California Orchids resistant to some diseases in habitat become susceptible in greenhouse

Peter A. Ark

Orchid growing in California is a multi- million dollar industry and in the process of expansion. Under natural conditions in the tropics and the pseudobulbs and invades made. orchids are attacked by both fungi and the whence it can spread to other viruses but there are no records to indi- in the clump. The disease is es- cate they are attacked by bacteria or that pecially noticeable after the plants are the and virus diseases cause seri- divided and repotted. Wounding of or- ous losses. chids provides portals of entry for zoo- Changes in the environment, both cli- spores and for new infections. Wounds matic and nutritional, are known to ag- left after flower spikes are cut are an ex- gravate certain diseases. Especially cellent place for the fungus to start the is this true in regards to conditions pre- d'isease. vailing in glasshouses. Control measures for the seedling Feeding grande or- phase of the disease consist in drenching chids with nitrogenous fertilizers usually the pots with copper sulfate solution, one leads to serious attacks of Gleosporium part in 100,000 parts of water. The zoo- spot. Here unbalanced and artificial spores and the fungus mycelium are very conditions-designed for accelerated sensitive to copper. This treatment will growth to produce maximum blooms in eliminate the disease in a short time. Re- the shortest period of time-tend to upset peated applications at the rate of about the natural equilibrium in the plant and One a week are not to the plants. Brown spot of orchids caused by Phytomonos increase the chances for contracting cer- Drenching or submerging the plants in cottleyoe on full-grown . tain diseases. 8-quinolinol benzoate-or sulfate-or the Glasshouse-grown orchids are subject sodium salt of o-hydroxydiphenyl-Natri- to many diseases caused by fungi, bac- phene-in a solution of one part in 2,000 teria and viruses. The most important parts of water, give very good results and from a commercial standpoint' are: Py- is recommended. These organic chemicals thium black rot, brown spot, brown rot, do not possess any residual effects since bacterial leaf scorch and pseudobulb rot. they are denatured by bacteria and light soon after the treatment. Full-grown plants having Pythium Pythium Black Rot black rot should be separated from the Pythium black rot is a disease of seed- healthy ones and dried thoroughly. They lings and of full-grown plants of Cattleya then can live for some time if watered orchids. carefully. Advanced cases of the disease The disease develops in seedlings when should be destroyed. they are planted in community pots. Caused by a fungus, with strict aquatic Brown Spot Bacterial scorch of Milltonio orchidr. Pseudobulb habits, called Pythium ultimum, the dis- rot originating from the rhizome. ease is favored by high humidity, over- Brown spot, a bacterial disease caused watering of the plants, and a high by Phytomonas cattleyae, mainly affects temperature-above 65" F. It commences Phalaenopsis and Cattleya orchids. as a small translucent spot and in a short In Phalaenopsis the disease attacks time extends into the growing point caus- both the seedlings and the full-grown ing the death of the plant. plants. Only full-grown plants of Cattleya The fungus grows only where there is are known to be attacked. free water. Under this condition it pro- On seedlings, water-soaked translucent duces numerous zoospores which swim spots of various sizes are formed, and the in the water. On reaching the surface of disease kills the plants eventually by rot- the plant the zoospores penetrate into the ting the growing point. The spots enlarge cells where they produce a mycelium that and soften the tissues so that the leaf be- spreads rapidly from cell to cell. The at- comes soft and often breaks. The exudate tacked plants wilt and sometimes die found in such cases contains large num- rather suddenly. Since plants in commu- bers of the bacteria which can be spread nity pots are close to each other, the to other parts of the plant or to other disease spreads with phenomenal rapidity plants. Brown rot of Cypripedium caused by Erwinio and losses can be considerable. Continued on page 14 cypripedii. Lesion on a leaf at left.

CALIFORNIA AGRICULTUREy FEBRUARYy 1951 7 TOMATOES 1:1,000. If necessary, the treatments may on the pseudobulb which turns at first a delicate yellow changing into orange red 'Continued from preceding page be repeated within a week or so. or red. The leaves finally drop off and the second hybrid-red crossed with tan- numerous orange red pseudobulbs can be gerine-only the Tt pair segregates, while Soft Rot seen in the pot. From the pseudobulb the all plants have both RR genes. Thus, the Soft rot of Cattleya orchids caused by organism migrates into the rhizome and T gene is dominant over the t gene. In the Erwinia carotovora is a rare disease. Its will travel from plant to plant. case of the third hybrid-yellow crossed advent is sudden and the results are devas- The disease is very infectious and re- with tangerine-both of the gene pairs tating. It is caused by the common soft quires immediate attention as soon as it segregate. rot bacterium-a soil inhabitant-which is first recognized. The control consists From a study of the three hybrids and attacks such crops as celery, carrots, and in applying 8-quinolinol or Natriphene their progeny, it is clear that at least potatoes. as for brown spot in Phalaenopsis. Sani- one R and one T is necessary for the for- The disease can start in fresh wounds tary measures must be observed and op- mation of the red carotenoid pigment, on Cattleya leaves and with high tempera- erators should disinfect their hands after lycopene. The precise role of these two ture and very high humidity it will handling infected Miltonias so as not to gene pairs in the formation of carotenoid rapidly change the leaf into a sack con- spread the disease. pigments is not entirely clear. There are taining liquefied tissues. The leaf wrinkles two uncertainties that will require further and droops. Later it breaks open and the Black Spot study. First, the double recessive rrtt has contents leak out. not yet been identified. Second, certain Plants affected by it can not be saved. The shippers of orchid blossoms also yellow tomatoes collected in Mexico are Control consists in early recognition of have troubles. Vanda blossoms shipped definitely anomalous. When collected, the disease and burning all the affected from a distance sometimes develop a they were classified as doubtful yellows. plants. The room in which the trouble oc- black spot right in the throat of the blos- When grown in Berkeley, they did not curred should be promptly and thoroughly sdm. Sometimes minute black spots scat- behave as pure yellow varieties, but were disinfected. tered on the petals of the flower ruin its consistently intermediate between red market value. The trouble is due to Gb- and yellow. merella sp., a fungus similar in its habits Independent studies carried on at Brown Rot to the gray mold fungus Botrytis cinerea Riverside suggest that three gene pairs Cypripediums are the only orchids in which sometimes attacks the flowers of determining pigment differences segre- California subject to the brown rot dis- Cattleya in greenhouses and in transit. gating in the species cross-Lycopersicon ease in orchid houses. It is caused by a Black spot infection of Vanda flowers esculentum crossed with L. peruvianum- bacterium, Erwinia cypripedii, which occurs before they are cut and shipped. When it reaches its destination the flower one of the green-fruited wild species. This prefers a temperature of 65' F or above fact together with the two uncertainties and a humidity of 70% or higher. begins to lose its color and black spots appear. The fungus develops slowly at mentioned indicate that only a beginning The disease is characterized by small to has been made in the study of inheritance medium-sized circular, somewhat greasy the low temperature prevailing in ship- ment but in the higher temperature of of tomato pigment differences. spots which, on running together, form the sales room the fungus grows and pro- J. A. Jenkins is Associate Professor in Ge- large sunken patches. The color varies duces the black spot. Black spot infection netics, University of California College of Agri- with the age of the lesion and in the culture, Berkeley. of Vanda blossoms has been prevented final stages is deep chestnut brown. The by the use of 8-quinolinol benzoate G. Mackinney is Professor of Food Tech- spots are frequently located close to nology, University of California College of Agri- 1:2,000 as a spray. This concentration of culture, Berkeley. the base of a leaf. Under favorable con- the chemical did not injure the appear- ditions the organisms migrate into the ance of the flowers. crown and thence into other buds causing death of all the living components of the Peter A. Ark is Associate Professor of Plant clump. To save plants already attacked Pathology, University of California College of ORCHIDS Agriculture, Berkeley. Continued from page 7 the treatment described above for brown spot of Phalaenopsis should be applied. The above progress report is based on Re- In full-grown plants the symptoms are search Project No. 973 being conducted by the . similar to those on seedlings. Infections Division of Plant Pathology. start on any part of a leaf but the most Bacterial Scorch dangerous location is at the base because Bacterial scorch and pseudobulb rot DECIDUOUS then the infection quickly moves into the has been observed recently on Miltonia Continued from page 12 growing point and the plant is doomed. Phalaenopsis plants of all ages are orchid hybrids. It is most severe under vail, a serious shortage could result. Such equally susceptible to the disease. conditions favorable for Miltonia grow- a shortage would undoubtedly give an up- In Cattleya brown spot is often con- ing-cool and moist greenhouses. ward boost to the price structure on fined to the upper part of the leaf. The The disease starts in wounds which are canned fruits all the way from the grower progress of the disease is not as rapid as always present on the brittle leaves of to the consumer. in Phalaenopsis. The lesion has well-de- Miltonia. The bacteria are exuded copi- With January and February tempera- fined margins and the color is nearer ously on the surface of the leaf and may ture playing a critical role in the final black than brown. The disease very sel- be spread by the water in syringing opera- outcome, it will be mid-March or early dom causes death in . tions from one pot to another, thus cre- April before accurate estimates of the Control of brown spot is achieved by ating an epidemic. situation can be made. By then the blos- the use of one part 8-quinolinol benzoate The affected leaves are water soaked som periods for most species will have to 2,000 parts of water or the sodium salt in early stages of the disease but later been reached or past and a real estimate of o-hydroxidiphenyl-Natriphene-also turn gray and even light brown and ap- of the fruit actually set can be made. pear scorched or blighted. Sometimes the at the rate of 1:2,000. In Cattleyas brown Dillon S. Brown is Assistant Professor of spot may be treated locally by swabbing disease is in the form of a narrow or wide Pomology, University of California College of it with a solution of corrosive sublimate, streak, terminating in the growing point Agriculture, Davis.

14 CALIF OR N I A' AGRICULTURE, FEBRUARY, 19 5 1