchoe. PI. Dis. Reprtr. 56: 405.
131. Mathur, R.L., S.L. Jharmaria and L.N. Daftari 1972. Efficacy of fungicides for control of powdery mildew CErysiphe polygoni DC) of pea. Indian J. of Agricultural Sci. 42: 919.
132. Me Cain, A.H. 1970. Fungic. & Nematic. Tests 1970. Fungic. & Nematic. Tests 25:114.
133. Me Callen, S.E.A. 1969. History of fungicides. In Fungi cides I . , D.C. Torgeson ed., Academic Press, New York, London.
134. Me Coy, N.L. 1979. Fungic. & Nematic. Tests 34: 109.
135. Me Kittrick, R.T. and J,L. Troutman 1969. Fungicides tested against powdery mildew in Arizona cantalope fields. PI. Dis. Reprtr. 53: 467-470.
136. Me Millan, R.T. 1973. Control of anthracnose and powdery mildew of mango with systemic and non-systemic fungicides. Tropical Agriculture 50: 245.
137. Meister, R.T. 1979. Farm Chemicals Handbook 1979. Meister Publishing Co., Willoughby Ohio.
138. Menzies, A.R. 1978. Fungic. & Nematic. Tests 33: 30. 139. Mollgard, H. 1975. A European vlew-Rieger begonias. Ohio Florists' Assn. Bull. 551.
140. Netzer, D. and I. Dishon 1970. Field control of powdery mildew on muskmelon by root application of benomyl. PI. Dis. Reprtr. 54: 232-234.
141. Nichols, L.P. 1975. Fungic. & Nematic. Tests 30: 142.
142. Northover, J. and M.G. Howard 1977. Fungic. & Nematic. Tests 32: 28.
143. Olofsson, B. and K. Qvarnstrom, Bekampning av myoldagg pa prydnadsvaxter. (Control of mildew in ornamental plants). Vaxtskyddsnotiser 36: 37-42.
144. O'Riordain, F. 1974. Control of powdery mildew (Oidium sp.) of Antirrhinum (snapdragon) with fungicides. PI. Dis. Reprtr 58: 12.
_1979. Powdery mildew, caused by Oidium begoniae, of elatior begonia-fungicide control and cultivar reaction. PI. Dis. Reprtr. 63: 919-922.
146. Palti, J. 1974. Powdery mildew of mango. PI. Dis. Reprtr. 58: 45-49.
147. Paulus, A .O., 0. Harvey , D. Maire, and Shibuya 1974. Controlling powdery mildew in outdoor roses. Calif. Ag. 28: 4.
148. Paulus, A.O., J.A. Nelson, O.A. Harvey, and R.G. Maire 1976. Rose powdery mildew control in outdoor roses. Calif. Ag. 30: (3) 9.
149. Pearson, R.C. and F.W. Meyer 1977. Fungic. & Nematic. Tests 32: 29.
150. Pearson, R.C. and C.E. Ruggles 1979. Fungic. & Nematic. Tests 34: 47.
151. Pecknold, P.C. 1977. Fungic. & Nematic. Tests. 32: 144.
152. Peterson, J.L. and S.H. Davis 1970. Suppression of Erysiphe polygoni and Botrytis cinerea on hydrangea with benomyl. PI. Dis. Reprtr. 54: 606-607.
153. ______and ______1972. Fungic. & Nematic. Tests 154. Peterson, J.L. & S.H. Davis 1972. Fungic. & Nematic. Tests 27: 118.
155. and 1973. Fungic. & Nematic. Tests 28: 108.
156. and 1973. Fungic. & Nematic. Tests 28: 112.
157. and 1975. Fungic. & Nematic. Tests 30: 143.
158. Potter, H .S. 1977 . Fungic . & Nematic. Tests 32: 100.
159. Powell, C,.C. 1971 . Fungic . & Nematic. Tests 26: 130.
160. 1973. Fungic. & Nematic. Tests 28: 112.
161. 1975. Fungic. & Nematic. Tests 30: 144.
162. 1977. Fungic. & Nematic. Tests 32: 135.
163. 1977. Fungic. & Nematic. Tests 32: 138.
164. Powell, C., C., and J.A. Chatfield 1978. Fungic. & Nemat: Tests 33: 121.
165. and 1978. Fungic. & Nematic. Tests 33: 128.
166. and 1979. Fungic. & Nematic. 125.
167. and 1979. Fungic. & Nematic. Tests 34: 127.
168. Powelson, R.L. 1971. Fungic. & Nematic. Tests 26: 116.
169. Prowidenti, R. and E.D. Cobb 1974. Fungic. & Nematic. Tests 29: 70.
170.^ Quinn, J.A. and C.C. Powell 1980. Fungic. & Nematic. Tests 35; to be published.
171. Raabe, R.D. and J.H. Hurliman 1974. Experiments to control powdery mildew on Rieger begonia. Calif. PI. Pathology No. 22; 2-5.
172. Ragsdale, N.N. 1975. Biochem. Biophys. Acta 380: 81-96 173. Ragsdale, N.N. and H.D. Sisler 1973. PesCic. Biochem. Physiol. 3: 20-29.
174. Reed, H.E. and A.Y. Chambers 1979. Fungic. & Nematic. Tests 34: 110.
175. Rich, S. and J.G. Horsfall, 1949. Fungicidal activity of dinitrocaprylphenyl crotonate. Phytopathology 39: 19.
176. Rappel, E.G., D.L. Mumford, and F.J. Hills 1974. Epidemio logical observations on sugarbeet powdery mildew epiphy- totic in western USA in 1974. PI. Dis. Reprtr. 59: 283.
177. Sampson, M.J. 1969. Proc. 5th Br. Insectic. Fungic. Conf. 2: 347-353.
178. Schneider, C.L., R.L. Sims and H.S. Potter 1978. Fungic. & Nematic. Tests 33: 66.
179. Schroeder, W.T. and R. Prowidenti 1969. Resistance to benomyl in powder]' mildew of cucurbits. PI. Dis. Reprtr. 53:4.
180. Semeniak. P., and J.G. Palmer 1970. Eradication and preven tion of powdery mildew on rose seedlings by dip and soil application of fungicides. PI. Dis. Reprtr. 54: 598-602.
181. Shabi, E. and S. Elisha 1979. Fungic. &. Nematic. Tests 34: 16.
182. Shabi, E ., A. Elisha and Y. Pinkas 1974. Fungic. & Nematic. Tests 29: 33.
183. Shahin, E.A., D.L. Beckelman and L.E. Claflin 1978. Fungic. 6c Nematic. Tests 33: 90.
184. Shaner, G. 1972. Performance of two fungicides against wheat powdery mildew in the field. PI. Dis. Reprtr. 56: 358-362.
185. Shaner, G . , R. Finney and D. Cox 1979. Fungic. and Nematic. Tests 34: 110-111.
186. Shephard, M.C., K.J. Bent, M. Wooner and A.M. Cole 1975. Proc. 8th Br. Insectic. Fungic. Conf. 1: 59-66.
187. Sijpestein, A.K. 1972. In Systemic Fungicides, R.W. Marsh ed., Longman, London, pps. 132-155. 188. Smith, P., W.H. Read, and F.T. Last 1969. Effects of powdery mildew on cucumber yields and its chemical control. Ann. appl. Biol. 63: 403-414.
189. Spotts, R.A. and F.R. Hall 1974. Fungic. & Nematic. Tests 29: 34.
190. ______and______1977. Fungic. & Nematic. Tests 32: 31.
191. ______and______1978. Fungic. & Nematic. Tests
192. Staub, T., F. Schwinn and P. Urech 1979. CGA 64251, a new broad-spectrum fungicide. Proc. Intl. Congr. PI. Protection, abstract ir 310.
193. Steiner, P.W. and W.H. Shaffer 1977. Fungic. & Nematic. Tests 32: 31.
194. Strider, D.L. 1974. Resistance of Rieger elatior begonias to powdery mildew and efficacy of fungicides for control of disease. Pi. Dis. Reprtr. 58: 875.
195. ______1976. Resistance of Kalanchoe to powdery mildew and efficacy of fungicides for control of disease. PI. Dis. Reprtr. 60: 45.
1976. Activity of pyrethroid insecticides against powdery mildew fungi. Pi. Dis. Reprtr. 60: 512-514.
1978. Reaction of recently released Rieger elatior begonia cultivars to powdery mildew and bacterial blight. PI. Dis. Reprtr. 62: 22-23.
______1979. Fungic. & Nematic. Tests 34: 128.
199. Szkolnik, M . , L.N. Henecke, and J.R. Nevill 1979. Fungic. 6t Nematic. Tests 34: 19.
200. Szkolnik, M . , J.R. Nevill, and L.N. Henecke 1974. Fungic. & Nematic. Tests 29:33.
201. Sztejnbe, A. D. Woodcock, and R.J.W. Byrde 1975. Antisporu- lant action of fungicides against Podosphaera leucotricha on apple seedlings. Pestic. Sci. 6: 107.
202. Taschenberg, E.F. and A.J. Braun 1977. Fungic. & Nematic. Tests 32: 64. 228 203. Taschenberg, E.F., R.C. Pearson and A.J. Braun 1978. Fungic. & Nematic. Tests 33:56.
204. Thayer, P.L., C.D. Hobbs and L.D. Dohner 1972. Control of powdery mildew of squash with a-(2,4-dichlorophenyl)-a- pheny1-5-pyrimidinemethano1 (triarimol). PI. Dis. Reprtr. 56: 45-49.
205. Thompson, H.S. Control of powdery mildew on tuberous begonia in Canada. Can. J. PI. Sci. 41: 227-230.
206. Thomson, W.T. 1967. Agricultural Chemicals IV. Fungicides. Thomson Pubs., Davis, Calif.
207. Valaskova,Eva 1964. Ochrana begonii proti padli. Acta Pruhoniciana 9: 63-75.
208. Vargas, J.M. 1973. A benzimidazole resistant strain of Erysiphe graminis. Phytopathology 63: 1366-1368.
209. Wells, J.M., J.A. Payne and N.E. Me Glohon 1975. Pecan scab and powdery mildew applications of benomyl and tryphenyl- tinhydroxide. PI. Dis. Reprtr. 59: 448-451.
210. Wheeler, B.E.J. 1975. Rose powdery mildew: field trials with systemic fungicides 1970-1972. Ann. appl. Biol. 79: 177-188.
211. Wicks, T. and D. Voile 1978. Fungic. & Nematic. Tests 33:35.
212. Wolfe, M.S. 1973. Proc. 7th Br. Insectic, Fungic. Conf. 1: 11-19.
213. Worf, G.L. 1978. Fungic. & Nematic. Tests 33:130.
214. Worf, G.L. and R.W. Ahrens 1972. Fungic. & Nematic. Tests 27: 112.
215. and ______1972. Fungic. & Nematic. Tests 27: 113.
216. and ______1973. Fungic. & Nematic. Tests 28: 112.
217. and ______1973. Fungic. & Nematic. Tests 28: 113.
218. Worf, G.L. and J.E. Kuntz 1975. Fungic. & Nematic. Tests 30: 137. 229
219. Worf, G.L. and J.E. Kuntz 1977 Fungicide & Nematic. Tests 32: 141.
220. and 1977 Fungic. & Nematic. Tests 32: 142.
221. and 1978. Fungic. & Nematic. Tests 33: 126.
222. and 1978. Fungic. & Nematic. Tests 33: 128.
223. Worf, G.L., J.E. Kuntz and B.H. Ambuel 1977. Fungic. & Nematic. Tests 32: 141-142.
224. Yarwood, C.E. 1951. Fungicides for powdery mildews. Proc. 2nd Intl. Congr. PI. Protection, London 1949, pps. 500-521.
225. Yoder, K.S., A.E. Cochran and R.M. Cubbage 1978. Fungic. & Nematic. Tests 33: 37.
226. Yoder, K.S., R.M. Cubbage and K.F. MacFarlane 1979. Fungic. & Nematic. Tests 34: 20-21.
227. Yoder, K.S., R.M. Cubbage and J.R. Warren 1979. Fungic. & Nematic. Tests 34: 22.
228. Yoshii, K. and 0. Morales 1975. Fitopatologica Bogata 10: 28-31.
229. Zaracovitis, C. 1965. Attempts to identify powdery mildews by their conidial states. Tr. Brit. Mycol. Soc. 48: 553- 558.
Late Additions
230. Lewis, G.D. 1978. Fungic. & Nematic. Tests 33: 90
231. Saxena, S.C., A. Singh and Y.L. Nene 1971. Fungic. & Nematic. Tests 27: 118
232. Emmell, L. and M. Czech 1960. Anz. Schadlingsk. 33: 145.
233. Powell, C.C. 1977. Fungic. & Nematic. Tests 32: 141.
234. Abdel-Rahman, M. 1979. Fungic. & Nematic. Tests 34: 62
235. Powell, C.C. and J.A. Chatfield 1978. Fungic. & Nematic. Tests 33: 123. 230
236. Drake, C.R. 1978. Fungic. & Nematic. Tests 33: 15
237. Johnston, S.A. and J.K. Springer 1978. Fungic. & Nematic. Tests 33: 89
238. Hartman, J.R. & S. Bale 1978. Fungic. & Nematic. Tests 33: 129.
239. Powell, C.C. and J.A. Quinn 1978. Preventing powdery mildew on Rieger begonias. Ohio Florists' Assn. Bull. 589: 5. 231
6. BIOLOGICAL CONTROL AND SYNECOLOGY 07 POWDERY MILDEW OF BEGONIA
Introduction
Hyperparasitism of powdery mildews by other fungi has been known for some time. Ampelomvces quisqualis Cesati (Cicinno- bolus cesatii De Bary) was first described as parasitic on powdery mildew by Cesati in 1852 ( 1). Several studies have been done on biological control with A.quisqualis ( 3, 9,11). This fungus is known to attack powdery mildews only under humid conditions and has never been shown to be totally efficacious
(13). However, Yarwood believes that this fungus is one of the important factors in inhibition of powdery mildews by free water (14). Tilletiopsis sp. was also recently mentioned as a possible biological control agent ( 2). Yarwood in his 1957 review noted that Thea cineta. raycophagous larvae ( 8), snails
(10), and thrips (12) while possessing antagonistic qualities did not efficaciously control powdery mildew. To this list I can add mycophagous mites which got into double Petri plates on rare occasions and ate enough Oidium begoniae to be a nui-
rhe field of biological control of leaf inhabiting pathogens is excitingly open. Little or no work has been done past identification, and even in identification of these interactions, much work is still to be done. A practical approach to these problems is lacking. No work has been done on efficacy of various formulations for instance. Formulating a medium to give the antagonist a head start may be important as in nature many mycoparasites do not "control" a pathogen until
the pathogen is well established. Biological control may be particularly useful in the greenhouse where the environment can be largely controlled to favor these organisms.
The understanding of leaf surface dwelling fungi is very
fragmentary- The existence of most of them was not suspected
until recently (4)- The importance of interactions between
host, pathogen and saprophyte in roots has been known since the early 1900s but the same three membered balances are not as well documented on leaves (5). Bacteria are the first organisms
to colonize newly emerged leaves, especially Erwinia herbicola
(4). These resident bacteria can include pathogenic bacteria which multiply on leaves or in buds without causing the host damage (6 ). The next organisms to colonize leaves are the yeasts such as Sporobolomyces sp. and Tilletiopsis sp. The
last to colonize the leaves are the fillamentous fungi such as
Cladosporium herbarum (4 ). Alternaria, Verticillium, Acre- monium, and Epicoccum spp. are also common.
Several studies were made that are preliminary to utili
zation of biological control of powdery mildew of Rieger begonias.
The purpose of these studies was to identify phylloplane
fungi on begonias, test their antagonism to 0. begon'ae and
note the effects of chemical or heat treatments used to kill
0. begoniae on leaf surface dwelling fungi. Unfortunately time
did not permit extremely detailed investigations of antagonism
at the time of writing of this dissertation. Work continues. 233 Detenuinatlon of genera and relative numbers of fungi found on begonia leaves in the greenhouse and in double Petri places.
Purpose: To determine what fungi are present in quantity on begonia leaves. These fungi can then be tested for antagonism to Oidium begoniae. Such antagonism may be used as a biological control if proved efficacious in later experiments. Comparison of the genera and quantity of fungi in greenhouses with those in double Petri plates gives an indication of the similarity of the environmental conditions at the leaf surface. This would indicate the degree of sameness in occurrence of leaf surface phenomenon such as germination and shriveling of conidia in double Petri plates and in greenhouses.
Procedure: Leaves from various greenhouses (List 1) and from double Petri plates were collected in November and December 1979 and January, 1980. These leaves were firmly appressed once to the surface of Difco cornmeal agar in Petri plates. Excised leaves in double Petri plates were originally from the Botany-Zoology building Genetics Department greenhouse and had been in double Petri plates for 7-10 days (no mildew) to
14-17 days (with mildew treatment), tn 21-24 days (mildew eradi cated) t o '28-56 days (leaf dead) when appressed to agar. After 1 week incubation at room temperature* (20-23 C) the cornmeal agar plates were examined and fungi were identified to genus.
A tally was also kept of the number of colonies per plate of each genus found. There were 4-5 plates per treatment and the experiment was repeated as indicated in List 1. Specimens 234 were prepared for electron microscopy in the same way as reported
on page 107.
Results and discussion: Cladosporium sp. (Cladosporium
herbarum? ) and Pénicillium spp. were the most common genera found
(see List 1) . Verticillium sp.,usually mixed with Streptomyces sp.,
became prevalent 7-20 days after powdery mildew infection in
double Petri plates.
It is interesting to note that the microflora on begonia
leaves was similar in all the experimental greenhouses in the
Botany-Zoology building at The Ohio State University List
1 . Very few fungi were found on rose leaves in the B-Z
greenhouses, relative to the number found on begonia. This
is likely to be due to the fact that there is little elevation
of relative humidity in the microclimate of rose leaves ( 7).
In Huenefeld's greenhouse, Cincinatti, Ohio and Engel's
greenhouse, Columbus Ohio the leaf surface dwelling fungi
on Begonia rex-culturum, B. manicata-crispa, Kalanchoe bloss-
feldiana, and rose was similar to that of B . x hiemalis in
the B-Z greenhouses. The genus Stysanus was frequently found
in Engels greenhouses too. Excised B. x hiemalis leaves
infected with 0. begoniae sent to me from Maryland also had
Cladosporium and Pénicillium as the prevalent genera.
The genera found in double Petri plates were the same as
found in greenhouses. Due probably to high relative humidity
these fungi were found in greater numbers in double Petri 235
plates. This is especially true of Verticillium sp. and Strepto- myces sp. If these fungi prove antagonistic, then high humi
dity may actually help eradicate powdery mildew on begonias!
Ampelomyces quisqualis was never found on begonia.
Verticillium sp. and Pénicillium sp. often were seen with the naked eye growing in old powdery mildew lesions (figs. 30,31).
Pénicillium , Verticillium and Streptomyces are all genera known to contain species antagonistic to some fungi. Strepto- myces sp., the only bacterial organism tested here was also
observed in a tangential section of powdery mildew hypha using
transmission electron microscopy (fig. 29). This bacterium more typically was observed coating the mycelium and conidia of
0. begoniae. Pénicillium sp. was also observed growing from
conidia of 0. begoniae (fig. 30). Whether the conidia were alive
or dead at the time of Pénicillium attack is unknown. Figure 29. Streptomyces sp. (S), surrounded by and presumed
internal in a hypha of Oidium begoniae. Trans
mission electron micrograph x 7800.
Figure 30. Pénicillium sp. growing on surface of Begonia x
hiemalis leaf among conidia of Oidium begoniae.
The 0. begoniae conidia germinated poorly, and
the presence of free water on the leaf surface is
suspected, x 450.
Figure 31. Verticillium sp. on c o m meal agar. List 1: Genera and number of fungi found on begonia leaves
from various places and isolated on commeal agar.
Location of leaf fungi isolated from, genera found and number per leaf. Bacteria and yeast not looked for unless mentioned.
Botany-Zoology(B-Z) building. Dept, of Plant Pathology back greenhouse. Trial one(5 leaves/treatment): Pénicillium 6. Cladosporium 11, Botrytis 1, Aspergillus 1, Epicoccum 1, (November, 1979).(B. x hiemalis) Trial two (4 leaves/treatment): Pénicillium 8, Clado sporium 9, Aspergillus 3, Epicoccum 1, Piptocephalus 1, (November, 1979). Trial three (4 leaves/treatment): Pénicillium 10, Cladosporium 9, Epicoocum 2, yeasts 7.
Botany-Zoology building. Dept, of Plant Pathology middle green house. Trial one: Pénicillium 1, Cladosporium 5, Botrytis 5, Epicoccum 1. (November, 1979).(B. x hiemalis) Trial two (4 leaves/plate): Pénicillium 1, Cladosporium 4 -, Aspergillus 5, Altemaria 3, Pink apothecium former 1, Rhizopus 2, (December, 1979).
Botany-Zoology building. Dept, of Plant Pathology front green house. Trial one: Penicilliin:i 1, Cladosporium 14, Epi coccum 5, Rhizopus l7( Ë. x hiemalis leaves/treatment). Trial two (4 leaves/treatment’) Pénicillium 1 ^ Clado sporium 12, Epicoccum 1, Altemaria 1, Streptomyces 1, Cunninghamiella 1, (November, 1979)
Botany-Zoology building. Genetics Dept, middle greenhouse. Trial one: PerIcillium 7, Cladosporium 29, Epicoccum 5, Botrytis 1, Rhizopus 1, (Noyember, 1979).B. x hiemalis. Trial two: Pénicillium 5, Cladosporium 43, Epicoccum 8, Botrytis 2, Streptomyces 6, Aspergillus 1. (November, 1979).
Walter J. Engel Inc. greenhouses, Columbus OH, Begonia rex- culturum. recently sprayed with triforine. Pénicillium 9 , Cladosporium 5, Botrytis 1, Stysanus 3, bacteria and yeasts 3.(4 leaves/treatment, January, 1980)
Walter J. Engel Inc. greenhouses, Columbus OH, Begonia manicata crispa. Pénicillium 9, Cladosporium 2, Botrytis 1, Stysanus 1, Rhizopus 1, Curvularia 1, bacteria and yeasts 8. (4 leaves/treatment, January, 1980). List I, continued.
Huenefelds greenhouses, Cincinatti, Ohio, Begonia rex-culturum. (2 leaves/treatment) Pénicillium 29, Cladosporium 100, Aspergillus 3, Botrytis 1, yeast and bacteria 13. (January, 1980).
Botany-Zoology building. Dept, of Plant Pathology back green house. Pénicillium 2, Cladosporium 4, yeast 3 (4 leaves/ plate; 4 plates/treatment, December, 1979).
Walter J. Engel Inc. greenhouses, Columbus Ohio. Pénicillium 4, Cladosporium 9, Botrytis 1, Altemaria 1, Stysanus 1, Rhizopus 1, bacteria and yeast 11 (4 leaves/plate; 4 plates/treatment, January,-1980).
Kalanchoe
Huenefelds greenhouses, Cincinnati, Ohio (2 leaves/treatment). Pénicillium 23, Cladosporium 73, Stysanus 22, Verticillium 13, Aspergillus 5.Rhizopus 1, yeast and bacteria 54, (January, 1980).
B . X hiemalis in double Petri plates
No powdery mildew present (5 leayes/treatment) Pénicillium 7, Cladosporium 20, Verticillium 11, Streptomyces 50.Epi coccum 1. (November, 1979)
Live powdery mildew present. Trial one (5 leaves/treatment): Pénicillium 28, Cladosporium 10, Epicoccum 1, Verticillium 20, Streptomyces 50, Botrytis 1. (November, 1979) Trial two (4 leaves/treatment): Pénicillium 28^ Clado sporium 9, Epicoccum 1, Verticillium 50, Streptomyces 65, Botrytis 1. (November, 1979)
Heat killed powdery mildew. Trial one (5 leaves/treatment): 12, Cladosporium 23, Verticillium 30, Strep tomyces 47, Alt e m a r i a 1. (November, 1979) Trial two (4 leaves/treatment): Pénicillium Clado sporium 7, Verticillium 75, Streptomyces 65. (November 1979)
Leaves dead. Trial one (5 leaves/treatment): Pénicillium 42. Cladosporium 15, Streptomvces 1, Choanephora 1 .Epicoccum 3, Botrytis 10. (November 1979) Trial two (4 leaves/treatment): Pénicillium 21. Clado sporium 4, Verticillium 26, Streptomvces 1, Aspergillus 3, Botrytis 2, A l t e m a r i a 4. (November 1979) List 1, continued.
Excised B. x hiemalis leaves from Maryland Cladosporium 140, Pénicillium 25, Verticillium 2, Streptomyces 6, Altemaria 2, bacteria and yeasts 50. 3 leaves, January 1980.
Kalanchoe leaves in double Petri plates
Pénicillium 11, Cladosporium 9, Altemaria 5, Epicoccum 2, Rhizopus 1,yeast 4. 3 leaves, December, 1979. 242
EfZect of fungicides on begonia phylloplane fun^i
Purpose: To cast the relative ability of commonly found phylloplane fungi to withstand fungicide treatments. An integrated control program would combine chemical and biolo gical control measures to maximize control of the fungus at minimum cost to the grower and the emvironment. Thus if efficacious antagonists to 0. begoniae are found fungicides which do not kill them would be preferred. Fungicide efficacy against Botrytis cinerea was also tested. A fungicide which simutaneously controls both 0. begoniae and B. cinerea could be useful in some situations. Benomyl was once such a fungicide.
Procedure: in-vitro trials : 0.5ml of fungicide suspensions of the rates noted in Tables 55 and 5 6 were applied to the surface of Difco cornmeal agar in Petri plates with a sterile glass
rod. After drying, an agar plug containing mycelium of one of the various fungi tested was placed in the center of each plate. There were five plates per treatment. The fungi were incubated for 7 or 9 days in the dark at room temperature (20-23 C ) .
The experiment was repeated once (Table 5 7) with some difference in fungi and fungicides tested between the two treatments.
in-vivo trials : Leaves (B. x hiemalis 'Schwabenland Red') in double Petri plates (Tables 57 and 58)or on plants on shaded greenhouse benches (Tables59 and 60), were dipped, sprayed or soildrenched with fungicides at rates given in Tables
58-60 • preventive trials fungicides were applied to 243 uninfected begonia plants or leaves. One week later the genera and number of phylloplane fungi were assayed for by appressing leaves to Difco c o mmeal agar plates. The plates were then inspected one week after leaf appression. In 'eradicative' trials naturally infected (0. begoniae race 2) greenhouse
B. X hiemalis 'Schwabenland Red' plants (Table 59) or Schwaben land Red leaves with 1 week old 0. begoniae race 2 infections in double Petri plates (Table 59) were used. These were dipped, sprayed or root drenched (see Tables 58-60 for specific method used with each treatment) with fungicides at rates given in
Tables 58-60. One week after fungicide application leaves were appressed to Diffco com m e a l agar plates. A week later the agar plates were examined for genera and number of phylloplane fungi. Fungicides are detailed further on pages
Results and Discussion: In-vitro trials revealed that the ergosterol inhibitors (fenarimol, triforine, triademefon) all had little effect on the common phylloplane fungi, Cladoporium sp., Pénicillium s p . Verticillium sp., Botrytis cinerea and
A. quisqualis (Table 55 and 56). Of the possible combinations of these fungicides and fungi only fenarimol on Verticillium sp. was inhibitory (Table 56). Fenarimol also had bacteriocidal proper ties in-vitro and exhibited activity against Streptomyces sp., often
found associated with Verticillium on begonia leaves in double
Petri plates. Dinocap was inhibitory against Cladosporium (fig. 32),
Pénicillium and Verticillium. Benomyl had the greatest amount of inhibitory affects against leaf surface dwelling fungi. It 244
Table 56; Radial growth of colonies of begonia phylloplane
fungi in centimeters after 9 days on c ommeal
agar amended with fungicides.
Fungicide Rate^ A. quis- Péni Verti- (per lOOgal) sporium cillium cillium^ Control 2.3 cm 2.2 2.5 1.0 Dinocap 20WD 1.2 2.0 0.6 0.9 Benomyl 50WP 0.0 0.0 1.6 0.0 Triforine 20%EC 8oz 2.3 2.2 2.5 1.1 Triademefon 25WP 1.5oz 2.3 2.3 2.5 1.0 Fenarimol 12.5%EC 40ppm 2.3 2.3 2.5 0.6 y) Some Streptomvces sp. mixed in with Verticillium sp. z) 0.5 ml of these solutions added to 15 ml of agar by streaking over surface with sterile glass rod.
Table 57: Radial growth of begonia phylloplane fungi in centimeters
after 7 days on commeal agar amended with fungicides.
Fungicide Rate^ Clado Botrytis Péni (per 100 gal) sporium cillium sp Control 1.2 cm .4.0+ 0.7 Dinocap 20WD 0.5 4.0+ 0.5 Benomyl 50WP 8oz 0.0 4.0+ 0.6 Triforine 20%EC 12oz 1.2 4.0+ 0.7 Fenarimol 12.5%EC 40 ppm 1.1 4.0+ 0.7
z) 0.5 ml of these solutions added to 15 ml of agar by streaking fungicide over surface with sterile glass rod. 245
Table 58 : Effects of preventative spray of dinocap (Karathane 20WD
6 oz/100 gal) on numbers and genera of begonia phyllo
plane fungi in the greenhouse and double Petri plates.
Number of fungi per leaf Fungal genus^ Greenhouse Double Petri Plate Control Dinocap Cladosporium 15 4 13 Pénicillium 4 3 21 Epicoccum 4 3 1 Botrytis 4 17 1 Aspergillus 19 0 0 Altem a r i a 11 2 0 Rhizopus 8 0 0 Verticillium 0 0 14 Choanephora 0 0 0 Streptomyces 0 0 52 bacteria y y unknown ^ 4 0 x) of course Streptomyces and bacterial are not fungi, y) very numerous z) a pink apothecial fungus with inoperculate asci. Figure 32. Inhibition of Cladosporium sp. by various
fungicides on cornmeal agar. 247
strongly inhibited Cladosporium. A. quisqualis and Verticillium.
Benomyl slightly inhibited Pénicillium sp. too. No fungicide
inhibited B. cinerea at a ll, including benomyl which is perhaps the most widely used fungicide for control of diseases caused by this pathogen.
In-vivo trials showed no significant differences between controls and fungicide treated leaves in genera and number of phylloplane
fungi present with the possible exception of triademefon (Tables
59-60). Even benomyl showed little effect on these fungi. Apparently
the fungicides are more diluted or break down faster on leaves than
in the in-vitro tests. 0 . begoniae then is more sensitive to these
fungicides than are the common phylloplane fungi (e.g. Cladosporium sp., Pénicillium or Botrytis). This is to be expected with the
systemic fungicides since internal fungitoxins would be inaccessible
to leaf surface inhabitants. The delicate balance between 0. begoniae and the host would probably be more susceptible to disruption by
fungicides too. Table 59; Number and genera of phylloplane fungi isolated
from 0 . begoniae inoculated Begonia x hiemalis
'Schwabenland Red' leaves in double Petri plates
after leaves were treated with eradicative and
preventative sprays of benomyl (Benlate 50WP, 8 oz/
100 gal).
Numbers of fungi per 4 leaves Control Preventive^ EradicativeY Trial 1 Trial 2 Cladosporium 17 10 4 1 Pénicillium 46 5 9 13 Epicoccum 2 2 1 0 Streptomyces 50 0 3 6 Botrytis 13 19 2 0 bacteria and yeasts
x) Fungicide applied one week before appressing leaf to cornmeal agar plate. y) Fungicide applied 1 week after inoculation of leaf. Leaves appressed to cornmeal agar one week after fungicide applica- z) very numerous, especially in benomyl treatments. Table 60: Effect of preventative fungicide treatments on number and genera of begonia
phylloplane fungi on whole plants in the greenhouse.
Numbers of fungi/ 3 leaves Genera Control DinocapZ Benomyl^ Fenarimolz Triforine z Triademefonzu
Cladosporium 11 6 10 8 16 4 Pénicillium 3 8 2 7 7 3 Alt e m a r i a 8 4 5 4 3 0 Aspergillus 14 19 11 21 37 0 Botrytis 3 1 0 0 8 0 Epicoccum 3 2 1 4 1 0 Rhizopus 0 0 Stemophyllium 0 0 2 0 0 0 Trichoderma 0 0 1 0 0 1 0 0 0 0 0 bacteria and yeasts v 18 17 12 12 3 Verticillium 0 0 0 1 0 0 unknown^ 3 0 7 0 10 0 u) This treatment on different greenhouse bench than other treatments. v) Very numerous w) grew very quickly and covered plates x) some present y) pink apothecial fungus with inopercula z) rates used: dinocap (Karathane 20 WD) 6 oz/lOOgal; benomyl (Benlate 50WP) 8oz/100gal; fenari- mol (EL 222, 12 .57.EC) 40ppm; triforine (20%EC) 12oz/100gal; triademefon (Bayleton) applied as a drench to soil at 2.5 g a.i./lOO 1iters-100 ml of solution per 6 inch 250 Effect of temperature on growth o£ phylloplane fungi on cornmeal
Purpose: To see how the relative amount of. growth of the most common phylloplane fungi changes with temperature. If these
fungi prove to be antagonistic to 0 . begoniae, then this infor mation could be valuable in determining at what temperature the attempts to control 0 . begoniae with these fungi should be made.
Procedure: An agar plug from stock cultures of Pénicillium sp., Cladosporium sp., or Verticillium sp. was placed on Difco cornmeal agar in Petri plates. The plates were placed in the dark (trial 1) or 12h light per day (trial 2), at 15,21,24 or 23
C and measured from time to time (Table 61).
Results and Discussion: All fungi tested had growth optima near 20-25 C and were capable of growth at all temperatures tested
(Table 61). The diameter of the colonies were more than doubled after 2 weeks at 21 C when compared with 15 or 28 C in Trial 1.
0 . begoniae has a similar temperature distribution but is more adversely affected by 24 and 28 C than are these fungi. In trial
2 Cladosporium sp. and Pénicillium sp. grew somewhat faster at 24
than 21 C. It appears likely that higher temperatures would favor
these fungi over 0. begoniae. Indeed, the Pénicillium sp. was often
found growing out of dying powdery mildew lesions at 24 and 28 Cl Table 61: Effect of temperature on radial growth (mm) of some phylloplane fungi on cornmeal
14 days 4 days______7 days______Fungi 15 C 21 C 28 C 15 C 21 C 24 C 28 C 15 C 21 C 24 C 28 C
Cladosporium sp. 10^ 31 6 1 3 4 2.5 1 7 7.5 6.5 Pénicillium sp. 3 16 7 2 4 5 5 3 7 7.5 6.5 Verticillium sp. 4 8 6 z) Each number represents the mean of 5 plates. Growth was fairly uniform from plate to plate and most standard deviations were much less than 1 . 252 Efficacy of some phylloplane fungi against 0. begoniae.
Purpose: To test the efficacy of Verticillium sp., Péni
cillium sp., Ampelomyces quisqualis and Cladosporium sp., phyllo
plane fungi found on B. x hiemalis. as biological control agents
of Oidium begoniae. Such control could be used as long-term
treatment agents,since unlike chemicals,they do not break down.
Furthermore, such organisms could have the potential to grow into
buds and protected places and inhibit 0 . begoniae therein.
Such organisms also are less potentially harmful to the environ-
Procedure: Pénicillium sp., A. quisqualis, Cladosporium sp.
or a mixture of Verticillium sp. and Streptomyces sp. were obtained
from begonia leaves,with the exception of A. quisqualis.which was
from sunflower, and was found for me by C.W. Ellett. These fungi were grown for 2 weeks at 21 C on c o m m e a l agar. Three plates of
each fungus were scraped of m y celia and conidia into 75 ml of
distilled water. Five ml aliquots of these suspensions were sprayed
onto mature, excised B. x hiemalis 'Schwabenland Red' leaves.
There were 8 leaves in each treatment. These leaves were incubated
for 3 days in double Petri plates at 21 C and 12h light per day.
Then the leaves were inoculated with 0. begoniae race 2 by blowing
air over infected stock leaves with 7 day old infections of the
pathogen. After 7 days the number of 0. begoniae colonies on
each leaf was counted. Unfortunately, due to variability and
small sample size this experiment can only be considered a pre - liminary one.
Results and discussion: No significant differences in
0 . begoniae colony numbers were observed due to the large variability in the data (Table 62). However, Pénicillium sp. and Cladosporium look promising and A. quisqualis may be somewhat efficacious. This experiment is currently being repeated. 254
Table 62 : Efficacy of antagonists as sprays against number of
Oidium begoniae colonies.^
Treatment Number of co]
Control 202 Verticillium-Streptomyces 215 Pénicillium sp. 134 Ampelomyces quisqualis 171 Cladosporium sp. 125
y) number of visible 0 . begoniae colonies 1 week after inoculation, z) 3 plates of each potential antagonists was scraped into 75 ml of distilled water and 5ml of the resulting solution was sprayed on each treated leaf. Three days later the leaves were inocu lated with 0 . begoniae. Notes to Chapter six.
1. Emmons, C.W. 1930. Clcinnobolus Cesatli. a study in host- parasite relationships. Torrey Bot. Club Bui. 57: 421-442.
2. Hoch, H.L, and R. Provvidenti 1979. Mycoparasitic relationships: Cytology of the Sphaerotheca fuliginea-Tilletiopsis sp. inter action. Phytopathology 69: 359-362.
3. Jarvis, W.R. and K. Slingsby 1977. The control of powdery mildew of greenhouse cucumber by water sprays and Ampelo myces quisqualis. Plant Dis. Reprtr. 61: 728-730.
4. Lacey, J. 1979. Serial dispersal and development of microbial communities. In Microbial Ecology: A Conceptual Approach, J.M. Lynch and N.J. Poole eds. Blackwell Scientific Pubs., Oxford, London, Edinburgh and Melbourne, pps 140-170.
5. Last, F.T. and R.C. Warren 1972. Non-parasitic microbes colonizing green leaves: their form and function. Endeavor 31: 143-150.
6 . Leben, Curt 1974. Survival of plant pathogenic bacteria. Ohio Agr. Res. and Development Center Circular 100.
7. Rogers, M.N. 1959. Some effects of moisture and host plant susceptibility on the development of powdery mildew of roses caused by Sphaerotheca pannosa var. rosae. Cornell Ü. Agr. Expt. Sta. Memoir -r 363.
8 . Salmon, E.S. 1904. Mycological notes. Jour. Bot. 42: 182-186.
9. Sztejnberg, A. 1979. Biological control of powdery mildews by Ampelomyces quisqualis. Abstracts of Papers, IX International Congress of Plant Protection, # 159.
10. Wolf, F.T. and F .A. Wolf 1939. The snail Polyphaga thyroidus as a mycophagist. Torrey Bot. Club Bui. 6 6 : 1-5.
11. Yarwood, C.E. 1932. Ampelomyces quisqualis on clover mildew. Phytopathology 22: 31.
_1943. Association of thrips with powdery mildews. Mycologia 35: 189-191.
13. 1957. Powdery mildews. Bot. Review 23: 235-301. 14. Yarrzood, C.E. 1978. History and taxonomy of powdery mildews. In The Powdery Mildews, D.M. Spencer e d ., Academic Press, London, New York. 257 7. EPIDEMIOLOGICAL MODELING AND INTEGRATIVE CONTROL OF OIDIUM BEGONIAE ON BEGONIA
Since VanderPlank's Plant Diseases: Epidemiology and Control
in 1963 ( 9 ) , quantitative models predicting rates of disease
progress or amounts of disease over time have come into vogue.
Such models have been used to show effects of sanitation ( 1 ,
9 ), plant spacing ( 8 ), fungicide effects (3 ,9 ), effects
of host resistance ( 3 , 10 ) , and effects of temperature and
moisture ( 7 )• The basic formula that predicts the amount
of diseased tissue is x^= x^e^*:, where Xq is the original number
of colonies or infected plants, , e is the base of the natural
log system, r is the rate of increase and t is time elapsed.
This formula suggests that diseases can be controlled by measures
that reduce the original inoculum (x q ), slow the rate of disease
increase (r), or shorten the time of disease exposure (t)(l).
Models based on this system can be helpful in contrasting and
comparing the effects and efficacy of the various control measures. The models can suggest how most efficiently the control measures can be combined too. The models will suggest new
experiments to be carried out. Finally, a model of powdery mildew
of begonia is a convenient way of integrating the previous chapters
of this dissertation.
The basic formulas used in this model are VanderPlank's
compound interest formulas:( 9 )
xt=* Xo(l-Xo)e^*^ r » l/t(logg(xt/l-Xt)-loge(xo/l-Xo))
There are some modifications. The first modification is caused of a spore and sporulation of the colony derived from the spore.
At 21 C this period is 4 days with 0. begoniae. During this time Xj.= Xq and r=0. Next, at 21 C comes a period of simple interest in which only the one colony is sporulating. This occurs until t= 2p, or from 4-8 days after initial inoculation.
During this period r = l/t(logg(l/l-Xc)-loge(l/l-XQ)). Note that many of the readings in double Petri plates were taken seven days after inoculation and would be in this time period.
Spread is minimal in double Petri plates with lids on. Thus a more significant indication of r is amount of sporulation, not number of colonies or percent leaf surface covered. After
8 days the disease enters the continuous compound interest phase in the greenhouse.
Comparison of r values
A comparison of r values of Oidium begoniae on begonias by cultivar is difficult to make from the data at hand. However, approximations can be made. Sail (7 ), modeling grape powdery mildew, determined the effects of temperature on rate by extrapo lating from data showing temperature effects on germination, penetration and hyphal growth. Although such parameters are not proportional to infection rates in begonia and are probably not so in grape. Sail obtained a model that was adequate for pre dicting infection by UncinUla necator. A more useful parameter is number of infectious conidia produced, as these will directly determine the number of daughter colonies. As in Sail's studies this information is not at hand for 0 . begoniae in evaluations of r by culcivar. Evaluations of r were therefore evaluated by
comparing hyphal lengths of the cultivars at 0,2 and 7 days and
by noting if any conidia were present at all. Amount of
sporulation then was estimated from comparing Tables 3 and
28 so that r = hyphal length^'^^/1.0547 x 10^®. If no conidia were produced r= 0, even if hyphal growth occurred. Results
are given in Table 63 and figure 33 , where they can be compared with effects of temperature and fungicides. The 'vertical'
race-specific resistance prevents penetration of race 1 into
Aphrodite cultivars and therefore causes both Xq and r to
equal 0. The non-race specific resistance also causes a drop
in germination rates by about 50% which means x^ as well as r
is effected by this type of resistance. In cultivars, such as
B. X richmondensis at 15 C, in which a small amount of sporulation
occurs and then death follows soon thereafter, t is shortened
The effects of temperature, moisture, drought stress and nutri
tion can similarily be calculated (Table 63 , fig.33) • For
temperature many more parameters, including number of conidia
produced, can be included in the determination of r. In double
Petri plates conidia form in chains and total, not daily conidial
production per lesion can be found. In double Petri plates,
host growth is minimal and need not be calculated. There are two ways of calculating the change in r caused by temperature.
The first is a modification of the formula Sail used with Uncinula
necator ( 7). This formula would be, for Oidium begoniae: Table 63: Comparative r values of 0. begoniae on different
cultivars or on treated B. x hiemalis leaves.
12 C B. X hiemalis .0001 15 C B. X hiemalis .04 18 C B. X hiemalis .10 21 C B. X hiemalis .28 25 C B. X hiemalis .002 28 C B. X hiemalis 0 15 C B. X richmondensis .0038 18 C B. semperflorens 'charm' .13 21 C B. rex-culturum , susceptible tissue .27 15 C B. thurstonii .0049 21 C B. X hiemalis, drought stressed .63(probably high) 21 C B. X hiemalis, dinocap preventative 0 21 C B. X hiemalis, triforine eradicative 0 "Ë" l>ienialis 21 C, drought stress
litemal is. 21 C
■B.semperflorens.18C
Ti.x liiemalis 18 C
“ B. X liienial is. 15 C . X riclimondensis
Fig. 33. Comparison of r values (l/t (log^.(xt/l-xt) - log^Cxo/l-Xo))) of various treatments of Oidium begoniae. (j increase) rh = .28 (.000257) (28.5-1^)0.73 where .28 = the rate of disease progress at 21 C on B . x hiemalis
under optimum conditions in a growth chamber and is the mean
hourly temperature. The temperature values can be combined to
give a daily reading too;
24 r = C.fh /24 (7) h=l
These values, like Sail's,are not totally accurate as they are
calculated from germination, infection,and growth rates. A better determinant of r is number of conidia produced x proportion
of conidia germinating per unit area per unit time. A comparison
of the curves of these parameters is given in figure 34 . Sail's
calculations are derived from work done by Delp, and Sail indi
cates germination, infection,and growth all can occur to some
extent up to 35 C. Delp (2) , on the other hand, indicated that
the infection maxima was 31 C and that no sporulation occurred
above 30 C. If Delp's findings are correct, r would clearly be 0 in
cases of prolonged 30-35 C and Sail's model will give erroneous
answers at these temperatures. Sail's model also assumes that
the inhibitory (28.5-1^) and the stimulatory (.000257
functions of temperature are discrete and continuous over the .
entire temperature range. This fact will be discussed in the
next chapter. It is also interesting to me that her model will
give negative values to temperatures above 35 C with U. necator or
above 28.5 C with 0_. begoniae . This will be discussed later in 1.0
75
50
g 25 g*
0 5 10 15 20 25 30 Degrees Centigrade Fig.34. Comparison of effects of temperature on germination, hyphal growth and sporulation.
— germination (48h); hvnlial length(7 days); sporulation(9 days) . 264 this chapter, when eradication is considered.
A second method of evaluating temperature effects is to consider the three parts of the temperature-spore production curve separately. From 7-20.5 C the curve is rising. From
20.5-28.5 it is descending. From 28.5 and up eradication occurs and r=0. This model would take into account temperature thresholds that Sail's model would not, but does not take into account physiological interactions between the causes of temperature effects. The slopes of r of the second and third parts of the curve are different in 0 . begoniae and this is not taken into account in Sail's model either. For 0. begoniae the following equations are proposed to predict r values:
From 0 to 7 C r-0. From 7 to 20.5 C r- Th^‘^/1.66 x 10^ From 20.5 to 28.5 C r- (28.5 - /29,300 From 28 C and up r- 0, however there are effects on Xg that lower subsequent r values when the temperature is again lowered
(see next section).
Eradication of infection with temperature and fungicides
The proportion of survivors (s) of an eradicative treatment with known efficacy f will be: s- XQ(l-f), (reference 4 ), where 1=100% survival and x^ is number of treated colonies. If means of escaping a treatment exist, such as occurs when coverage of fungicide is incomplete or survival in protected places occurs then the formula is amended to:
s=(individuals not contacted by biocide + individuals surviving contact with biocide) s=Exo + Xo(l-E)(1-f) =Xo(.l+Ef-f) where E- the proportion of the population that is not contacted by the treatment.
Combining heat and fungicide treatments or using two fungi cides in combination will eradicate as the product of the 2 s values if the effects of the treatment are caused by different modes of action and if the treatments do not interact with each other. The f values for some treatments are listed in Table 62.
Heat treatments are more effective over a period of time and the above f values for heat treatments were determined by me as follows: f= hk/ 1-hk where f = proportion of colonies killed and hk = haustoria observed killed in time t and 1 - 1007, of haustoria counted. When hyphae no longer could be induced to grow when replaced in the 21 C incubator hk was considered to be
1. Adding Th s together, as done to determine r in the previous section would inaccurately describe eradication as this process gives a linear, not a logarhythmic curve of survivors over time.
A more accurate approximation of eradication over time by heat is
(1-s) = (1.34(Ttn ■ 27.5))^ /825, where t-1ime in days that heat was applied, T^ - mean temperature during that time and all T„i values are greater than 27.5 C.
Eradicative temperatures or fungicides are accompanied by infection rates close to r-»0 ('Table 64). The eradicative treatments therefore induce discontinuous periods of infection. The theory of discontinuous infection, complicated by variable r rates is Table 64: Comparative f values for different eradicative
treatments, (f- proportion of eradicated colonies).
Treatment f
Dlnocap 20WD, 5 oz/lOOgal. 0.999 Triforine 20%EC, 12 oz/lOOgal. 0.99 28 C, 4 days, double Petrl plate 0.28 28 C, 7 days, double Petrl plate 0.63 28 C, 14 days, double Petrl plate 0.99 28 C, 14 days, whole plants, young leaves 0.78 28 C, 28 days, whole plants, young leaves 0.99 32 C, 2 days, double Petrl plates 0.10 32 C, 4 days, double Petrl plates 0.99 32 C, 3 days, whole plants 0.92 32 C, 6 days, whole plants 0.999 267 beyond the scope of chis work and was not covered by Van der Plank either (9). The real effect of eradication on infection rate is by lowering Xq for subsequent infection periods as seen in the formula r- l/t(logg(X[/l-xt) - logg(xo/l-Xo))• Thus, the reduc tion of Xq is calculated as Xq - (1-(1-s))( X q ), where " the number of colonies for the next favorable period of infection,
1-s is the proportion of colonies surviving as calculated on the preceding page and XQi-S che number of colonies at the start of the heat treatments. This formula can be integrated into the formulas used to determine x^. and r at 0-28.5 C as in the following example:
Example : What proportion of leaves would be diseased given 1% infected tissue was treated with 7 days of 21C, 5 days of 30C and 5 more days of 21C? Assuming that the compound interest stage has already begun, x^-XQe^c - 2.718-28x7 - 7 . % . Next Xq-^ (1-(1.34(T- 27.5))"/825 - 7.1(1-(1.34(2.5))5 /825- 3.48%. Finally, X[= x^erc =3.48 X 2.718-28x0 = 23.2%.
Of course, effects of fungicides could be calculated in the same way. This is a different method than is used for calculating protectant fungicides by van der Plank (9), but I believe this method proposed by Kable and Jeffrey (4) could be used for protectant fungicides to the extent that they lower x^. Most protectant fungicides will also lower r by reducing growth and sporulation of some surviving conidia. This model has been only partly validated.
The value of r= .28 at 21C is likely to be low.
Integrated Control of Oidium begoniae.
The following control measures are suggested by the research and the model: 1. Measures chat lower x q (These treatments delay or stop
an epidemic from occurring).
a. Disease free stock plants. This is an essential. Segre gate newly arrived stock plants from disease-free stock plants until new arrivals are assurred disease-free. Stock plants should be kept in several small rooms not a large one so that infection of one does not easily lead to infection of all. Do not grow different susceptible species next to one another in commercial greenhouses if possible as infection from one may spread over the entire range of species.
b. A protective spray program is a necessity. Today a biweekly spray of -’inocap seems to be the most effective registered treatment. Bayleton soil drenches look promising.
c. Free water reduces germination. Keep plants well watered. Water just before spore release (late morning watering in most cases).
d. Temperatures of 28.5 C lowers germination and penetration. Changing the onset of light (i.e. turn on lights before sunrise) may make germination occur in the hottest part of the day. Germination would be inhibited and the heat would be free. More shading of benches would be required.
e. Temperatures of 30 C or more stops penetration. The comments in d apply here, but the lights must be turned on earlier.
f. Temperatures of 32 C or more absolutely stops all stages of the fungus. Comments in d apply here. Even more shading needed.
g. Low relative humidity can help dessicate spores before germinating. Don't water until just before period of spore release (about noon).
h. High boric acid delays epidemics, but is phytotoxic.
i. 'Vertical', race-specific resistance lowers Xq 100% on Aphrodite Rieger begonias.
j. Horizontal, race non-specific resistance lowers X q about 50% on the most resistant species at 21 C. This resistance will be difficult to incorporate into B. x hiemalis.
2. Measures that lower r (slow rate of spread of infection). Many of the things that lower x^ lower r also. This, in fact, has caused the terms horizontal and vertical resistance to be somewhat controversial (5,6). Xg and r are not indepen dent of one another as they are tied together by infection frequency ( 6 ). The amount of spores in the air (xq) at loca tion A are largely dependent upon the r of conidial production at location B . Measures that work by lowering r are suspect, because no powdery mildew can be tolerated in susceptible begonia. There are some exceptions, though, such as the 'horizontal' resistance.
a. Temperatures other than the optimum (20-21 C) lowers r. Raising the temperature from 21 to 25 C can lower r 100 fold.
b. Temperatures of less than 7 or greater than 28 C cause inhibition of sporulation and cause r -0 after a period of several days.
c. Low relative humidities may slightly increase sporulation and hence raise r. Low winds may stop spread of disease.
d. Pyrimidine fungicides, such as fenarimol, triademefon and triforine greatly reduce r. These fungicides reduce Xg too.
e. "Horizontal" resistance decreases r. This resistance is greater at higher temperatures.
f. Eradicative treatments of fungicides such as dinocap, also reduce r, by inhibiting growth and sporulation, and by inhibiting formation of new colonies.
g. Greater spacing between plants will reduce r.
h. Removing infected plants from the greenhouse will reduce r.
i. Early in an epidemic removal of the few, infected leaves seen will reduce r. However, it is better to remove the whole
j. Addition of Viterra-2 hydrogel and keeping plants watered slows an epidemic, probably by lowering germination-
k. More than one source of inoculum will increase Xg and conse quently increase r.
3. Measures that reduce t (time of exposure to the pathogen).
Obviously, things that reduce Xg will also reduce t if Xg
can be made to equal 0. If r^O, an epidemic is stopped and 270 t will be lowered. Cultural practices that reduce the time that it takes to grow a crop to maturity will also lower t, especially where several types of susceptible begonias are grown in the course of the year.
All of the above control measures can be combined. In cases where interactions are few or non-existent the reduction in r value will be the product of the two r values. However, many, if not the majority of the cases above, are interrelated in some way and the effect on the r value will usually be less but may occasion ally be more (synergy) than the products of the r values.
The future: A new model proposed for calculating infection rates from growth chamber studies.
Some difficulties arise when growth chamber studies are used
to calculate or predict r values. The transfer of knowledge gained in the laboratory to the field would be facilitated if a model could be devised that would allow one to use data gathered
from experiments on germination rates, hyphal growth, sporulation
in vitro,etc to predict occurrence of epidemics in the field
in an exact way.
Examination of the data from these studies and studies done by others on powdery mildews (e.g. see reference 8 , page
show that daily hyphal growth, number of haustoria per length
of hyphae, number of conidiophores/day/hyphal length all increase
in a linear manner. Although this increase in itself is suffi
cient to cause an exponential increase in number of colonies. single colony. This increase can be predicted though by finding the rate of branching of hyphae per day (y). Y is very simply calculated; for instance ^if one branch gives rise to 3 on the average each day,then y- 3. The daily spore production of a single colony is then predicted by wz(yif conidia are not produced in chains where w^number of germtubes or branches formed in the first 24h after germination; z~ number of conidiophores per day per hyphal length, t- time after germination and p is the latent period between germination and spore production, z is only measured after sporulation has begun. The total number of conidia produced in a non-chain former (in this case I mean a conidiophore that produces only one spore in its life time) and the daily spore production of a chain forming colony is predicted by the equation x^-wz ^ y^ . Finally, the total spore production of a colony which produces more than one conidium per conidiophore (e.g. Oidium begoniae) is predicted by the equation: ^ Xg " (2_ + y^“^+ y ^ " ’ .,+ y""^)wz 0
To predict how many spores are produced per colony,then,
the following must be determined: number of branches at t -1 which is w in the above equations; number of spores per branch per day after time p (z); and number of daily branchings formed
from previously formed branches (y). Naturally this assumes that environment is constant. This deterministic model also assumes
that the host is equally susceptible as growth procédés. 272 Application of this approach from a single colony to growth of entire populations involves more complicated equations. First, the percentage or proportion of conidia which land on susceptible tissue, which are capable of germination, penetration and growth to formation of sporulating colonies must be estimated from growth chamber experiments. Effects of wind and plant density should be accounted for. Xq, the original amount of inoculum present can be determined. The effects of several colonies growing simultaneously and out of synchrony must be predicted. Given an origin of all colonies from a single inoculation with 1 or several conidia the equations that predict the amount of conidia and colonies formed are:
1) Daily spore production of non-catenulate fungus:
%c - Xgvwz (1) ( + (1+2+3+. . .t-3p)
(y'=‘3P)) + (l+(l+2) + (l+2+3) + (l+2+3+4)+. . . (1+2+3. . .+
t-4p))(y"'^P); + (l+(l+(l+2))+(l+(l+2) + (l+2+3))+(l+. . .
(1+. .. t-5p))) (y*'"^P) etc. Disregard negative n'jmbers.
2) Total spore production of non-catenulate fungus and Daily spore production of catenulate spore producer;
t-p t-2p+l t-2p «c "= ^ y") + Z (i)( ^ y") + 0 10 and so on as above
3) Conidiophores produce spores catenulately, total spore production; As in 1 but substitute the equation for total production for one colony given on the preceding page into the equftion, e.g. t-p
xt -= X q V w z C ^ y" + y " " ^ + y""^+. . .+y'^"^ ) +
(t-2p+l) c_2p 2(1) ( 2 yn+y"-V ecc. 1 0 273 The infection rate r can be determined from these formulas
too by determining at time 1 and then later determining
%t at time 2 and then r - l/t2 -ti(logg Xt2 - logg X;.^) . Curiously use of these equations shows avery good fit with van der Plank's hypothesis of logarhythmic increase for two generations but
then begins to give higher r values than van der Plank's
system in later generations. Since this model, unlike Van der
Plank's,is biologically based it is not necessarily less accurate.
Validation of this model continues. Effects of disease proportion on rate of increase are yet to be calculated.
Preliminary indications are that the y value of 0. begoniae may be more complicated than indicated herein. Hyphal growth may not be strictly linear and the rate of growth of all hyphae may not be the same. Notes to Chapter seven.
1. Berger, R.D. 1977. Application of epidemiological prin ciples to achieve plant disease control. Ann. Rev. Phyto- pathol. 15; 165-183.
2. Delp, C.J. 1954. Effect of temperature and humidity on the grape powdery mildew fungus. Phytopathology 44:
3. Fry, W.E. 1978. Quantification of general resistance of potato cultivars and fungicide effects for integrated control of potato late blight. Phytopathology 68: 1650- 1655.
4. Kable, P.P. and H. Jeffrey 1980. Selection for tolerance in organisms exposed to sprays of biocide mixtures: a theoretical model. Phytopathology 70: 8-12.
5. Nelson, R.R. 1978. Genetics of horizontal resistance to plant disease. Ann. Rev. of Phytopathol. 16: 359-378.
6. "parlevliet, J.E. 1979. Components of resistance that reduce "the rate of epidemic development. Ann. Rev. of PhytopthoL. 17: 203-222.
7. Sail, Mary Ann 1980. Epidemiology of grape powdery mildew: a model. Phytopathology 70: 338-342.
8. Strandberg, J.O. and J.M. White 1978. Cercospora apii damage of celery-effects of plant spacing and growth on raised beds. Phytopathology 68: 1650-1655.
9. Van der Plank, J.E. 1963. Plant Disease: Epidemics and Control. Academic Press, New York and London.
10. Van der Plank, J.E. 1968. Disease resistance in plants. Academic Press, New York and London. 275
APPENDIX: A PHYSIOLOGICAL MODEL OF CONTROL OF 0. BEGONIAE
In this brief chapter I am going to speculate on the mode
of action of host resistance, resistance in B. x hiemalis at
temperatures above 21C and resistance induced by pyrimidine and
triazole fungicides. The morphological features of control by
these various means are similar (see pages 207-208). Such a model
can be useful in evaluating control measures and suggesting
useful experiments.
Evidence for this model was gleaned from the literature.
Permeability changes have been suggested as being the basis of
compatability and incompatibility in a recent review of powdery mildews (2). The importance of an intact matrix membrane to the
compatibility of the host and pathogen was alluded to earlier
(pages 110-111). The resemblance of resistant reactions to cellular
senescence has been alluded to in other studies (2,5). The chief
difference between senescence and normal response of the host to wounding is that the latter is more local and more rapid (3).
An outline of the model follows. The steps of the resistant
response are given in the approximate order in which they have
been found to occur in the literature. Steps 1-5 are the compatible
reaction. Steps 6-11 are the incompatible reaction, which in some
cases follow steps 1-5 and are sometimes induced directly. The
key to induction of the resistant reaction is continued loss of
permeability after wound repair has been attempted. This reaction
will occur in all wounds too large or persitenc for the cells
repair mechanism to cope with. Before Infection-. Negative charge on inner surface of plasmalemma relative to outside. pH is relatively high inside the cell membrane.
Infection occurs (application of auxin, heat, light, wounds and addition of chemicals including HCl, CaCl2 and KCl can also induce steps 1-5 (11,12).
1. H"*" Ions enter cell via wound caused by pathogen. Interior of cell becomes more acid. ions enter cell.
2. binds with agents such as cytochrome and phytochrome which are bound to proteins in the plasmalemma. These proteins change in configuration decreasing membrane permeability. Unsaturated carbon atoms in membranes tend to become saturated. Saturated lipids in the membrane are relatively impermeable. Some enzymes are activated by the lowered pH.
3. Gibberellic acid is activated. Respiration increases (lower pH enhances as in the Mitchell chemiosmotic theory).
4. RNA synthesized(2). Stomates open (K+ high, abscissic acid: gibberellins low) (11).
5. Protein synthesis induced (11)- Callose secretion and transport increased (16). Cell elongation induced by gibberellins and auxins (11) - a.Membrane synthesis, repairs made, matrix membrane made, H pumped out at expense of ATP (13) .
b. Incompatible reaction begins when attempt to repair membrane does not succeed before the following events occur.
6. Gibberellic acid; Abscissic acid ratio declines. Senescence begins (11) .
7. Stomates close, K"*" and levels decline. Membrane permeability increases.(11) .
8. Cellular respiration, protein and RNA synthesis slowed by repression of genes by abscissic acid (11).
9. Release of contents of lysosomes and other packets containing anabolic enzymes, phenols or phytoalexins may occur,
10. The Shikimic pathway is favored over glycolysis. Phenols are 277
made(e.g. trans-cinnamic acid). Abscissic acid may be involved in inducing chis pathway (9). Phytoalexins and flavanoids are made from the phenolic compounds. Phenols reverse effects of auxins (9) . 11. Lignins are made, partly from phenolic compounds. Cell is dead and lignins form barrier to separate toxins from live cell.
If a chemical causes atoms to penetrate only at specific
sites then more specific (oligogenic) effects are encountered. It is possible that pyrimidine and triazole fungicides may act this way.
Pyrimidine fungicides act directly on fungi in-vitro, inhibiting ergosterol synthesis (1,4,14,15). Few, if any, tests have been made on effects of these chemicals in-vivo. It is suspected that the
relationship between fungicide, host and parasite is more complex in nature. The giberellic acid inhibiting function (1) of these
chemicals could cause a quicker switch from the compatible to incom
patible reaction when infection occurs. It is possible that the
fungicides cause some sterol inhibition in the host leading to
some increased permeability. Resistance to these fungicides would
probably then be less complete and rarer in nature than in-vitro,
since the effects of increased permeability in the host are wide
spread and the products of many genes as presented in this model.
Resistance after penetration occurs could also be a result of more rapid switching from the compatible to incompatible physiological
state. This could be caused by many different modifications of the model that cause faster rate of metabolism (e.g. more hydrogen bonding
sites, more enzymes, more lysosomes) or that cause imbalances
favoring the incompatible reaction (e.g. less auxin (17) or less
gibberellic acid relative to abscissic acid (17).). The key to
resistance being successful is that this reaction occurs before the fungus can form new haustoria or sporuLate.
The involvement of temperature with resistance of this type can come as no surprise. Increased permeability of membranes as tempera ture rises has been known for many years (12). This effect could be induced by changing saturated lipid components into more fluid and permeable unsaturated forms (10). Note the release of ions that would accompany this change.
Relevance of Model to the present study.
In begonias this model may explain several observations;
-Eradication could occur faster in excised leaves compared with
whole plants because of greater abscissic acid levels.
- Eradication occurs faster in older leaves compared with younger
leaves because of higher abscissic acid levels.
-High temperatures cause fluidity of membranes with resulting
increase in permeability (10). This could induce the production
of inhibitory compounds. High temperatures could also act on the
fungus' membrane causing leakage of compounds into the host that
increase rate of senescence. This possibility is not part of the
model, however lit cannot be discounted.
-Light should enhance gibberellic acid levels via the phytochrome
system. This could increase the amount of metabolic activity in
the host and bring about the resistant reaction more rapidly than
in the shade. This may be part of the inhibitory nature of full
sun on powdery mildews.
-Teleologically, the diurnal cycle of fungal activity would be
desirable to the fungus to avoid the high-light-high temperature 279 portion of the day. If the initial stage of metabolic activity is a necessary part of the resistance reaction and if phytochrome aids in this reaction via induction of gibberellic acid and decreased membrane permeability (11), then the diurnal cycle of fungal activity makes even more sense.
- Areas of leaves of B. rex-culturum high in anthocyanins and
presumably also high in phenolic precursors of anthocyanins
are more resistant than other areas of the leaf (see page 46).
Such compounds are often found together (6).
- Gibberellic acid and abscissic acid are made in the same pathway
in both the plasmalemma and chloroplast (7,11). This could
account for electron micrographs which picture chloroplasts near
haustoria in epidermal cells. These chloroplasts have been seen
'emitting' some electron dense substance (fig. 18). However,
resistance has not yet been shown to ever be transmitted via
chloroplast DNA (8).
- The involvement of plant hormones in the resistance reaction are
involved in many other events of importance to plants. High
abscissic acid 7 evels may also be anti-florigenic. Thus, the
high temperature and high fungicide rates which cause reduction
in flower size or number could be possibly causally related to
the resistance mechanism. This could be particularly discouraging
to breeders who wish to have horticulturally desirable and
resistant crops.
- High levels of abscissic acid are associated with dormancy in
green plants. 0. begoniae may also become dormant as a result 280 of interaction with products of the resistant reaction between
27-31C. Abscissic acid does not have any direct effect on germina
tion of 0. begoniae conidia at physiological concentrations
(unpublished obseirvation) . Gibberellic acid also had no effect
but auxins and cycloheximide were inhibitory at 10"^ to 10
to germination of conidia floated on drops of solutions containing
these compounds.
- Drought stress should lead to an increase in membrane permeability
with a loss in IT^ and K"*" and a rise in abscissic acid levels. That
drought stress leads to increased susceptibility is therefore a
paradox for this hypothetical model. Perhaps rate of infection
is increased but longevity of life of infections is decreased.
This seems to be the case but needs testing. Notes to Chapter 8.
1. Buchenauer, H. 1977. Mode of action of triademefon in Usti'.ago avenae. Pestic. Biochem. and Physiol. 7: 309-320.
2. Bushnell, W.R. and John Gay 1978. Accumulation of solutes in relation to the structure and function of haustoria in powdery mildews. In The Powdery Mildews, D.M. Spencer, ed., Academic Press, London, New York, pages 183-235.
3. Day, P.R. 1974. Genetics of Host-Parasite Interaction. Freeman and Co., San Francisco.
4. DeWaard, M.A. and J.G.M. van Nistelrooy 1979. Mechanism of resistance to fenarimol in Aspergillus nidulans. Pestic. Biochem. and Physiol. 10: 219-229.
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