- Information Bulletin 201 R. Kenneth Horst is a professor in the Department of Plant Pathology, New York State College of Agriculture and Life Sciences, at Cornell University, Ithaca, NY 14853. Paul E. Nelson is a professor of plant pathology. Pennsylvania State Univer- sity, University Park, PA 16802, and an adjunct professor in the Department of Plant Pathology, Cornell University.

Cover drawing by Robina Maclntyre. Diseases of Geraniums'

Contents

Introduction 2

Pesticide Legislation 2

Bacterial Diseases 3 Bacterial Blight Bacterial Fasciation Southern Bacterial Wilt Bacterial Leaf Spot References

Fungus Diseases 9 Botrytis Blight Rust Alternaria Leaf Spot Blackleg Cutting Rots Thielaviopsis Root Rot Verticillium Wilt References

Virus Diseases 17 Tobacco Ringspot Tomato Ringspot Pelargonium Leaf Curl Pelargonium Ring Pattern Pelargonium Line Pattern Pelargonium Flower Break Pelargonium Mosaic Yellow Net Vein Other Viruses General Control Measures References

Nonparasitic Diseases 24 Edema References

Fungicides

Use of Soil Fungicides 26 References

Soil Treatment Introduction

Geraniums are one of the most versatile and widely used flowering plants in the floriculture industry. They are produced as pot plants and bedding plants by the commercial florist. Geranium plants may be used as spring pot plants, in window boxes, in border plantings, and in mass exhibits in outdoor landscape designs. They may be vegetatively propagated from cuttings or apical shoot-tip culture or grown from seed. Extensive use of culture-indexed and virus-indexed cuttings, the use of plants produced from apical shoot- tip culture, and the use of seed-propagated cultivars have reduced the number of disease problems for both the commercial producer and the consumer. According to the United States Department of Agriculture Crop Reporting Board publication Floriculture Crops, Production Area and Sales, 1980 and 1981, Intentions for 1982,geraniums grown for potted plants in the United States in 1981 had a total wholesale value of approximately $49,518,000, In the northeastern United States the wholesale value of this crop was $1 5,479,000. and New York State produced a crop valued at $5,927,000. Geraniums are also grown from seed in bedding plant production. Although the percentage of geraniums used as bedding plants is not known, it is a significant number in an industry with a wholesale value of $1 50,388,000 in the United States in 1981. From this information one can see that geraniums are a commercial crop of considerable value, and disease problems can result in significant economic losses. The diseases discussed in this bulletin are primarily those that affect the florist, garden, or zonal geranium, Pelargonium x hortorum. Some of these pathogens may also affect other species of Pelargonium such as the Martha Washington geranium, Pelargonium domesticum, and the ivy geranium, Pelargonium peltatum, as well as the geranium cultivars propagated from seed. Such cases are noted in the text when their occurrence can be documented.

Pesticide Legislation

In accordance with recent New York State legislation, pesticides that are highly toxic and those that are persistent and accumulative are placed on a restricted-use list and may be purchased and used only by certified applicators or under the direct supervision of certified applicators. Restricted-use pesticides recommended in this publication are identified throughout by a dagger (t). The Federal Environmental Pesticide Control Act (FEPCA) was signed into law October 21, 1972. Under this law, all pesticides are to be classified into restricted-use and general- use categories. This law also prescribes that all users of restricted-use pesticides be certified. In New York State, all commercial applicators of pesticides must be certified, even if they use only general-use (nonrestricted) pesticides. Also, all private applicators who use restricted-use pesticides must be certified. In most situations, private certificajion is sufficient for greenhouse operators and their employees. If in doubt, check with the Department of Environmental Conservation pesticide inspector for your region. Classification of pesticides in the restricted-use category may change. Therefore, the chemicals listed in this bulletin under control measures should be taken as examples of compounds that will control the specific disease. These recommendations may not be current. Consult your Cooperative Extension agent if in doubt of their current status. To avoid confusion, common names of pesticides are used when approved for use on the label by the Office of Pesticides Program of the Environmental Protection Agency (EPA). Otherwise, trade names are used because they also appear on the label. No endorsement of pesticides or other named products is intended, nor is criticism of unnamed products implied. More-detailed information on the rules and regulations governing the handling and use of these chemicals in New York State can be obtained from the Department of Environmental Conservation, Bureau of Pesticide Management, 50 Wolf Road, Albany, NY 12201. Bacterial Diseases

Bacterial Blight underside of the leaf (fig. 1 ). After a few Xanfhomonas pelargonii days, these spots become slightly sunken and well defined. These symptoms are Importance followed by wilting (fig. 2) and death of the Bacterial blight, or bacterial stem rot and affected leaf. Bacteria from the leaf spots leaf spot, is one of the most important may spread down the water-conducting diseases of geranium in the United States. (vascular) tissue in the leaf petiole into the Until the introduction of culture-indexed stem and then back up to the top of the cuttings on a commercial scale, this plant, eventually causing the death of the disease was the limiting factor in the plant from the stem rot phase of the production of cuttings from field-grown disease. geraniums in California. Disease epidemics In other cases affected leaves wilt at the occurred regularly, and losses of 10 to 100 margins of the leaf blade, and the leaf percent were experienced by plant petiole initially remains turgid. Infected producers. areas of the leaf rapidly die in angular areas The disease is not restricted to California bounded by the veins. This symptom IS and apparently occurs wherever geraniums common in geranium and may be caused Figure I. Bacterial blight. Small water- are grown. Before the introduction of by several other factors. But wilting of the soaked spots developing on the lower leaf culture-indexed cuttings, bacterial blight affected leaves IS always associated with surface. occurred throughout the northeastern bacterial infections. The affected leaves United States and often resulted in losses may drop off immediately or may hang on of 50 to 60 percent to local growers. The the plant for a week or more. disease is widespread in the United States, Stem symptoms. The stem rot phase of and because geraniums are one of the this disease is often called black rot. In leading pot-plant crops in this country, plants attacked in this manner the water- bacterial blight is of considerable economic conducting tissues in stems and branches importance. For many growers the control become brown to black 2 to 4 weeks after of this disease makes the difference the plant is infected. At this time one or between a loss or a profit in geranium more leaves on a branch usually wilt (fig. production. 3). Later the bacteria spread from the In addition to the United States, bacterial water-conducting tissue, located just under blight occurs throughout the world and has the cortex, inward to the pith and outward the potential to cause severe losses to the cortex, causing a brown to black wherever geraniums are grown. The discoloration in the stem. At thls stage the disease has been reported from Australia, exterior of the stem is gray and dull in Denmark, England, France, Greece, and appearance, and defol~at~onof the plant West Germany and probably occurs in continues until only the tips of the branches other countries as well.

Figure 2. Bacterial blight. Later stage in Symptoms the development of leaf symptoms The bacterium causing this disease showing slightly sunken, well-defined attacks leaves, stems, and cuttings of spots. Notice that the leaf has begun to Pelargonium hortorum as well as other wilt. species of Pelargonium. The stem rot stage is the most common symptom seen in commercial greenhouses, and most commercial cultivars of P. hortorum are susceptible. The leaf spot symptom is seen less often. This may be because environ- mental factors are not often favorable for its occurrence or some cultivars are resistant to leaf infection. Leaf symptoms. Two types of symptoms Figure 3. Bacterial blight. Early symptoms occur on leaves infected with the bacterial of the stem rot phase on the cultivar blight organism. In one case symptoms first Irene. Notice the leaves wilting on the left appear as small water-soaked spots on the side of the plant. have leaves (figs. 4, 5). The stem rapidly ture to disease development was studied blackens and shrivels into a dry rot, leaving by Kivilaan and Scheffer. They found that the stem fibers and epidermis intact but plants held at a night temperature of 28" C destroying the rest of the stem tissue. At (81 O F) showed first symptoms 7 days after this time affected plants consist of almost inoculation, whereas 3 weeks were completely defoliated, blackened branches required for first symptoms to appear on with only a few tufts of leaves on the tips inoculated plants held at night tempera- (fig. 6). The roots are blackened, but not tures of 18" C (60" F), and symptom decayed. Occasionally a plant will recover expression was still slower at 1O0 C (50" partially and produce new healthy F). Disease development and symptom appearing branches. In a few months these expression increased with increasing branches also will die. temperatures from 1 O0 to 28O C (50' to Cutting symptoms. Infected cuttings fail 81 " F). In general, at low temperatures Figure 4. Bacterial blight. Continued devel- to root and slowly rot from the base (1 0"-18" C [50a-60" F]) symptoms are opment of the stem rot phase on the upward. The leaves wilt and often show suppressed, at high temperatures (21 "- cultivar Irene. Notice that more leaves are typical wilt symptoms as described 30" C [70°-85" F]) symptom development wilting and there is a black lesion (arrow) previously. A few weeks after such cuttings is enhanced, and at very high temperatures on the stem. are placed in a rooting medium, the stems (32"-38" C [90°-100" F]) symptom become a dull black-brown as with typical development is again suppressed. stem rot, and the cuttings die. Nutrition. Kivilaan and Scheffer studied the effect of plant nutrition on disease Pathogen development in geranium plants inoculated This disease is caused by the bacterium with X. pelargonii. They found that the Xanthomonas pelargonii. The pathogen disease developed most slowly in plants was reported to cause a bacterial leaf spot receiving balanced nutrients. In plants of geranium in the eastern United States in receiving unbalanced nutrients disease 1923. At that time the bacterium was development was most severe in plants named Bacterium pelargonii. The stem rot receiving high nitrogen and low potassium phase of the disease was reported in 1932 or low nitrogen and high phosphorus. In by Dodge and Swift, but they did not general, plants in a high-nitrogen solution conclude that the two symptoms were developed symptoms more rapidly than did caused by the same bacterial pathogen. In plants in a low-nitrogen solution. Everything 1952 Hellmers showed that the same else being equal, one can expect fastest Figure 5. Bacterial blight. Continued bacterium caused both the leaf spot and disease development in plants highest in defoliation and development of stem the stem rot phases of the disease. In 1937 organic nitrogen. les~onson the cultivar Springfield White. Burkholder had described a bacterial leaf Survival and spread of Xanthomonas spot on several species of Geranium. He pelargonii. In cuttings. The bacterium is called this bacterium Phytomonas geranii, most commonly carried over in infected and the name was later changed to cuttings by bacteria that occur in the Xanthomonas geranii. Starr and co- water-conducting tissue (vascular workers did comparative biochemical and elements) of the cuttings. Cuttings taken cultural determinations and cross- from plants with stems with obvious inoculation tests with authentic cultures of symptoms Invariably are infected with 6. pelargonii and X. geranii and concluded bacteria and often rot during propagation. that the two species were identical. They Cuttings taken from infected plants that recommended that the two species be show no external symptoms may serve as combined as X. pelargonii. symptomless carriers and are of much At the present time the major stem rot greater danger to both the propagator and and leaf spot disease of Pelargonium is the grower. caused by one organism. X, pelargonii. In cutting propagation. The propagation medium is ideal for spread of the pathogen Environmental Relations when cuttings are rooted in a common Figure 6. Bacterial blight. Blackened and Temperature. Symptoms of bacter~al shriveled areas (arrows) on stems of rooting bench. Cuttings are stuck close blight appear on infected plants more together, and the bacterium can spread geranium resulting from infeclion by rapidly at higher temperatures (fig. 7). The Xanthomonas pelargonii. from an infected cutting into the rooting relationship of greenhouse night tempera- medium and infect adjacent noninfected Figure 7. Bacterial blight. Plants inoculated with Xanthomonas pelargonii and grown at different temperatures to show the effect of temperature on symptom expression. Plants on the left grown at 18" C (60" F) and plants on the right grown at 27" C (80" F). Note the severe symptoms on the plants grown at 27" C (80" F).

cuttings. When cuttings are rooted singly in on healthy geraniums. Leaf blisters and leaf individual containers, this is not a problem spot symptoms appeared on some of these because there is very little spread from plants after 17 days, and after 2 weeks container to container, and each cutting symptoms appeared on all the plants on can be handled as an individual unit. which whiteflies fed. The pathogen was On infested cutting knives. The most isolated from these lesions, and it Infected efficient means of transmission of X. healthy geraniums when the plants were pelargonii from plant to plant is by the inoculated. cutting knife. Because of this the preferred method of taking cuttings is to break them Host Range and Susceptibility of from the stock plants. If knives are used, Cultivars they should be disinfested by dipping in Apparently none of the commercial 70% alcohol and flaming or by soaking for cultivars of P. hortorum are immune from 5 minutes in sodium hypochlorite, 10% stem rot. Cultivars vary in their susceptibility Clorox (1 oz Clorox plus 9 oz of water). to X. pelargonii, but none possess useful In soil. Munnecke (1956) studied the resistance to the bacterium. Knauss and survival rate of X. pelargonii in a heavy Tammen tested 41 species, hybrid species, loam soil infested by plowing under and cultivars of the genus Pelargonium. Figure 8. Bacterial blight. Cross section of geranium plants that had been inoculated Resistance occurs in P. domesticum, P. a geranium stem showing the early devel- with X. pelargonii. Immediately after acerifoliurn, P. Torento, P, tomentosum, opment of a bacterial pocket in the xylem plowing and at intervals of 1, 3, 6, and 12 and P. scarboroviae. Pelargonium and surrounding tissue. months, rooted cuttings of a susceptible graveolens exhib~tsintermediate resistance cultivar of P. hortorum were planted in the or susceptibility, and P. peltaturn is highly field. Results of these tests showed that the susceptible. The pathogen was found to bacteria have slight survival ability in moist persist in root-inoculated, symptomless soil, although 100 percent loss may occur plants of P. domesticum maintained for 2 if cuttings are planted immediately in months under conditions optimum for infested soil. Survival depends on the rate disease development. Thus, although this of decay of the infected plant parts, and 6 species is highly resistant, it is not immune months is probably the time limit for and may serve as a symptomless carrier of survival of the pathogen under these X. pelargonii. conditions. Anatomy of Plants Infected with In overhead water and by plant contact. Xanthomonas pelargonii Spread of X, pelargonii readily occurs in pots of closely spaced geraniums on a The mode of spread of the pathogen is greenhouse bench. Dougherty, Powell, and the same in all Pelargonium species, Larsen showed that in blocks of 108 plants whether they be resistant or susceptible. with 5 infected plants spaced throughout Initially, movement of the pathogen occurs the block, all plants were infected and throughout the plant in the xylem vessel showed symptoms within 44 to 50 days elements, followed by lateral movement when overhead watering was used. In a into adjoining parenchyma cells. The relative numbers of bacteria and the similar experiment using Chapin-type Figure 9. Bacterial blight. Cross section of number of vascular bundles initially Irrigation tubes, all nonincubated plants a geranium stem showing disintegration of invaded are low in resistant species and remained symptomless after 50 days. xylem tissues as the bacterial pocket high in susceptible species. In susceptible Munnecke reported that his field continues to enlarge. observations indicated that rapid spread of species, bacterial pockets form around the bacterium occurred when it rained or affected xylem vessels (fig. 8), enlarging to when overhead watering was used. Spread encompass all the xylem cells in the bundle also occurred by plant contact and on farm (fig. 9) and, finally, portions of the cambium, equipment. phloem, cortex, and epidermis. In resistant By insect transmission. Bugbee and species the plants respond to infection by Anderson ~solatedX. pelargonii from live forming a ring of cells around affected whiteflies (Trialeurodes vaporariorum) that portions of the vascular bundles. This had fed 24 hours on geranium with leaf appears to be a secondary defense spot symptoms. Whiteflies that had fed 24 reaction, restricting the lateral spread of the hours on diseased geraniums were placed pathogen. Control Several important steps are involved in control of bacterial blight of geraniums. The bacterium causing this disease is most commonly carried over in infected cuttings. Cuttings taken from stems showing obvious symptoms of the disease are almost always infected and serve as a means of infecting clean cuttings during propagation. Cuttings taken from infected plants showing no obvious symptoms may also be carrying bacteria. These cuttings may carry the organism for some time before showing symptoms, especially during cool weather. Because of this all cuttings used should be derived from culture-indexed plants. The Figure 70. Bacterial fasciation. Short, culture-indexing procedure is described in fleshy, thick aborted stems with the section "Detection of Fungus and misshapen leaves developed at the soil Bacterial Vascular Wilt Pathogens by level on an infected plant. Culture-Indexing." Soil or artificial growing media and containers used for geranium production should be treated with steam or chemicals before use. Because the bacterium can survive in old geranium plant tissue in soil, the soil should be free of any such debris before it is treated. Good sanitation and cultural practices should be followed throughout the growing period. The bacterium can be spread rapidly by overhead watering or splashing water, by plant contact, and through the use of infested tools and equipment. Plants should be watered carefully and not crowded together on the bench to avoid spread in this way. Bacterial Fasciation Figure 1 1. Bacterial fasciation. Short, Corynebacterium fascians fleshy, thick aborted stems with misshapen leaves developed at the soil Importance level on an infected plant. The bacterial fasciation disease was first found in the United States on sweet pea in 1935. Before then it had been observed in England in 1927. The disease has also been reported to occur in Germany, Denmark, and Sweden. The disease occurs throughout the United States and affects many other plants in addition to geranium. At present this disease is of minor importance, but has the potential to cause problems in the culture of geranium. The bacterial pathogen that causes this Figure 12. Bacterial fasciation. Gall-like disease has a wide host range, which growth that has developed at the soil level includes such diverse plants as lily, on an infected plant. Figure 13. Bacterial fasciation. Aborted shoots and gall-like growths that have developed below the soil level on an infected plant. The soil level is indicated by the arrow.

Figure 14. Bacterial fasciation. Aborted shoots and gall-like growthy that have developed below the soil level on an infected plant. gladiolus, Viburnum, carnation, chrysan- organism is waterborne and carries over on themum (C. indicum, C. morifolium, C. the surfaces of affected buds and in maximum), dahlia, Pelargonium hortorum, infested soil. It can be spread in watering, in sweet pea, hollyhock, forsythia, phlox, handling infected plants, and by infested primula, delphinium, and petunia. This list soil. The bacterium may also be spread in includes many plants commonly found in cuttings from infected plants, but at present the home garden. Infected geraniums there is no conclusive evidence to support purchased by the homeowner as bedding this belief. plants may serve as a source of the bacterium to infect other plants in the Control home garden as well as to infest the soil. Control measures consist of sterilizat~on of all propagating media and potting soil Symptoms and careful sanitation practices to prevent Symptoms of the disease on geranium the reintroduction of the bacterium into the are distinctive and usually quite obvious. treated soil. Any stock plant exhibiting Many short, fleshy, thick, aborted stems symptoms of this disease should be with misshapen leaves develop at or below discarded. Culture-indexed cuttlngs should the soil level on affected plants; The mass be used whenever possible. of fasciated growth on old plants resembles a "witches'-broom" and may reach a Southern Bacterial Wilt diameter of 1 to 3 inches (figs. 10, 11). In Pseudomonas solanacearum few days and finally necrosis of affected other cases the growth at the base of the leaves. Two weeks after inoculation most Importance plant resembles a gall, and no aborted leaves are necrotic, but the terminal shoots shoots develop (fig. 12). The aborted shoot Southern bacterial wilt is a widespread usually remain green until the stem turns or gall development is usually visible above and destructive disease on many host black at the soil line and the plant ground, but in some cases the major plants in warm climates. Among the many collapses. Flowers fail to open normally. portion occurs below the soil level and little plants attacked by Pseudomonas Vascular discoloration is present in the or no growth is visible (figs. 13, 14). solanacearum are tomato, potato, tobacco, stem 5 days after root inoculation. The aboveground portions of the aborted pepper, eggplant, peanut, banana, and shoots and galls are a normal green color ornamental plants such as dahlia, marigold, Pathogen similar to the color of the rest of the plant. and zinnia. In addition a diverse group of Southern bacterial blight of geranium is The underground portions are usually pale weeds are also attacked by the bacterium. caused by Pseudomonas solanacearum. yellow. Apparently the main stem of an The first report of this disease on geranium Although this bacter~umwas not reported affected plant continues to grow normally, was in 1980 from North Carolina. In a large on geranium until 1980, it has been known but there is some evidence to indicate that planting of the cultivar Sprinter about one- to attack other host plants, such as tomato this growth is stunted or dwarfed and that half of the plants wilted and died. These and potato, since the 1890s. The bacterium the number of flowers is reduced. Affected plants were large and otherwise in good has been given several different names, plants seem to live as long as healthy condition. Subsequent investigation by D. among them being Bacillus solanacearum, plants. L. Strider, North Carolina State University, Bacterium solanacearum, and Xantho- showed that Pseudomonas solanacearum monas solanacearum. Pathogen was the causal organism, and the informa- Bacterial fasciation is caused by the tion given here is based on his published Environmental Relations bacterium Corynebacterium fascians. The report. Disease development and symptom organism was first named Phytomonas expression occur most readily at tempera- Symptoms fascians in 1936, then Bacterium fascians, tures between 25" C (75" F) and 38" C and finally changed to the current name in Initial symptoms of southern bacterial (100" F). Overhead watering also favors 1942. The bacterium affects only the wilt appear in 5 days after plants have been disease development and symptom meristem tissue of buds and causes the root inoculated. Although plants can be expression. buds to break dormancy. Wounds are not inoculated by dipping roots in a bacterial required for the bacterium to initiate suspension, by puncturing stems at the Control d~seasedevelopment. junction of petiole and leaf with infested To date no cultivars resistant to toothpicks, or by pouring a bacterial Pseudomonas solanacearum have been Environmental Relations suspension over cut roots, disease devel- found. The use of cuttings derived from Conditions that favor aborted shoot and opment is most uniform with the method culture-indexed stock, growing geraniums gall development are moisture and involving cut roots. The initial wilting of from seed, and soil treatment are still moderately warm temperatures. The lower leaves is followed bv chlorosis in a probably the best control measures. Bacterial Leaf Spot Pathogen rot of Pelargonium hortorum. Pseudomonas cichorii This disease is caused by the bacterium Phytopath. 64:1081-83. Dowson, W. J. 1942. On the generic name Importance Pseudomonas cichorii. The organism has also been called Phytomonas cichorii and of the gram-positive bacterial plant Bacterial leaf spot is a new disease of Bacterium cichorii. pathogens. Trans. Brit. Mycol. Soc. geranium described recently by Arthur W. 25:311-14. Engelhard of the University of Florida, Environmental Relations Engelhard, A. W. 1980. Bacterial leaf Bradenton, and most of the information Disease development and symptom spot-a new geranium disease. given is taken from his report. This disease expression are favored by high humidity Florists' Rev. 166 (4311 ):53-55. may become a major production problem and the presence of free water on the Harrison, D. E. 1961. A bacterial disease of with geraniums in the southeastern United foliage. Disease development is rapid on pelargoniums in Victoria. J. Agric. States. The disease is serious because the plants grown outdoors and exposed to Victoria 59:493-95. same causal bacterium causes leaf spot of frequent rains. The disease will also Hellmers, E. 1952. Bacterial leaf spot of chrysanthemum and also attacks other develop in plants grown under cover if they Pelargonium (Xanthomonas pelargonii ornamental plants such as Philodendron are exposed to overhead watering or dew. (Brown) Starr & Burkholder) in panduraeforme, Aglaonema, Monstera, Denmark. Trans. Danish Acad. Tech. Scindapsus, Gerbera, and larkspur. In Control Sci. 4:l-40. addition the bacterium can also infect Control of bacterial leaf spot on geranium Kelman. A. 1953. The bacterial wilt caused vegetable crops such as celery, cabbage, is based on starting with plants free of by Pseudomonas solanacearum. broccoli, cauliflower, and endive. Pseudomonas cichorii and excluding the North Carolina Agric. Exp. Sta. Tech. pathogen from the growing area. Strict Bull. 99. 194 pp. Symptoms sanitation procedures must be followed in Kivilaan, A.. and R. P. Scheffer. 1958. The symptoms are leaf spots on the geranium production areas. Workers Factors affecting development of geranium and vary depending on weather must be careful coming from areas where bacterial stem rot of Pelargonium. conditions. Plants grown outdoors where other susceptible plants are growing Phytopath. 48:185-91. they are exposed to rain exhibit dark brown because the bacterial pathogen may be Knauss, J. F., and J. Tammen. 1967. to black, irregularly shaped necrotic areas, carried on clothes, hands, sprayers, and Resistance of Pelargonium to 5 to 10 mm or larger. These spots may other equipment. Any measures to protect Xanthomonas pelargonii. Phytopath. extend or enlarge along the veins, and plants from rain or dew and avoidance of 5711 178-81. coalescing spots may encompass large overhead watering will aid in controlling the Knight, B. C., A. A. Nichols, and C. W. sections of a leaf. Chlorosis develops in disease, for moisture is necessary for Leaver. 1962. Bacterial leaf spot of tissues adjacent to the lesions within a few disease development. No effective Pelargonium. Plant Path. 11 :29-30. days, and when spots become numerous, chemical control is known at this time. Lacy, M. S. 1939. Studies in bacteriosis. general leaf chlorosis occurs. Severely Vol. 24, Studies on a bacterium affected leaves become necrotic and may References associated with leafy galls, fasciations, fall off the plant. and "cauliflower" disease of various Under environmental conditions less Baker. K. F. 1950. Bacterial fasciation plants. Part 3, Further isolations, favorable for disease development and disease of ornamental plants in inoculation experiments, and cultural spread, such as when plants are only California. Plant Dis. Rept. 34:121-26. studies. Ann. Appl. Biol. 26:262-78. exposed to occasional overhead watering. Brown, N. A. 1923. Bacterial leaf spot of Maviat, M. F. 1948. On a new bacterial sunken lesions develop on both upper and geranium in the eastern United States. disease of the Pelargonium. Compt. lower leaf surfaces. These lesions may J. Agric. Res. 23:361-72. Rend., Paris 226:115. have slightly raised tan centers surrounded Bugbee, W. M., and N. A. Anderson. 1963. Munnecke, D. E. 1954. Bacterial stem rot by a dark margin with or without a chlorotic Whitefly transmission of Xanthomonas and leaf spot of Pelargonium. halo. Eventually, most of the leaf spots pelargonii and histological examination Phytopath. 44:627-32. of leaf spots of Pelargonium hortorum. become dark brown to black. Continued -, 1956. Survival of Xanthomonas development of symptoms and spread of Phytopath. 53:177-78. pelargonii in soil. Phytopath. 46:297- the disease may stop in dly weather. Burkholder. W. H. 1937. A bacterial leaf 98. The flowers may also become infected, spot of geranium. Phytopath. 27: 554- Nelson, P. E. 1962. Diseases of geranium. and individual flowers may fail to open 60. N. Y. State Flower Growers Bull. when infected in the bud stage. Occasional Dodge, B. O., and M. E. Swift. 1932. Black 201 :1-10. infection of the flower peduncle occurs, but stem rots and leaf spot of pelargonium. Nelson, P. E., L. P. Nichols, and J. Tammen. natural stem infection has not been J. N.Y. Bot. Garden 33:97-103. 1971. Vascular wilts. In Geraniums, a observed. Dougherty, D. E., C. C. Powell, and P. 0. manual on the culture, diseases, Larsen. 1974. Epidemiology and insects, economics, taxonomy, and control of bacterial leaf spot and stem breeding of geraniums, ed. J. W. Fungus Diseases

Mastalerz, pp. 232-40. Penn. Flower Botrytis Blight Growers, University Park. Botrytis cinerea Nichols, L. P., and P. E. Nelson. 1974. Geranium-programmed disease Importance control. Penn. State Univ., University The fungus Botrytis is one of the most Park. common and widely distributed pathogens. Panagopoulos, C. G. 1963. Bacterial stem It can colonize dead organic matter readily rot of Pelargonium in Greece. Inst. and attacks many differenl plants. Under. Phytopath. Bendki. Ann., n.s. 5:117- proper conditions the fungus can attack 21. geranium flowers, leaves, and stems, Starr, M. P., Z. Volcani, and D. E. Munnecke. causing petal blight, leaf spots, and stem 1955. Relationship of Xanthomonas cankers. The blighting of flowers and the pelargonii and Xanthomonas geranii. leaf spots can be a serious problem on Phytopath. 45:613-17. plants being finished for sale. In addition, Strider, D. L., R. K. Jones, and R. A. spores may lodge on the surface of stems Haygood. 1980. Southern bacterial of stock plants; and when cuttings are Figure 7 5. Botrytis blight. Geranium flower wilt of geranium caused by taken and stuck in the propagating bench, attacked by Botrytis showing the matted, Pseudomonas solanacearum. Plant the spores germinate and cause a basal rot fuzzy growth of the fungus on the petals. Dis. 65:52-53. of the cuttings. Tilford, P. E. 1936. Fasciation of sweet peas caused by Phytomonas fascians Symptoms n. sp. J. Agric. Res. 53:383-94. Flowers. Flowers attacked by Botrytis Wainwright, S. H., and P. E. Nelson. 1972. exhibit premature fading and drying of the Histopathology of Pelargonium petals. The central florets are often the first species infected with Xanthomonas to be affected. These flowers turn dark, wilt, pelargonii. Phytopath. 62:1337-47. and drop prematurely. During periods of high moisture, the affected blossoms may be covered with grayish brown masses of spores, and the florets may be matted together (fig. 15). Leaves. The leaf spot phase of Botrytis blight often appears when petals from affected flowers fall on the leaves and the fungus grows into the healthy tissue (fig. 16). Often the leaf spot assumes the outllne of the infected flower part that falls Figure 16. Botrytis blight. Leaf spots on it. When the leaves are wet or the caused by the fungus Botrytis. Notice the hum~dity is high, the spots enlarge and rotted flower petals that have initiated the become irregular in shape, brown, and leaf soots. water-soaked. If the high humidity cont~nues,the spots become covered with grayish brown masses of spores. Cuttings. Spores on the surface of stems of the stock plant often remain dormant until cuttings are taken and stuck in the propagation bench. During the rooting process, these spores germinate and infect the tissue of the cutting, causing a cutting rot. This rot may be in the form of lesions on the cutting stems (fig. 17) or a complete basal rot of infected cuttings (fig. 18). Botrytis cutting rot usually is a light to dark brown rot of the cutting in contrast to the shiny coal black rot caused by Pythium. Botrytis spores are very light and can be Figure 17. Botrytis blight. Lesions on the carr~edto nearby plants by air currents or stems of cuttings caused by Botrytis. .- -. by splashing water. The stubs of the stem refuse in the greenhouse or field tha p- ... - left when cuttings are removed often are serve as a good base for sporulatlon ot infected in this manner. Infected cutting Botrytis. Fungicides applied as sprays or stubs exhibit a brown rot that may extend thermal dusts on a regular schedule may several inches down the stem. prevent colonization of senescent tissue. System~c fungicides may be helpful in Pathogen controlling Botrytis blight on stock plants Botrytis blight is caused by the fungus and in preventing cutting rot during Botrytis cinerea. This fungus is capable of propagation. Heat and ventilate to prevent penetrating tissues directly, but penetration dew formation in the greenhouse. is more common when the host plant is grown under unfavorable conditions or if Rust wounds are present. Senescent tissues are Puccinia pelargonii-zonalis penetrated and colonized readily. Importance Environmental Relations Geranium rust is caused by the fungus Senescent or dead tissue on the plant Puccinia pelargonii-zonalis, The disease and dead organic matter around the areas was discovered first in South Africa and where geraniums are grown support the later spread to Australia, New Zealand, the growth of Botrytis and, hence, add to the Hawaiian Islands, and Europe. In Europe potential inoculum. Senescent florets and the disease has been reported in Britain, petals of geraniums are particularly Belgium, France, Germany, Italy, and susceptible to attack by this fungus. Switzerland. Rust was first reported in the Botrytis sporulates readily on flowers, United States in New York State in 1967. leaves, and dead organic material, Shortly after this discovery the disease was especially during periods of high humid~ty. reported from the Monterey Bay area of The spores are easily detached and California. Since that time geranium rust disseminated by air currents or splashing has been found in Pennsylvania, Florida, Figure 18. Botrytis bltght. Stem rot of water. Moisture is necessary for and Minnesota. This disease is a serious cuttings caused by Botrytis. qermination and ~enetration,and moisture problem for the grower because of the of condensation is ideal disfiguring of the foliage by the rust Temperatures from 1 0" C to 21 ' C (50" pustules. In addition, many states have a F to 70" F) favor disease development. quarantine against this disease, which with the optimum temperature occurring at prevents growers from moving or selling about 18" C (65" F). When large numbers their crop until the disease is eradicated. of spores are present, the occurrence of epidemics is favored by high day tempera- Symptoms tures, low night temperatures-16" C to The first symptoms on infected lezlves 18" C (60' F to 65" F)-and high relative are very small, pale yellow spots, mainl!! on humidities. Under these conditions dew will the undersurface of the leaves. These form on the flowers and leaves. spots increase in size, reaching a diameter The size of leaf lesions caused by of 5-8 mm, and turn rusty brown, indicating Botrytis and the amount of sporulation on the development of masses of urediospores these lesions increase with increasing in uredia In these spots (figs. 19, 20). On temperature from 10" C to 25' C (50" F to the upper leaf surface, the spots are pale 76' F). At 30' C (86" F) typical Botrytis yellow, and their size corresponds to the lesions and sporulation do not occur. There rust spot on the undersurface (fig. 20). Figure 7 9. Rust. Yellow spots with rusty is no significant difference in the size of leaf Although, occasionally, rust spots with brown centers on the lower surface of a lesions or in the sporulation on these uredia may be seen on the upper leaf geranium leaf. The rusty brown centers lesions when produced ~nlight or darkness surface, the majority are always on the are masses of urediospores. at 20' C (68" F). undersurface. Leaves that are heavily infected and contain many rust pustules Control turn yellow and drop prematurely, leaving Control measures consist of sanitation, the plants partly defoliated. Not only are the the use of fungicides, and the manipulation brown dusty spots objectionable, but loss of the environment. Clean up all plant of leaves makes the plants unsightly, Figure 20. Rust. Upper surface of a geranium leaf (left) showing the pale yellow spots that occur above the rust pustules on the lower surface. Lower surface of a geranium leaf (right) showing typical yellow spots with masses of rusty brown urediospores in the center. to rust. McCoy tested 17 Pelargonium species and cultivars for susceptibility to the rust fungus Puccinia pelargonii-zonal~s under controlled-environment conditions. The Pelargonium hortorum cultivars Irene, Improved Ricard, Enchantress Fiat, Pink Enchantress Fiat, and Summer Cloud were highly susceptible, with heavy pustule formation occurring within 14 days of inoculation when the inoculated plants were held at 21 " C (70" F). Pelargonium domesticum 'Graf Zepplin', P. radula, and P. limoneum developed heavy flecking and occasionally small pustules within 14 days of inoculation. Pelargonium domesticum 'The Princess' and 'Madam Laval'. P. peltatum 'Etincelant', P, fulgidbm,' P. reduces their value for customer sales, and Control monstrum, P. torento, and P. radens were may result in serious financial loss for the When geranium cuttings or plants are highly resistant or immune, producing at grower. purchased, they should be examined most, a moderate fleck after 4 weeks. Pathogen carefully for evidence of infection by P. Harwood and Raabe tested the P. pelargonii-zonalis. If evidence of infection, hortorum cultivars Springtime Irene, Dark The disease is caused by the rust fungus such as a rust pustule, is found, the Red Irene, Irene, Improved Blaze, Salmon Puccinia pelargonii-zonalis,which produces shipment should be rejected. The reason Irene, Cardinal, Genie, Snowball, Better only the uredial stage on geranium. The for such action is that even if only a single Times. Springfield Violet, Fiat Enchantress, telial stage is rare and apparently has been plant in a shipment is infected, odds are Appleblossom, and Penny Irene. All found only on plants in Switzerland and good that other plants in the shipment have cultivars were susceptible, with Penny South Africa. On geraniums grown in the incipient infections that have not yet Irene and Appleblossom being the most greenhouse, the urediospores serve as resulted in the appearance of symptoms. susceptible and Springtime Irene, Dark Red repeater spores and perpetuate the If rust becomes established on plants in Irene, and Irene, the least susceptible. disease. the greenhouse, the disease may be Species that were tested and found to be Environmental Relations controlled with regular sprays of fungicides. susceptible were P. endlicherianum, P. Because the recommendations for inquinans, P. quercifolium, P. salmoneum, If a supply of urediospore inoculum is fungicides to use for control of this disease P. tabularae, and P. zonale. Another 29 present, infection of susceptible plants may vary depending on the locality, consult your Pelargonium species were tested and occur readily under environmental local plant pathology extension personnel found to be resistant. conditions that favor the development of for current recommendations. free moisture on the plant Spores of the Hot water treatment of geranium cuttings Alternaria Leaf Spot fungus are effectively spread by splashed infected with P. pelargonii-zonalis may also Alternaria tenuis water, but they may also be disseminated be a method for eliminating the rust fungus. Importance by air currents. A small source of infection Experimental tests have shown that rust may endanger plants at considerable did not develop on cuttings of Pelargonium Alternaria leaf spot has been reported to distances, not just those in the immediate hortorum infected with the rust fungus after occur in California and Florida in the United vicinity. they were dipped for 90 seconds in water States and in some parts of Europe. Germination of urediospores of P. at 50" C or 52O C (120' F or 124O F) or Although economic losses due to Alternaria pelargonii-zonalis requires liquid water and were held for 24 or 48 hours in a water- leaf spot are minor, the disease is occurs within 3 hours at the optimal saturated atmosphere at 38' C (100" F). important, for it may be confused with the temperature of 16" C (60" F). The The hot water treatment permitted leaf spot stage of bacterial blight caused by pathogen usually forms appressoria and germination of some urediospores, Xanthomonas pelargonii, and plants may penetrates through the stomates 5 to 6 particularly those taken from poorly be unnecessarily rogued because growers hours after the leaves are inoculated with growing plants. Hot water treatment assume the leaf spotting is caused by the urediospores. The hyphae are intercellular damaged the small expanding leaves, but blight bacterium. with intracellular haustoria. Symptom did not retard subsequent plant growth. development is most rapid and extensive at Symptoms Several research workers have tested 21 C (70" F). " cultivars of Pelargonium hortorum and Initial symptoms appear as water- other Pelargonium species for resistance soaked areas, less than 1 mm in diameter. on the lower surface of the leaf. These may Blackleg Pathogen enlarge to necrotic areas 2 to 3 mm in Pythium species This disease is caused by several diameter with slightly sunken centers, specles of fungi in the genus Pythium such Importance which are less evident on the upper leaf as P. ultimum, P. debaryanum, and P. surface. When viewed by transmitted light, Blackleg has been known since the late irregulare. These fungi are sometimes these necrotic areas have small brown 1800s and occurs wherever geraniums are called water molds. They are soil centers surrounded by a yellow diffuse grown when environmental conditions inhabitants, occurring in most agricultural band, and they may enlarge or remain the favor disease development. Although soils, and are unspecialized parasites same size. Those that enlarge become losses due to this disease do not occur capable of attacking a wide range of host large, irregular, necrotic areas, 6 to 10 mm frequently, it is not unusual to find 90 to plants. in diameter. The fungus may sporulate 100 percent of the cuttings in a propagating while the leaves are still on the plant, but bench killed as a result of blackleg. This is Environmental Relations the most abundant sporulation occurs on primarily a disease of cuttings and young In general, Pythium blackleg is favored fallen leaves. Often the spots coalesce and plants. The disease is becoming rare by any factor of the environment not form very irregular necrotic spots, usually because of the use in propagation of favorable for plant growth. High soil limited by the large leaf veins. Severe perlite, wood pulp blocks, and expandable moisture or low oxygen content of the infestations may kill the leaf or cause peat pellets, all of which are essentially free propagation or growing medium is mosr' considerable leaf fall. Dead leaves shrivel of Pythiurn species when new. Direct important. Dissemination of the pat hogen and support abundant sporulation by the rooting in peat or pots filled with rooting mix occurs by movement of plant debris fungus. and treated with steam in place on the colonized by the pathogen or in t,its of bench also is a factor in reducing disease infested soil carried by running watel Pathogen incidence. tools and growing containers. This disease is caused by the fungus Alfernaria tenuis. It produces multicellular, Symptoms Control brown, flask-shaped conidia in short chains The fungus may attack geranium during Control measures involve several steps. on conidiophores. The conidia are usually any stage in the growth of the plant. Treat the propagation and growing media beaked. Cuttings, rooted cuttings, and young plants with steam. In addition, pots, benche!s, and are particularly susceptible to the disease, tools should also be treated with st(?am. If Environmental Relations whereas older plants are more resistant. the pots are to be set on benches The disease usually occurs when Symptoms appear initially at the base of containing gravel or cinders, this material growing conditions are not favorable for the stem at the soil line. Affected stem should also be treated with steam. good growth of geranium plants. In tissues turn coal black (fig. 21 ). The fungus Drenches with soil fungicides may be used California these conditions occur during may continue to colonize the stem, and the to prevent the colonization of steam- the winter when low night temperatures black rot continues to progress up the treated soil by Pythium species. These slow the growth of plants growing outdoors. stem. The epidermis of the affected tissues treatments are not a substitute for the In addition, the foliage may be Wet for long remains intact, but is a shiny, black color. original steam treatment. Consult \lnllr,--. periods of time because of the frequent Eventually the tops of affected cuttings and local plant pathology extension persoInnel rains and fogs that prevail during the winter. plants wilt and die (f~g.22). for current recommendations. Destroy all In contrast, the disease is most common in affected plants and crop residue. Florida in warm weather when tempera- tures are too high for good growth of Cutting Rots geraniums, usually in April or later. Rhizoctonia species, Fusarium Control species The disease occurs primarily on field- Importance grown plants and usually is not severe Diseases caused by Rhizoctonia and enough to require specific control Fusarium species are of minor importance measures. If it is severe, fungicide sprays in the industry as a whole. Individual should be applied at regular intervals. growers may experience extensive losses Consult your local plant pathology from the diseases caused by these extension personnel for current organisms. Although cutting rots caused by recommendations. these fungi have been reported for ttIe past Figure 21. Pythium blackleg. Brown to 50 years, these diseases are rarely s;een at black wafer-soaked rot at the base of mesent. geranium cuttings. Figure 22. Pythium blackleg. Coal black basal rot of cuttings in a propagation flat. Note the wilting and collapse of affected cuttings.

Environmental Relations In general, Rhizoctonia and Fusarium are most severe in attacks on plants that have been injured or when conditions are unfavorable for growth of the plant. High soil temperatures and low soil moisture are particularly favorable for Rhizoctonia. Control Control measures consist of several steps that must be integrated to achieve successful control. These steps follow: Steam treatment or chemical fumiga- tion of the propagating medium, growing medium, tools, benches, and containers to eradicate the pathogens. Strict sanitation practices to prevent contamination of treated soil. Use of soil fungicides to prevent colon~zation of treated media, should contamination occur. Contact your local Symptoms Pathogens plant pathology extens~on personnel for Unrooted cuttings infected with either Rhizoctonia solani is the species most current recommendations. Fusarium or Rhizoctonia during propagation frequently associated with this disease. appear stunted with yellowed, wilted leaves The imperfect stage produces no spores, Thielaviopsis Root Rot and small new leaves that do not enlarge. and the perfect stage is seldom seen in Thielaviopsis basicola Below the soil line, cuttings exhibit an nature. The fungus is disseminated as Importance extensive dark black, soft, basal stem rot vegetative mycelium or as resistant that frequently involves all of that portion of structures capable of surviving long Thielaviopsis root rot, or black root rot. the cutting below the soil line. On partially periods when conditions are unfavorable may occur on geranium and many other rooted cuttings, Rhizoctonia causes a for growth of the fungus. Rhizoctonia is a plants such as poinsettia, cyclamen, basal stem rot that is usually restricted to soil inhabitant, capable of colonizing soils in begonia, tobacco, cotton, and bean. This about 1 cm of the basal portion of the stem. the presence of other organisms, and disease is not common on geranium, but it A combination of Fusan'um and Rhizoctonia occurs in most agricultural soils. It is an does occur in several eastern states where infecting cuttings results in an increase in unspecialized parasite that attacks a wide geraniums are grown. The incidence of the extent of the basal stem rot. Isolates of range of host plants. Thielaviopsis root rot on geranium is often Fusarium species alone cause decay of Fusarium species, such as F. oxysporum high when geraniums are grown in some roots and browning of the surface of and F. solani, have been implicated as greenhouses after poinsettias, for the the basal portion of the stem. Rhizoctonia causal organisms in cutting rots. These fungus occurs on both hosts and is carried may also cause root rot of some of the fung~produce two types of conidia called over in the soil as chlamydospores. In older roots of fully rooted cuttings. macroconidia and microconidia. The general, this disease is of minor importance Several cult~varsof seedling geraniums macroconidia are large multiseptate in the production of geranium. seem to be very susceptible to Rhizoctonia spores, wh~ch are slightly curved and Symptoms root and crown rot. Rhizoctonia is often a shaped somewhat like the blade of a more severe problem when the crop is scythe. Microconidia are small conidia with Infected plants are slow growing and subject to a certain amount of stress one or two cells and oval to oblong in stunted, and the lower leaves may turn caused by drought or excess salts. On shape. These fungi also produce yellow and drop. Roots of plants showing seedling geraniums Rhizoctonia will move chlamydospores, which are small, round, these foliar symptoms have dark brown to up the stem and cause the plant to thick-walled cells produced in plant tissue black lesions. Unrooted cuttings planted in collapse from the crown upward. The and soil and are capable of surviving for soil infested with the Thielaviopsis root rot lesions and rotted tissue appear brown, long periods of time when the environment fungus exhibit delayed or reduced rooting, and the brown weblike mycelium of the is unfavorable for growth and reproduction or rooting is prevented completely. The fungus can often be seen growing on the of the fungus. All types of spores can be base of the new cuttlng is susceptible for leaf and lower stem tissue that is rotting. disseminated in soil, in plant debris, or by only 1 to 2 weeks and becomes highly splashing rain and wind. resistant or immune following this period. Pathogen first the petioles of the affected lea ~ - This disease is caused by the fungus remain turgid, but after a day or two Thielaviopsis basicola. This fungus wedge-shaped areas on affected leaves produces single-celled hyaline endo- turn yellow, or the entire leaf may turn conidia. Black, thick-walled chlamydo- yellow, and the petioles wilt (fig. 23). This is spores are also formed in root lesions on followed by a gradual drying of the affected infected plants. The pathogen has a limited leaves and petioles, resulting in leaf drop. distribution in agricultural soils; but once it As the disease progresses, more leaves is introduced, it may persist for long periods collapse, yellow, dry up, and drop, L" ly of time in the absence of a susceptible in the defoliation of the affected area of the plant by means of chlamydospores. plant (fig. 24). The growth of affec:ted plants may also be stunted. The progr.ess ,- Environmental Relations of the disease varies from plant to plr-rllll. . Ill Thielaviopsis basicola is probably a cases where progress is rapid, the plant facultative parasite and requires some soon loses most of its leaves, and the remaining, unwilted leaves are pale green predisposing factor to allow it to invade ,. . geranium roots and cause typical root and may show interveinal yellowing, wnlcn Figure 23. Verticillium wilt. Leaf from looks very much like a nutrient defic:iency infected geranium plant, cultivar lesions and foliar symptoms. If cuttings or a virus disease symptom. In thc ? final Springfield White, showing wilting and become infected in the propagating bench, a rot of the cut end may occur. More stages, an affected plant may show yellowing, one of the early symptoms of dieback or drying of the t~psof branc hes this disease. important, though rooting is reduced, many infected cuttings will root without showing and brown to black discoloration of the any symptoms. Later, during production, main stem and branches (fig. 25). These root growth on these plants is poor and the stem areas are completely discolore(3 in plants are stunted. cross section, but above the externally discolored area the browning is limited to Control the vascular tissue. Control measures consist of steam In some cases an infected plant may not treatment of the propagation medium, the show symptoms for weeks or even months. pottlng medium, pots, potting bench, But after this period of time, the entire plant production bench, and tools. Strict may suddenly collapse (fig. 26). Shortly sanitation measures must be followed to thereafter, the plant will die. prevent infestation of steam-treated media. Symptoms of Verticillium wilt (3n P. domesticum are much the same as those Verticillium Wilt on P. hortorum, except that the wilting and oadili~ Verticillium albo-atrum collapse of leaves do not occur as r-, 'U", (fig. 27). Affected leaves turn yellow, but Figure 24. Verticillium wilt. Cultivar Importance often do not wilt for several days. 1Nith Springfield White with healthy plant (right) The Verticillium wilt disease occurs on some cultivars the defoliation is not severe, and infected plant (left) showing extensive many crops and may be a problem on but the stunting of infected plants is -64-n defoliation of the infected plant greenhouse-grown crops such as pronounced. geranium, chrysanthemum, rose, and The P, hortorum cultivars Better 1 snapdragon. This disease may also occur Olympic Red, Ricard, Wendy Ann, Dic on geraniums and other crops grown Improved Picardy, Springfield White, naalo outdoors. The pathogen attacks both Red, Penny, Genie, Irene, and Dark Irene Pelargonium hortorum and P. domesticum, have been inoculated with V. albo-: jtrum, and the disease has been reported to and all cultivars were found to be suscep- -. occur in New York, California, and Oregon. tible. The P. domesticum cultivars I he Recently the disease has been observed Princess, Marie Vogel, Graf Zeppelin, and primarily on P. domesticum. Mrs. Layal were also tested, and all found to be susceptible to attack by the fungus. Symptoms The symptoms of Verticillium wilt aiid the The first symptoms of the disease on P. stem rot phase of bacterial stem rc )t and hortorum consist of the collapse of a few leaf spot are similar in many respect:;. The leaves in the middle or upper portion of the early symptoms of both disease!3 are main stem or side branches of the plant. At wilting of leaves, followed by defoliation, dieback of branch tips, and brown to black Third, only cuttings derived from carefully discoloration of the main stem and maintained culture-indexed stock should branches. It is possible that plants infected be used in geranium production. with V. albo-atrum have been mistakenly diagnosed as cases of bacterial stem rot References and vice versa. It is also possible that one Baker, K. F., W. C. Snyder, and H. N. plant could be infected by both organisms Hansen. 1940. Some hosts of at the same time. The only way to make a Verticillium in California. Plant Dis. positive diagnosis is by making a culture Rept. 24:424-25. from the plant in question. Bolay, A. 1966. La rouille du Pelargonium Pathogen zonale L'Herit. Rev. Hort. Suisse 39:2- 5. This disease is caused by the fungus Braun, H. 1924. Geranium stem rot caused Verticillium albo-atrum, which produces by Pythium complectens n. sp. Host large numbers of single-celled hyaline Figure 25. Verticillium wilt, Infected plant resistance reactions: Signif~canceof conidia on verticillately branched of cultivar Springfield White showing Pythium-type sporangial germination. severe symptoms. Notice the brown to conidiophores. The conidia are short lived and are seldom seen in nature on infected J. Agric. Res. 29:399-419. black discolored areas on the branches. - 1925. Comparative studies of plants. The fungus also produces . microsclerotia, which are long-lived Pythium debaryanum and two related resistant structures capable of carrying the species from geranium. J. Agric. Res. 30:1043-62. fungus over long periods of time when Dimock, A. W. 1940, Importance of environmental conditions are unfavorable Vert~cillium as a pathogen of for growth and reproduction. Under some ornamental plants. Phytopath. conditions microsclerotia have survived 14 30:1054-55. years in the absence of a suitable host. In 1959. Effective control of addition to survival by means of . geranium diseases. Florists' Exchange microsclerotia, the fungus may also survive Hort. Trade World 132:15. on infected, symptomless weed hosts. One Dimock, A. W., R. E. McCoy, and J. F. such weed host is Solanum sarachoides, a Knauss. 1968. Pelargonium rust. A nightshade weed. Infected nightshade new geranium disease in New York plants, despite extensive infection of the State. N.Y. State Flower Growers Bull. xylem by the fungus, are symptomless, 268:1, 3. Figure 26. Verticillium wilt. Plant of cultivar except for occasional reddish brown Dodge. B. 0. and M. E. Swift. 1932. Black Irene showing a healthy plant (right) and vascular discoloration in older plants. an infected plant (left). Note the collapse stem rots and leaf spot of Pelargonium. of all the foliage on the infected plant. Environmental Relations J. New York Bot. Garden 33:97-103. The fungus Verticillium albo-atrum can Doidge, E. M. 1926. A preliminary study of be spread in infested soil and in the South African rust fungi. Bothalia symptomless but ~nfected cuttings. The 2198-99. pathogen can survive in so11 for long Englehard, A. W. 1982. Alternaria leaf spot. periods of time in the absence of a suitable In Geraniums Ill, ed. J. W. Mastalerz host. Therefore, treatment of propagating and E. J. Holcomb, pp. 193-94. Penn. media and potting media is necessary for Flower Growers, University Park. disease control. Evans, S. G. 1966. New or uncommon plant diseases and pests. Rust Control (Puccinia pelargonii-zonalis) of The control of this disease involves Pelargonium. Plant Path. 15:144. several steps. First, all propagation and Fletcher, J. T., and C. L. J. Ryan. 1966. A growing media should be treated with leaf spot of Pelargonium caused by steam. In addition, the containers, produc- Alternaria tenuis Nees ex Pers. Plant tion benches, tools, and the like used in Path. 15:69-70. geranium production should also be Forer, L. B., and L. P. Nichols, 1982. Rust. Figure 27. Vertic~lliumwilt. Healthy plant In Geraniums Ill. ed. J. W. Mastalerz (right) and infected plant (left) of treated with steam. Second, careful sanitation should be practiced to prevent and E. J. Holcomb, pp. 206-7. Penn. Pelargonium domesticum showing Flower Growers, University Park. stunting of the infected plant. contamination of steam-treated media. Gibbs, J. G. 1936. Pelargonium rust. New . 1962. Verticillium control must be Stoddard, E. M. 1957. A Fusarium rot of Zealand J. Agric. 3:142-47. considered when indexing geraniums. geraniums and its control. Plant Dis. Gill, D. L. 1936. Geranium cutting rots and Florists' Rev. 130(3363):11-12, 21. Rept. 41 536. their control. Florists' Exchange Hort. Manning, W. J., and M. Glickman. 1969. Torgeson, D. C. 1952. Observations of Trade World 87:17. Effectiveness of several systemic and Verticillium wilt of geranium in Oregon. Grouet, D. 1936. La rouille du Pelargonium nonsystemic fungicides in the preven- Plant Dis. Rept. 3651. zonale. Traitement par thermotherapie. tion of Botrytis blight of geranium Wehlburg. C. 1970. Pelargonium rust Ann. Epiphyt. 16:315-31. cuttings and stock plants. Plant Dis. (Puccinia pelargonii-zonalis) in Florida. -. 1967. La rouille du Pelargonium Rept. 53:412-15. Plant Dis. Rept. 54:827-28. zonale. Rev. Hort. 139:221-25. Manning, W. J.. P. M. Vardaro, and E. A. Haas, J. H. 1962. The pathogenicity of Cox. 1973. Root and stem rot of Thielaviopsis basicola (Berk. & Br.) geranium cuttings caused by Ferraris to Pelargonium x hortorum B. Rhizoctonia and Fusarium. Plant Dis. Ph.D. thesis, Penn. State Univ., Rept. 57:177-79. University Park. 104 pp. Miller, H. N.. and R. J. Sauve. 1975. Haas, J. H., and J. Tammen. 1962. Etiology and control of Pythium stem Pathogenicity of Thielaviopsis basicola rot of geranium. Plant Dis. Rept. to Pelargonium hortorum. Phytopath. 59:122-26. 523 2. Munnecke, D. E. 1956. Alternaria leaf spot Harwood, C. A,, and R. D. Raabe. 1979. of geranium. Plant Dis. Rept. 40:452- The disease cycle and control of 54. geranium rust. Phytopath. 69:923-27. Nelson, P. E. 1962. Diseases of geranium. Herr, L. J. 1966. Thermal dusting with N. Y. State Flower Growers Bull. tetrachloroisophthalonitrite for control 201 :I -1 0. of Botrytis on geranium. Plant Dis. Nelson, P. E.. and L. P. Nichols. 1982. Rept. 50:777-79. Verticillium wilt. In Geraniums Ill, ed. J. Horst, R. K., and P. E. Nelson. 1975. W. Mastalerz and E. J. Holcomb, pp. Diseases of chrysanthemum. Cornell 229-32. Penn. Flower Growers, Univer- Cooperative Extension Information sity Park. Bull. 85. Ithaca, N.Y. 36 pp. Nichols, L. P., and L. B. Forer. 1972. Hyre, R. A. 1972. Effect of temperature and Geranium rust discovered in Pennsyl- light on colonization and sporulation of vania. Plant Dis. Rept. 56:759. the Botrytis pathogen on geranium. Nichols. L. P., and P. E. Nelson. 1971. Plant Dis. Rept. 56:126-30. Foliage diseases. In Geraniums, a Jannone. G.. and G. Binaghi. 1966. Su due manual on the culture, diseases, gravi malattie delle piante ornamentali insects, economics, taxonomy, and di recente reperimento in Provincia di breeding of geraniums, 2d ed.. ed. J. Genova. Lanterna Agric. 6:81-88. W. Mastalerz, pp. 21 2-1 6. Penn. Lieber, E. 1966. Uber den Pelargonienrost Flower Growers, University Park. (Puccinia pelargonii-zonal; Doidge) - . 1982. Pythium blackleg. In und seine Bekampfung. Gesunde Pfl. Geraniums Ill, ed. J. W. Mastalerz and 18:245-48. E. J. Holcomb, pp. 208-9. Penn. Linderman, R. G. 1982. Thielaviopsis black Flower Growers, University Park. root rot. In Geraniums Ill, ed. J. W. Phillips, D. J., and A. H. McCain. 1973. Hot- Mastalerz and E. J. Holcomb, pp. 21 0- water therapy for geranium rust 16. Penn. Flower Growers. University control. Phytopath. 63:273-75. Park. Powell, C. P. 1982. Diseases of seedling McCoy, R. 1975. Susceptibility of geraniums. In Geraniums Ill, ed. J. W. Pelargonium species to geranium rust. Mastalerz and E. J. Holcomb. pp. 161- Plant Dis. Rept. 59:618-20. 65. Penn. Flower Growers, University McWhorter, F. P. 1962. Diverse symptoms Park. in pelargonium infected with Stienstra. W. C., B. J. Jacobsen, and N. A. Verticillium. Plant Dis. Rept. 46:349- Anderson. 1973. Pelargonium rust, 53. Puccinia pelargonii-zonalis, discovered in Minnesota. Plant Dis. Rept. 57:890. The economic significance of virus Tobacco Ringspot Virus diseases affecting commercial geraniums has been difficult to assess. None of the 12 Importance known viruses affecting geraniums will Tobacco ringspot virus (TRSV) was first alone kill a plant. Viruses may cause slower reported by Kemp on geranium In 1967 in rooting, fewer flowers, poor production, Ontarlo, Canada. The virus is transmitted nonuniformity, and unsightly appearance. by the nematode Xiphinema americanum. The assessment of the economic Seed transmission is characteristic of most importance of geranium virus diseases is nematode-borne viruses, and TRSV is complicated by the fact that many known to be transmitted in seeds of commercial cultivars are virus infected and soybean, petunia, and Gomphrena globosa also that symptoms may be relatively mild and to the seed of geranium through the or not apparent under certain environ- maternal tissue, but not by the pollen. mental conditions. Most commercial Symptoms production of geraniums has been primarily accomplished by vegetative propagation, Symptoms on the foliage may consist of which perpetuates the spread of virus. various combinations of vein yellowing and Figure 28. Tobacco and tomato ringspot. There has been increased interest in the yellow to necrotic spots, rings, and line Yellow rings and spots on leaves of commercial use of seedling geraniums. patterns (flg. 28). Symptoms are usually geranium caused by tobacco ringspot Because viruses are not generally masked at temperatures of 16°-180 C and tomato ringspot viruses. transmitted through seed, it has been assumed that virus problems could be avoided by this means of propagation; but it Table 1. Symptoms on geranium and indicators for viruses affecting geraniums is now known that some viruses that affect Virus Major symptoms Indicator species * geraniums are seed transmitted. Heat therapy and meristem tip culture Tobacco ringspot, Yellow to necrotic spots, rings 2-5 have been used to remove viruses from Tomato ringspot, and line patterns on leaves vegetatively propagated crops including Pelargonium ringspot many ornamentals. These procedures have been applied by a limited number of Pelargonium ring pattern Yellow ringspots on mature major suppliers of commercial geraniums. leaves of P, zonale The establishment of geranium propagating Pelargonium line pattern Yellow to green spots and line material free of speciflc viruses is based on patterns along veins on leaves the ability to index for viruses and the development of a suitable medium to Pelargonium leaf curl Yellow and brown asteroid spots regenerate shoots and roots from on leaves; leaf distortion meristematic tips, as well as adequate Cucumber mosaic Purple spotting along the veins of survival of heat-treated geraniums before leaves; yellow mottling, vein meristematic tip culture. Such procedures clearing, vein banding on leaves are now available for geranium certification [I programs. Certification programs are Pelargonium flower break Color streaking on flowers 1 I described under "General Control Tobacco mosaic Not established Measures." Although most of the described viruses occur in low concentrations in Tobacco rattle Not established pelargonium and may be difficult to isolate Tomato black ring Not established 1 -3,5-7 and identify in this host, all can be cultured in Nicotiana clevelandii, and most can then Not established 1-357 be assayed in Chenopodium quinoa. Table NO^ established Yellow vein clearing on leaves 9 1 summarizes symptoms on geranium and (yellow net vein) indicator plants for viruses affecting geranium. *I. Chenopodium amaranticolor (goosefoot) 6. Petunia hybr~ds(petunia) 2. C, quinoa 7. Phaseolus vulgaris (bean) 3. Cucumis salivus (cucumber) 8. Vigna sinensis (cowpea) 4. Nicot~anacleveland~i 9. Susceptible geranlum cullivars 5. N, tabacum (tobacco) (60'-65' F). Infection by TRSV also indexed geraniums and the efforts of Pelargonium Flower Break reduces the number of florets and growers to maintain geranium stock in increases the number of aborted buds per glasshouses where disease and pest Pelargonium flower break virus (PFBV) inflorescence. In England the disease is control is more readily attained. The has been reported to cause disease in rare called pelargonium ringspot. During the relationship of PLCV to petunia asteroid instances and is, therefore, thought to be of spring clear ringspot symptoms can be virus would indicate that this virus is little importance to commercial geranium observed in P. peltatum and a line pattern probably soilborne. Although the vector has production. PFBV is serologically different in P. zonale. Pelargonium ringspot is a not been identified, it is thought to be from all other viruses known to infect catchall name for diseases caused by the transmitted by a fungus. geraniums. The virus is difficult to detect in ringspot viruses resulting in similar infected cultivars, and most cultivars are symptoms. Stone in 1980 indicated that Symptoms symptomless after infection. Attempts to neither TRSV or tomato ringspot virus The symptoms of PLCV are distinctly infect P. peltatum have been unsuccessful. alone is capable of producing typical different from other geranium virus Susceptible cultivars show flower color pelargonium ringspot symptoms. However, diseases. Yellow stellate spots develop breaking, which appears as light-colored studies in the United States and Europe only during the winter months. These spots streaks in petals. Growth is retarded, reveal that those viruses independently become necrotic with age, distortion and a flowers are smaller, and rugose petals have can produce symptoms. crinkled appearance occurring as the been observed. leaves expand. PLCV is readily isolated Pelargonium Mosaic Tomato Ringspot Virus and sap transmissible from leaves showing symptoms, but is difficult to detect in Pelargonium mosaic is sometimes called Importance symptomless leaves, especially during leaf break. Mosaic has been reported in the Tomato ringspot virus (TomRSV) was summer months. PLCV induces distinct United States and Denmark. Cucumber first reported by Kemp on geranium in symptoms in N. clevelandii inoculated with mosaic virus (CMV) is thought to be the 1969 in Ontario, Canada. The virus is juice from infected geraniums. cause of mosaic and leaf break. Mosaic- transmitted by the nematode Xiphinerna like symptoms include chlorotic mottling, americanum. TomRSV is transmitted in Pelargonium Ring Pattern vein clearing, and vein banding in younger seeds of soybean, red clover, strawberry, and partly in older leaves of geranium and raspberry and to the seed of geranium Pelargonium ring pattern virus (PRPV) has been confused with pelargonium infected with CMV; CMV failed to induce through the maternal tissue and pollen. such symptoms in P. ilngspot caused by TRSV and TomRSV, P. peltaturn and zonale in England, but appeared to Symptoms but is now known to be a distinctly different virus. PRPV does not infect P. peltaturn, enhance symptoms of infections with other Symptoms on the foliage associated with viruses. In leaf break symptoms CMV TomRSV infections are similar to those and thus, it differs from TRSV and TomRSV. True ring patterns are produced in P. interferes with anthocyanin formation described for infections with TRSV (fig. 28). resulting in replacement of the normal leaf Florets of geraniums infected with TomRSV zonale, and it is fairly common in older cultivars. It has sometimes been isolated zone with purple spotting along veins. contain more aborted anthers and a There is no reduction in leaf size or from recently introduced seedlings. greater percentage of nonviable pollen distortion. Symptom development is grains than do florets of TRSV-infected Symptoms appear as yellow ringspots in the older leaves of plants 10 to 12 months seasonal with a masking of symptoms plants. Furthermore, plants infected with during the summer. TomRSV, when self-pollinated, produce after infection. No vector is presently fewer fruit per pollination, seed per known for this virus. Yellow Net Vein pollination, and viable seed than healthy Pelargonium Line Pattern self-pollinated plants. Yellow net vein is suspected to be Pelargonium line pattern virus (PLPV) caused by a virus, but none has yet been Pelargonium Leaf Curl was first described in England by Hollings identified. The symptoms are distinctive and considered beautifully ornamental by Importance and Stone in 1976 and 1977 and in Yugoslavia by Plese and Stefanac in 1980. some. Veins on both young and old leaves Pelargonium leaf curl (PLCV) has also PLPV has been characterized as distinctly are yellow throughout the year (fig. 29). been called crinkle. PLCV was first different from PRPV, TRSV, TomRSV, and The remainder of the affected plants described in the United States by Jones in flower break virus. Symptoms do not appear to be normal. Sometimes affected 1940 and later in England by Hollings in develop in pelargonium until 18 months plants exhibit a faded yellowing of leaf 1962. When this disease was first reported after infection and usually develop during veins. Manganese deficiency may cause and characterized, it was frequently found winter months. Leaves of infected plants similar symptoms. in pelargonium; but in recent years it has exhibit yellow to green spots and line not been frequently observed. This may be patterns, particularly along veins. associated with the availability of virus- Chart 1. Diagram for control of viruses and wilt pathogens in pelargoniums Other Viruses Several other viruses have been reported cuttings to increase to occur in geranium and include tobacco plots for produclion mosaic, arabis mosaic, tobacco rattle, tic- (' YO tomato black ring, an unidentified bacilliform, and an unidentified isometric virus. It is unclear whether these viruses incite specific diseases with which specific symptoms can be associated and whether they are economically important to commercial geranium production. -see char! 3 tr~pleculture General Control Measures Index (4 mo) -see chart 2 Detection of Viruses by Indexing Plants of any pelargonium cultivar can heat therapy and be freed of viruses known to lnfect them by shoot apex culture heat treatment and meristem tip culture (6-8 mo) procedures diagrammed in chart 5. The -see char! 4 CVI@ manner in which virus-indexing (charts 3 cert~f~cation and 4) fits into a complete program, which includes culture-indexing (chart 2) to vegetate to remove vascular pathogens, as well as supply Index heat therapy and shoot-tip culture to free pelargon~umsfrom known viruses, is illustrated in chart 1. The time required is illustrated in chart 1. The time required for plant renewal following this program to completion to for annual Index cuttings from obtain either culture-indexed (CI) production to certtfication or culture-indexed-virus- commercial growers nonlndexed plant indexed (CVI)R certification is indicated in the diagram as well as the complexity of such a program. A detailed description of virus-indexing follows: 1. Leaf samples are removed from young plant growth to obta~npelargonium sap for testing for viruses known to infect pelargonium (charts 3,C; 4.C). The sap is rubbed on Chenopodium quinoa, Gornphrena globosa, Vigna sinensis cv. Big Bov and Cucumis sativus cv. Chicaao pi&ling, which are used as test plants. - Figure 29. Yellow net vein. Prominent 2. One gram of leaf samples is ground in yellowing Iz~f veins of geranium leaf, which phosphate buffer (0.05-0.33 M) at pH 7- is thought to be caused by a virus not yet 7.2 containing 3-4% polyethylene glycol identified. MW 6,000 (PEG). 3. A fine carborundum powder is dusted on young test plant leaves, and the pelargonium sap is rubbed on the leaves. Hands must be washed before inoculating each plant, or care must be exercised by using tissue paper to handle leaves during inoculation (which can be discarded after Chart 2. Culture index each inoculation) to prevent viral contamination from plant to plant. 4. Inoculated leaves should be rinsed off with a fine water mist soon after inoculation. 5. Test plants should be maintained at low light (4,000 lux) and high temperatures (27" C [80° F]) to obtain good symptom development. 6. Test plants must be kept free of insects. 7. Final read-out can be made 14-30 days after inoculation (chart 3,D,E,F). Symptoms on test plants are conspicuous spots on leaves, which differ somewhat with infections from different viruses. Because virus titer may be low in geranium, N. clevelandii can be inoculated with juices from geranium for 14 days to build high virus titer, followed by inoculation to test plants for symptom expression for virus index. 8. Mother plants found to be infected with virus can be heat treated at 37" C (1 00" F) for 8-1 0 wk (chart 5J\,B); afterwards, shoot tips are removed (chart 5,C) and cultured as shown in figure 30. a) All influorescence must be removed from pelargonium plants to be heat treated. b) Heat therapy begins by placing Chart 3. Virus bioassay plants to be treated in a heat chamber with a 16-hr photoperiod. c) The initial temperature should be 24" C (75" F) day and 21 " C (70" F) night. d) A 3" C (5" F) daylnight differential beina maintained, the temperature within the chamber should be raised 3" C (5" F) daily until a final setting of 38" C (1 00" F) day and 35" C (95" F) night is obtained. e) Plants are maintained under these conditions until either 4 weeks of heat therapy has been achieved or serious plant deterioration occurs. f) Shoot tips (0.5-1.0 mm) are removed from the plants at this time. 9. Shoot tips are removed with the aid of a stereomicroscope (1 5x-40x) and sterile tools and placed on a defined nutrient culture medium. 10. By removing a shoot tip and growing a plant from this as shown in figure 30, one can obtain plants free of viruses that affect pelargoniums and are tested in the indexing system. Plants must be virus- Figure 30. Geranium meristem tip culture. A, Meristem tip with leaf initials (geranium X30). B, Proliferation before differentiation (geranium, 10 days after culture). C, Differentiation and regeneration oi shoots and roots. D, Plantlet transplanted to sterile Cornell peat-lite mix.

I ! indexed again to assure that they are clean (chart 5,D,E). Cuttings from these plants should be flowered to determine cultivar trueness. 11. All mother plants that test negative for the known pelargonium viruses must be maintained in an area isolated from all plant material that has not been virus-indexed. 12. Cuttings from mother plants certified to be free of specified pelargonium viruses are then used to plant and increase numbers of plants for production (charts 1; 3,G; 4,H). 13. An alternate method to index for viruses affecting pelargonium is enzyme- Chart 4. Virus seroassay linked immunosorbent assay (ELISA). This method is presently available for TRSV and TomRSV. a) Leaf samples are removed from young plant growth to obtain pelargonium sap for ELISA (chart 4,C). b) Virus specific antibody is adsorbed to ELISA plates (chart 4,D). 49J %pp c) Pelargonium sap containing l day possible virus is added (chart 4,E). triple culture d) Enzyme-labeled specific antibody is --I(-- _ added (chart 4,F). e) Finally, enzyme substrate is added (chart 4,G). f) Virus presence is determined by color formation resulting from hydrolyzation of enzyme substrate. All procedures that involve handling of cuttings to increase cuttings from CI- and VI-certif~ed plants plots for production must be done in a manner to avoid juice contamination. Tissue paper shields, sterile forceps, and (or) sterile razor blades can be CVl@ used to take these cuttings. Forceps, razor certification blades, or knives can be sterilized by dipping in alcohol and flaming before each cut. D = antibody adsorbed to plate Obvious improvement in geranium E = test sample (pelargonium sap) added containing possible virus flower production is shown in figure 31 of F = enzyme-labeled specific antibody added plants that have been VI certified. G = enzyme substrate added; color intensity indicates virus titer Figure 3 7. Comparative meristem tip cultured and nonmeristem tip cultured geraniums. Cultivar Wendy Ann on the left was grown from meristem tip cultured tissue and exhibits increased vigor and earlier flowering compared with the Wendy Ann on the r~ght,which was derived from nonmerlstem tip cultured tissue. The plants are the same age from the time of rooting.

Chad 5. Heat therapy and shoot tip culture References Abo El-Nil, M. M., and A. C. Hildebrandt. 1971. Differentiation of virus- symptomless geranium plants from anther callus. Plant Dis. Rept. 55:1017-21. Anonymous. 1959. Some diseases of geranium. Agric. Gaz.. N.S.W. 70(8):412-17. -- -- Baker, K. F., and S. H. Smith. 1966. I Dynamics of seed transmission of CI & CVl@ plant pathogens. Ann. Rev. Phytopath. VL&j(6 wk) ,certificat~on 4:311-34. \, virus Boettcher, A. D., and A. C. Hildebrandt. Index 1964. Growth and differentiation of geranium apical meristems in vitro. Phytopath. 54389. \flower check Fulton, J. P. 1962.Transmission of tobacco (8 wk) ringspot virus by Xiphinema americanum. Phytopath. 52:375. Gippert, R.. and K. Schmelzer. 1973. Erfabrungen mit Spitzenmeristem- Maintenance of Virus-Tested Plants medium; therefore, it is helpful to place a kultren von Pelargonieu (Pelargonium zonale -Hybrids). Arch. Phytopath. U. Plant material that has been freed of portion of a sponge-rubber test-tube plug specific viruses is not immune. Good in the medium to be sterilized along with Pflanzenschutz. Berlin. 9:353-62. Hollings, M. 1962. Studies of pelargonium management is required to prevent the medium on which the slices are placed reinfection. Control of insects and during culture. This plug absorbs the debris leaf curl virus. I. Host range, nematodes is important because they may so that bacterial growth can be identified transmission, and properties in vitro. Ann. Appl. Biol. 50:189-202. transmit geranium viruses. Plant handling below the plug in the nutrient medium. . 1965. Disease control through must be held to a minimum, for these Nutrient medium cultures showing any - viruses may also be spread by handling fungus (chart 2,H) or bacterial growth virus-free stock. Ann. Rev. Phytopath. first a virus-infected plant and then a clean (chart 2.1) after 14 days are discarded 3:367-96. plant. Sanitation is important because plant along with the cuttings from which the Hollings, M., and 0. M. Stone. 1965. Studies of pelargonium leaf curl virus. debris and unsterilized soil may serve as a slices were removed. Cultures exhibiting contamination source. Therefore, the best no fungus or bacterial growth (chart 2,G)in 11. Relationships to tomato bushy stunt and other viruses. Ann. Appl. Biol. method for geranium virus control and the nutrient medium indicate thal the maintenance of virus-tested plants is to cuttings have successfully passed the first 56187-98. Hollings, M., 0. M. Stone, and W. T. Dale. take cuttings only from symptomless plants culture index. These cuttings are rooted in 1972. Tomato ringspot virus in and to use a program that includes individual containers (chart 2,A) while pelargonium in England. Plant Pathol. sanitation and effective insect and awaiting the first culture readout. Cuttings 21 146-47. nematode control. are then removed from these rooted plants for a second culture (chart 2,C) to locate Horst, R. K. 1982. Controlling virus Detection of Fungus and Bacterial those infected cuttings that may have been diseases. In Geraniums Ill, ed. J. W. Vascular Wilt Pathogens by Culture- missed during the first culture index. Mastalerz and E. J. Holcomb, pp. 253- Indexing Culture-indexed successes are again 61. Penn. Flower Growers, University Culture-indexing is performed by rooted (chart 2,D) followed by cutt~ng Park. removing thin slices obtained aseptically removal for a third culture index (chart 2.E). Horst, R. K., H. T. Horst, S. H.Smith, and W. from the base of a cutting (charts 2,B;3,B; Culture-indexed successes from the third A. Oglevee. 1976. In vitro regeneration 4,B) and placing the slices in a nutrient culture index are rooted (chart 2,l)and thus of shoot and root growth from medium such as a nutrient broth become the CI-certified mother plants. meristematic tips of Pelargonium X supplemented with dextrose (chart 2,F). The culture-index procedure is done hortorum Bailey. Acta Hort. 59:131- Slices from pelargonium often have debris before heat therapy, for plants infected with 41. Jones, L. K. 1940. Leaf curl and mosaic of associated with them that can be confused X. pelargonii (bacterial vascular wilt with bacterial growth in the culture pathogen) will not survive heat treatment. geranium. Washington Agric. Exp. Sta. Bull. 39011-1 9. Kemp, W. G. 1966. The occurrence of geraniums of the cv Diddens Improved reproductive tissues of Pelargonium X yellow-net vein virus in geraniums in Picardy. Proc. Am. Phytopath. Soc. hortorum. Phytopath. 67:292-97. Ontario. Can. J. Plant Dis. Surv. 2:103. Smith, S. H., R. K. Horst, and W. A. 46(3):81-82. Paludan. N. 1968. Virus diseases in Danish Oglevee. 1974. Rooting of geranium - . 1967. Natural occurrence of geranium cultures - pelargonium leaf meristem-tip cultures reduced by virus tobacco ringspot virus in pelargonium curl virus (PLCV). Tidsskr. for Planteavl infection. Phytopath. 64:585-86. in Ontario. Can. J. Plant Sci. 47:295- 72121 1-1 6. Sodergren. W. 1967. Pelargonviroser- 300. - . 1976. Virus diseases in symptom, diagnostik och bekampning. - . 1969. Detection of tomato Pelargonium x horiorum especially (Pelargonium viruses-symptoms, ringspot virus in pelargonium in concerning tomato ringspot virus. diagnosis, and control). Vaxtskydd- Ontario. Can. J. Plant Dis. Surv. 49:l- Acta Hort. 59:119-30. snotiser 32 (3-4):51-63. 4. Pillai, S. K., and A. C. Hildebrandt. 1968. Stone, 0. M. 1974. Some properties of two Kivilaan, A. 1957. Ringspot and other Geranium plants differentiated in vitro spherical viruses isolated from disorders of pelargonium that are from stem tip and callus cultures. geraniums in Britain. Acta Hort. caused by graft-transmissible agents. Plant Dis., Rept. 52:600-601. 36:113-22. Phytopath. 47:29-31. Plese, N., and Z. Stefanac. 1980. Some -. 1980. Nine viruses isolated from Kivilaan, A,, and R. P. Scheffer. 1959. properties of a distinctive isometric pelargonium in the United Kingdom. Detection, prevalence, and virus from pelargonium. Acta Hort. Acta Hort. 110:177-82. significance of latent viruses in 1 10:183-90. Stone, 0. M., and M. Hollings. 1973. Some pelargonium. Phytopath. 49:282-86. Reinert, R. A,, A. C. Hildebrandt, and G. E. properties of pelargonium flower break Lawson, R. H. 1971. Virus diseases. In Beck. 1961. Geranium viruses: virus. Ann. Appl. Biol. 75:15-23. Geraniums, a manual on the culture, Geranium diseases are widespread; 1976.Ann. Rept. Glasshouse Res. diseases, insects, economics, here are the symptoms to look for. Inst. for 1975, pp. 119-20. taxonomy, and breeding of geraniums, Florists' Rev. (Aug.):l7-19, 75, 77-8. -. 1977. Ann. Rept. Glasshouse Res. ed. J. W. Mastalerz, pp. 252-62. Penn. -. 1962. Characterization of five Inst. for 1976, pp. 121 -22. Flower Growers. University Park. geranium virus isolates and in vitro Thorn-Horst, H., R. K. Horst, S. H. Smith, McWhorter, F. P. 1957. Virus diseases of growth of geranium stem tips. and W. A. Oglevee. 1977. A virus- geranium in the Pacific Northwest. Phytopath. 52:749. indexing tissue culture system for Plant Dis. Rept. 41 :83-88. -. 1963. Differentiation of viruses geraniums. Florists' Rev. 160:28. 29, Murashige, T. 1974. Plant cell and organ transmitted from Pelargonium and 72-74. culture methods in the establishment horiorum. Phytopath. 53:1292-98. Walsh, D. M., R. K. Horst, and S. H. Smith. of pathogen-free stock. A. W. Dimock Romaine, C. P., and R. H. Lawson. 1982. 1974. Factors influencing symptom Lecture no. 2. Cornell University, Viruses. In Geraniums Ill, ed. J. W. expression and recovery of tobacco Ithaca, N.Y. Mastalerz and E. J. Holcomb, pp. 242- ringspot virus from geranium. Murdock. D. J., P. E. Nelson, and S. H. 52. Penn. Flower Growers, University Phytopath. 64:588. Smith. 1976. Histopathological Park. examination of pelargonium infected Russo. M., M. A. Castellans, and G. P. with tomato ringspot virus. Phytopath. Martelli. 1979. Rhabdovirus-like 66844-50. particles in English ivy (Hedera helix) Newhart, S. R., C. P. Romaine, and R. and ivy-leafed geranium (Pelargonium Craig. 1980. A rapid method for virus- peltatum). Phytopath. Z. 96:122-31. indexing the florist's geranium. Hort. Ryden, K. 1972. Pelargonium ringspot-a Sci. 15:811-13. virus disease caused by tomato Nyland, G., and A. C. Goheen. 1969. Heat ringspot virus in Sweden. Phytopath. therapy of virus diseases of perennial Z. 7311 78-82. plants. Ann. Rev. Phytopath. 7:331- Scarborough, B. A,, and S. H. Smith. 1975. 54. Pollen viability and anther abortion in Oglevee-O'Donovan, W. A. 1982. Culture- tobacco ringspot and tomato ringspot indexing for vascular wilts. In virus infected geraniums. Proc. Am. Geranium Ill, ed. J. W. Mastalerz and Phytopath. Soc. 2:128. E. J. Holcomb, pp. 233-41. Penn. - . 1975. Seed transmission of Flower Growers, University Park. tobacco and tomato ringspot viruses Oglevee, W. A.. S. H. Smith, and R. K. in geranium. Phytopath. 65835. Horst. 1975. Isolation of tobacco -. 1977. Effects of tobacco and ringspot virus from symptomless tomato ringspot viruses on the Edema The control of plant pathogenic micro- organisms by fungicides involves protection Symptoms of edema appear on the and therapy. Treatment with fungicides lower leaf surface, on petioles and stems before the pathogen infects the plant is as tiny pimplelike blisters (fig. 32). Later protection, and treatment with fungicides these blisters enlarge and become brown after the plant is infected is therapy. and corky (fig. 33). Affected leaves may Protection involves killing the pathogen yellow and drop off. or limiting its growth before it attacks the Edema is a nonparasitic condition that plant; this can be accomplished in two often develops dur~ngperiods of cloudy, different ways. In the first case, the cool weather. The occurrence of this fungicide can be applied to the pathogen condition is thought to result from an after it is in contact with the host plant as a Imbalance of water uptake and water loss contact fungicide. In the second case, the by plants. A high level of water absorption fungicide can be applied to the host plant by the roots and a low level of water loss by before the pathogen is on the plant as a the leaves result in excessive water residual fungicide. The production of retention in the cells of the leaves, petioles, geraniums may involve the use of contact and stems. Because of th~s,some cells fungicides, but primarily involves the use of burst and form watery blisters, which later residual fungicides because attempting to become dry and corky (fig. 33). Figure 32. Edema. Early stage showing destroy the pathogen after it has been Edema is most likely to develop when small watersoaked and pimplelike blisters deposited on the plant is too risky under the the soil is moist and warm and the air is on the lower leaf surface of geranium. intense growing conditions of the molst and cool. Such environmental greenhouse or field. For this reason most conditions around the plants result in rapid fungicides used in the production of water absorption from the soil and slow geraniums are the residual type of water loss from the leaves, producing the protective fungicide. A good residual symptoms described. protective fungicide has the following The Irene cultivars of Pelargonium characteristics: horforum and P. peltaturn, the ivy geranium, Remains active for a relatively long are especially susceptible to edema. period of time. To apply a fungicide every Control of this condition is difficult, but day to maintain the necessary toxic~tylevel the following steps will help to minimize the around the clock would be nearly occurrence of edema: impossible and too expensive if it were Space the plants to allow good air possible. circulation. Has good adhesive properties. Do not overwater plants. This is Because the dissemination of most especially critical during periods of cloudy pathogens is favored by rainy weather, the cool weather. Water in the morning so that fungicide must resist the erosive action of the growing medium is not too wet Figure 33. Edema. Brown corky blisters water. overn~ght. on the lower leaf surface of geranium. Has good spreading properties. Avoid wetting the leaves because this Because it is necessary to completely will retard transpiration and favor excessive protect leaf and stem surfaces, the water retention. funaicide should s~readevenlv over the References sulace of the leaf.'~hisis usu~llyaccom- plished by adding a wetting agent to Moorman, G. W. 1983. Geranium edema. reduce surface tension. If too much wetting Penn. Flower Growers Bull. 3485. agent is added to a formulation, runoff may Nelson, P. E. 1962. Diseases of geranium. occur, resulting in lower concentrations of N.Y. State Flower Growers Bull. fungicide than required. Wetting agents 201 :I -1 0. also facilitate penetration of fungicides into Nichols, L. P., and P. E. Nelson. 1982. leaves or stems; if too much is used, Edema. In Geraniums Ill, ed. J. W. phytotoxicity may occur. Mastalerz and E. J. Holcomb, p. 204. Is stable against oxidation in sunlight. Penn. Flower Growers, University Is toxic to plant pathogenic micro- Park. organisms, but nontoxic to the plant. Table 2. The dithiocarbamates, listed by chemical, accepted common and trade names. Is active against a wide range of and their most effective uses pathogenic microorganisms. Actually, most of the presently available fungicides are Accepted Trade Chemical General rather specific. Thus, a particular fungicide common name names name use must be selected for control of a particular disease. Frequently, spray programs are ferbam Fermate, Carbamate ferric dimethyl Foliar diseases devised involving the use of more than one dithiocarbamate fungicide, either as combination sprays or zineb Dithane 2-78, Parzate zinc ethylene Foliar and flower in alternate applications. Formulations of bisdithiocarbamate diseases, rusts two or more fungicides in a single package are marketed for use in the home garden. maneb Dithane M-22. manganese ethylene Foliar disease Many different types of protective Manzate bisdithiocarbamate fungicides are in use today. The following maneb + Manzate D, Dithane manganese ethylene Foliar diseases discussion is limited to those chemicals of zinc M-22-Special bisdithiocarbamate plus zinc demonstrated effectiveness and to those sulfate presently in general use for control of geranium diseases. mancozeb Manzate 200, Dithane zinc ion-maneb coordinationFoliar diseases M-45, Fore product Organic Chemicals ziram Zerlate zinc dimethyldithio- Foliar diseases Organic chemicals were not commonly carbamate used until after World War II. Although they metiram Polyram zinc ethylene bisdithio- Foliar diseases are, in general, more specific than the carbamate complex earlier inorganic fungicides, the level of control they afford is much higher. The carbamates were the first organic chemicals to be commonly used. The first Table 3. Some fungicides other than carbamates carbamate, tetramethylthiuram disulfide (thiram), was developed by the rubber Accepted Trade Chemical General industry as a more-efficient sulfur common name names name use compound for vulcanizing. Although its cyclohexamide Actidione PM 3-(2-(35-dimethyl-2- Powdery mildew fungicidal properties were demonstrated in oxocyclohexy)-2- 1934, not until 1940 did it come into hydroxyethyl) glutarimide general use, and it was used as a base for developing other fungicidal compounds. benomyl Benlate {Methyl I-(butylcarbamoyl)- Leaf spots, Because the carbamates controlled many 2-benzimidazole-carbamate} powdery mildew, foliar diseases that could not be controlled root and crown with the existing fungicides, they are said to rots, and soil have revolutionized fungicidal control of drenches plant diseases. Some carbamates in captan Captan, Orthocide N-{(trichloromethyl) thio)-4- General foliar general use today are listed in table 2. cyclohexene-1,2- diseases Other organic fungicides of importance dicarboximide to the ornamentals industry are benomyl (Benlate), captan, Daconil, and folpet chlorothalonil Daconil 2787, Bravo Tetrachloroisophthalonitrile Foliar and flower (Phaltan) (table 3). These are all fungicides 75, Exotherm Termil, blights with broad-spectrum activity, used for the Termil control of foliar and flower diseases. folpet Phaltan N-(trichloromethyl-thio) Foliar diseases Karathane and Pipron (table 3) are specific phthalimide fungicides for control of powdery mildews. dinocap Karathane WD, Mildex Mixture of 2-(1- Powdery mildew Antibiotics Methylhepty1)-4,6- dinitrophenol and 2-(1- Antibiotics are chemicals derived from Methylhepty1)-4,6- the metabolic products of microorganisms. dinitrophenyl crotonate Penicillin was one of the first to be discovered. Although it has not been piperalin Pipron 3-(2-methylpiperidino) propyl Powdery mildew effective against plant pathogens, other 3,4-dichlorobenzoate Use of Soil ~ungicides!

antibiotics are active against both plant Use of Fungicides Recontamination Problem pathogenic fungi and bacteria. Actidione, used for control of powdery mildew, is an A frequent question regarding the use of The heat treatment of soils at tempera- example of an antibiotic chemical. fungicides is whether the grower should tures of 100" C (212" F) and above results use a spray or a dust. Sprays still take in the eradication of not only the plant Thermal Dusts precedence over dusts simply because the pathogens, but also most of the naturally coverage is better, particularly on the occurring soil microflora. Although certain Thermal dusts are prepared from undersides of the leaves. Although insects spore-forming bacteria and thermophilic chemicals with the characteristic of may move to chemical deposits on the fungi survive these temperatures, for all subliming when heated to about 427' C upper leaf surface, plant pathogens do not; intents and purposes, heat treatment (800" F). The vaporized chemicals and effective control is achieved only if results in a biological vacuum. This condense into particles that appear as a both leaf surfaces are covered thoroughly. vacuum persists for a few days after dense smoke penetrating the entire Proper use of a fungicide includes treatment. Then the surviving microorga- greenhouse atmosphere. The particles selecting the right chemical for a given nisms and the microorganisms introduced then settle out as an extremely fine dust disease. A pathogen can be spread by a from the air begin to colonize the soil. that deposits on plant surfaces. At present fungicide spray if the wrong fungicide is Within a few weeks, the population Daconil is available under the name Termil used. Good disease control requires the exceeds that of the original population. as solid tablets for use as a thermal dust correct chemical, applied to give adequate If plant pathogens are introduced during and under the name Exotherm Termil for coverage, without runoff, at the proper the period of the biological vacuum, when use in smoke generators. Both formulations time, and at regular intervals, determined the competitive, antagonistic, and antibiotic are very effective for control of Botrytis on by residual activities and weathering. activities of the soil saprophytic micro- the foliage and flowers of many ornamental organisms are essentially absent, the plants. pathogens colonize the soil mass rapidly Systemic Fungicides and in high population. The result, upon planting, is a greater loss than might have Systemic fungicides are absorbed by the been experienced if the soil had not been plant through the leaf, stem, or root surface treated. and translocated varying distances within The recontamination problem, the the plant. This distance may be as small as greatest one connected with the heat from one surface to the other or as far as treatment of soils, is a major reason for from the roots to the shoot apex. Benomyl crop failure after treatment. The critical is a systemic fungicide. Systemic fungicides nature of this problem dictates a strict, have the following advantages: (1 ) The continuous sanitation program. plant can be continuously protected Sanitation is and must be an integral part throughout the growing season without of greenhouse or nursery production. Soil repeated application of fungicides. (2) The treatment is the initial step in any produc- systemic can be taken up by the roots and tion practice. Sanitation begins as soon as transported to newly formed tissues. (3) soil treatment has been completed; it ends The systemic is not subjected to only when a crop has been finished and the weathering as are fungicides applied to the soil is again ready for treatment. Sanitation foliage. (4) Unsightly residues on flowers means, in effect, "keeping a clean shop" and foliage can be avoided. (5) The and is stressed because a "dirty shop" systemic can provide a means of affords the soilborne pathogens the means controlling and eradicating vascular wilt I of spread essential to their survival and diseases as well as other internal disorders increase. of plants. (6) Because the toxicant is in the The most-important plant pathogens plant, there is minimal toxic effect to people inciting root diseases are not airborne, but working in the greenhouse during the are dependent upon the mechanical growing season. transfer of infested soil or water or infected plant material for their spread. Soil fungicides offer another method for Table 4. Soil fungicides commonly used to limit colonization of steam-treated soils and controlling the recontamination problem. A pathogens against which they are effective soil fungicide can be defined as any chemical that prevents colonization of Fungicide Name Pathogen (fungi) steam-treated soil by plant pathogens when the chemical is applied to the soil in N-(2,6-dimethylpheny1)-N- rnetalaxyl Subdue 2E Pythium spp., the presence of living plants. Soil fungicides (methoxyacety1)-alanine methyl ester Phytophthora spp. can be distinguished from soil fumigants by N-[(trichloromethyl)thiol]-4- captan (WP)' Broad spectrum three criteria: (1) They are nonvolatile. (2) cyclohexene-l,2-dicarboximide They retain activity over a relatively long period of time. (3) They are not phytotoxic Sodium-p-(dimethylamino) Lesan (WP) Pythium spp.. and can be applied to soil in which plants benzenediazo-sulfonate Phytophthora spp., are growing. To classify a given chemical Aphanomyces into one or the other group may be difficult SPP. because the chemical may show properties Ferric dirnethyldithiocarbarnate ferbam (WP) Broad spectrum of both. An ideal soil fungicide would exhibit the Pentachloronitrobenzene Terraclor (WP) Rhizoctonia spp.. following characteristics: (1) broad- Sclerotinia spp., spectrum activity, that is, activity against Sclerotium spp. many different soilborne plant pathogens; Bis(dimethylthiocarbarnoyl)disulfide thiram (WP), Arasan Broad spectrum (2) little or no activity against the saprophytic microorganisms; (3) activity 'WP =wettable powder. against plant pathogens at concentrations not toxic to plants or man; (4) activity for the period of time required to produce a If a soil mix is used, the correct amount crop. There is no such soil fungicide today, must be weighed out, not measured in but all fungicides meet some of the tablespoons or cups. Not only do requirements (table 4). "tablespoons" and "cups" vary in size, but The activity of most fungicides in soils is the amounts of fungicide will vary fungistatic rather than fungicidal; that is. depending upon whether the material they limit or suppress the growth and "settled" in the package or is "fluffed" out. development of plant pathogens rather Mixing must be thorough to assure an even than kill them. For this reason, their use is distribution in the soil mass. Unless the restricted to application after the plant chance of recontamination is slight, mixing pathogens have been eradicated by steam fungicides with steam-treated soil should treatment to prevent or limit colonization be avoided. should the pathogens be introduced. In applying soil drenches, three steps are essential: Application of Soil Fungicides 1. Weigh or measure out the proper Soil fungicides are formulated as amount of fungicide. wettable powders or liquids. The wettable 2. If a wettable powder is used, powders can be used as a soil mix after continuously agitate the suspension. steaming or in a water suspension as a soil 3. Deliver the proper amount of the drench. The liquids are applied as a soil suspension per square foot of bench space drench and can be metered through or per container. fertilizer proportioners. The method of References application that best fits a given operation can be used. What is important is to apply Baker. K. F., ed. 1957. The University of the correct amount to each plant. Too little California system for producing healthy will result in poor control; too much will container-grown plants. Calif. Agric. result in plant injury. Exp. Sta. Manual 23. 332 pp. Soil Treatment

Heat Treatment As a result, losses due to disease may be for a given job or a larger area is treated in greater than if the soil had not been treated a given time. Poststeaming soil toxicity is The use of heat to eradicate plant at all. This phenomenon has been termed reduced because of the lower temperature. pathogens from infested soil or soil the recontamination problem. The purpose Finally, a portion of the established containers is based upon the fact that most of sanitation is to prevent this problem. saprophytic soil microflora remains and soilborne plant pathogens have low When organic materials high in organic luxuriates, providing competition and (or) thermal death points. Under ideal nitrogen are incorporated into a soil mix antagonism for any pathogens introduced. conditions most plant pathogens are killed and the mix is steam treated, ammonia by heating to 60' C (140' F) for a 30- toxicity may occur. Many of the organisms Chemical Fumigation minute period. that convert organic nitrogen to ammonia A soil fumigant can be generally defined as In general, the higher the moisture are spore formers, and some survive the content of living organisms, the greater a chemical that, when applied to soil, will steam treatment; the bacteria that convert volatilize and diffuse through the pore their susceptibility to killing by heat. In dry ammonia to nitrite and nitrite to nitrate are soils, fungi form resistant resting structures, spaces in amounts toxic to pathogenic not spore formers and are killed. As a microorganisms. Although it is considered which are less susceptible to heat than result, ammonia may accumulate in toxic their actively growing vegetative or to be a substitute for steam, steam does amounts within 2 weeks after treatment reproductive structures. not provide a practical means of treating and remain in toxic concentrations for up to large field areas. In addition, many nursery Although most plant pathogens are killed 8 weeks. Ammonia toxicity can be avoided at 60° C (140' F) when held for 30 and greenhouse establishments do not by using organic matter with no organic minutes, higher temperatures are required have a steam facility. Thus, chemical soil nitrogen. An increase in soluble salt and for killing some pathogens, soil insects, and fumigants have a decided advantage over manganese levels may also occur with steam in the production of field-grown cut weed seeds. Under ideal treating steam treatment. If the increase is great conditions, a temperature of 71 " C (160" flowers, in most nursery establishments, enough, injury may occur. and in small greenhouse operations. F) for 30 minutes should thus be used. Free-flowing steam is steam at Because ideal conditions are seldom Briefly, the advantages of chemical atmospheric pressure. It is at a temperature fumigants are the following: encountered in commercial production, of 100' C (212' F) and contains a temperatures higher than those required They are adaptable to most potential 1,152 Btu for heat transfer. When greenhouse or nursery operations and can must be employed to provide a margin of properly used, it affords the most-efficient safety. The recommended temperature for be used to eradicate plant pathogens from method of heat treating soils to eradicate treating soils is 82' C (180' F), to be held large acreages or small cans of soil. plant pathogenic microorganisms. Treatment does not alter the structure for 30 minutes. Assuming optimum conditions, it is not Steam treatment is the most efficient of the soil. possible to treat soil with free-flowing means of eradicating plant pathogens from Treatment does not result in the steam at less than 100' C (212' F). In release of toxic amounts of soluble salts or soils. Compared with dry heat, steam actual practice, the thermometer is placed imparts a large quantity of heat (over 1,100 manganese (though ammonia toxicity may at the coolest point in the soil mass, and still be a problem). Btu per Ib) at low intensity 100' C (212' F) timing is begun when this point reaches and flows through soil to the cold area. One There are, of course, certain 82' C (180' F). If the volume of steam is of the principal advantages of steam over disadvantages: then reduced to a "trickle," this temperature The fumes, in general, are phytotoxic, dry heat is that the Btu are released at the may be maintained at this point. It should point to be heated. Finally, when soil is and chemical fumigants cannot be used in be evident, however, that most of the soil is the vicinity of growing plants (see tmethyl heated with free-flowing steam, the natural treated at 100" C (212" F). heat ceiling of 100" C (212" F) precludes bromide in table 5). Aerated steam is produced when air is Fumigants may be highly selective in charring organic matter. In addition, steam injected into a free-flowing steam line. The is nonselective, killing all plant pathogenic regard to organisms killed. result is to disperse the steam vapor. When microorganisms at 100' C (212" F). They may be absorbed by the soil or this occurs, the temperature of the steam- by organic matter and leave residues toxic Steam treatment of soil is not without air mixture is reduced. Thus, steam-air at disadvantages. The greatest of these, from to plant growth. 82' C (180" F) will heat soil to 82' C the disease control standpoint, is that In general, they are highly toxic to (180' F) and no higher; condensation and humans. steam is nonselective in the microorga- heat transfer will only occur until the soil nisms it destroys. Not only the plant Principles of fumigant activity. The reaches 82' C (180° F). The same would activity of a soil fumigant is determined by pathogens are killed, but the soil be true of steam-air mixtures of 76.5' C saprophytes as well. If a pathogen is its ability to diffuse through soils and its (170' F), 71 ' C (160' F). 65.5' C (150' toxicity to microorganisms. introduced into newly steam-treated soil, it F), or 60' C (140' F). colonizes the soil mass rapidly and, in the The application costs are reduced absence of competitive and (or) because either a smaller boiler is required antagonistic microorganisms, luxuriates. Table 5. Soil fumigants useful in control of soilborne pathogens of geranium and application procedures

Pesticide Major uses Min. soil temp. Application procedure and precautions Larvacide Weeds, most fungi, nematodes, and 15.5' C (60° F) tchloropicrin usually applied to soil with a hand applicator Picfume soil insects. Controls Verticillium. or a small, self-propelled tractor fumigator. Use at the rate Acquimite of 2-2% cc on 10-in. centers. or 12 IbIl000 sq ft. Heavier Chlor-0-Pic soils require increased dosage. It is important that gas be Niklor confined by sprinkling soil surface thoroughly with water immediately after treatment or, preferably, by covering Tri-Clor with a gasproof cover, which should be left on the soil for (tchloropicrin) at least 24 hr. Soil must be well aerated for 10-21 days before planting. Bulk soil for potting can be sterilized in prepared bins, sealed garbage cans, or drums; use tchloropicrin at 5 cclcu ft of soil. tchloropicrin works best when soil temperature is between 15.5O C and 32' C (60" F and 90" F). Fumes toxic to plants; greenhouse must be empty. Vapan Nematodes, most weeds, and fungi. 10' C (50' F) Apply 1 qtI100 sq ft with a sprinkling can or a hose VPM proportioner. If sprinkling can is used, add required Sistan amount of chemical to can, fill with water, stir, and Trimaton distribute evenly over measured area. Treat only 100- (liquid carbarnate) 200 sq ft at a time; then water-in thoroughly to carry the chemical through the layer to be fumigated. Do not plant for 2-3 weeks after treatment; then make test plantings of seedlings or cuttings, and wait a few days before planting entire crop. If the soil is cold or excessively wet, wait 3-4 weeks. Fumes toxic to plants; greenhouses must be emotv. Vorlex Nematodes, weeds, and fungi. 10' C (50' F) Apply with hand injector or tractor-mounted equipment. Di-Trapex See label for rates. Wait 14-28 days before planting. (mixture of 80% dichloropropene-dichloro- Fumes toxic to plants; greenhouse must be empty. DroDane. 20% methvl isothiocvanatel Dowfume MC-2 Weeds, insects, and nematodes at 1 10' C (50" F) ?Methyl bromide is very poisonous to humans, but can be (98% tmethyl bromide. 2% lb1100 sq fl for 24 hr. At 4 lb1100 sq applied with complete safety when all precautions are tchloropicrin. Under It for 24 hr, bacteria and most fungi followed. It can be used in greenhouses where plants are growing although slight leaf injury has been observed on pressure in 1 - and 1 %-lb are killed. carnations, chrysanthemums, geraniums, and lilies. It is cans and 30-lb cylinders) sold in convenient 1-lb and 1%-lb tin cans for application Verticillium is not adequately with a Jifly applicator or in 30-lb cylinders for large-scale controlled even at higher rates; where treatment. A thoroughly tight seal with a plastic cover is control is desired, use tchloropicrin. necessary to prevent escape of the gas. For sterilizing bulk soil for potted plants, use 4 lbI100 cu ft or approximately 1 Iblcu yd. Soil must be thoroughly aired for 3-7 days before use. Length of airing will depend on soil type, moisture, and temperature. Normally, in clear. warm weather, sandy and other light soils, air in 3-4 days. heavier soils in 3-7 days. WARNING: Do not use bromine-containing fumigants (tmethyl bromide, tmethyl bromide mixtures, and ethylene dibromide) for treating soils to be planted to carnations or salvia. Fumes slightly toxic to plants; can be used in greenhouses with vents open. Brozone Insects and nematodes; fungi, weeds, 10' C (50° F) A preparation of tmethyl bromide in a solvent with a very (Mixture of 68.6% and bacteria only at higher rates. small amount of tchloropicrin, which serves as a warning tmethyl bromide agent, comes in 5- and 30-gal drums, and can be applied with 1.4% with a hand injector. Use 6-8 cc per injection on staggered 10-in. centers. After injection, step on the holes tchloropicrin in to prevent rapid escape of the gas. Use a water seal. Use petroleum solution) of a gastight cover laid over soil gives even better control. Soil should be well aired for several days before planting. Fumes slightly toxic to plants; can be used in greenhouse with vents open. Microfume. Prezervit, Nematodes, many weeds, and fungi. 10' C (50" F) Apply 2 Ib 25% granular Mylone1100 sq ft. Rototill and Crag Fungicide 974. water with 1-2 in. of water. Do not plant until 3 weeks Craa., Nematocide after treatment: then make test plantings of seedlings or (Mylone 25% G) cuttings, and watch for a few days before plantlng ent~re crop. Fumes tox~cto plants; greenhouses must be empty. Nole: Soil fumigants should be used with all precautions given on the label. TRestricted-use pesticide. The ideal soil fumigant would exhibit Soil preparation, temperature, and low chemical reactivity with soil aeration. Because a water seal or plastic components, cover is used in applying most fumigants to low diffusibility, prevent too rapid diffusion from the soil, soil high water solubility. conditions optimum for treating will be low vapor pressure, and those optimum for planting. low adsorption by soil components. The soil should be in good planting The ideal soil for fumigation would have condition, free of lumps or clods. low organic matter conted, Soil temperature should be in the low adsorptive capacity for fumigants, range of 18"-24O C (65'-75' F) for best low continuous air space, and results. low water content. After fumigation, the soil must be The biologic component of the soil thoroughly aerated so that all trace of the (microorganisms, weeds, insects) should fumigant is gone before transplanting. be metabolically active rather than in some 0 In general, the higher the temperature resting phase of the life cycle. and the lighter the soil, the shorter the Soil fumigants and their uses. Although aeration period. none of the presently known chemicals Always use a chemical fumigant on a fulfill all the requirements of the ideal small scale before treating the soil for an fumigant, many fulfill enough of them for entire crop. practical use. Several soil fumigants useful in control of soilborne pathogens of geraniums are listed in table 5. In addition to the specific recommendations for each fumigant, there are a few general rules concerning use of fumigants. Restrictions on usage of these materials vary from state to state. Consult your Extension Service for further information. Special chemicals and dosages required to kill fungi. Fungi are, in general, more difficult to kill than insects, nematodes, and most weed seeds. This fact must be kept clearly in mind when selecting a chemical and when deciding the rate at which it should be used. Growers often make the mistake of assuming that if weeds and nematodes are killed, the fungi are also. Although this is not true, it is a general rule that if the fungi are killed, the weeds and nematodes will be killed as well. Nematocides or herbicides cannot be used to control soil fungi. Acknowledgments

We are grateful for the help and expertise of Howard Lyon in the photog- raphy work. In addition we are indebted to Dr. Wendy A. Oglevee-O'Donovan for her review of the section on virus diseases. Table 1 contains information from the chapter on viruses by C. P. Romaine and R. H. Lawson in Geraniums 111, published by Pennsylvania Flower Growers in 1982. The New York State College of Agriculture and Life Sciences is a statutory college of the State University, at Cornell University, Ithaca. N.Y. 1985

Design: Lorraine Heasley Produced by Media Services at Cornell University Cornell Cooperative Extension-Helping You Put Knowledge to Work This publication is issued to further Cooperative Extension work mandated by acts of Congress of May 8 and June 30, 1914. It was produced with the cooperation of the U.S. Department of Agriculture, Cornell Cooperative Extension, New York State College of Agriculture and Life Sciences, New York State College of Human Ecology, and New York State College of Veterinary Medicine, at Cornell University. Cornell Cooperative Extension offers equal program and employment opportunities. Lucinda A. Noble, Director.