University of California, Department of Plant Pathology, Riverside 92521
University of the Virgin Islands, Agricultural Experiment Station, St. Croix 00850
Intercontinental Growers Association, Inc., P.O. Box 1628, Kingshill, VI 00851
Commercial cultivation of lettuce (Lactuca sativa L.) under hydroponic conditions was initiated in August 1997 in the U.S. Virgin Islands. One week following transplant into an outdoor growout system, several plants wilted and died. Over the next 2 weeks, approximately 50% of the 30,000 plants died and the remaining plants were severely stunted. Temperature of the nutrient solution ranged from 28 to 30°C. Root specimens received for diagnosis consistently yielded pure cultures of Pythium myriotylum Drechsler. To confirm Koch's postulates, lettuce seedlings were reared hydroponically in a greenhouse. Temperature of the nutrient solution ranged from 27 to 29°C. After 14 days, one plant in each hydroponic chamber (which contained six lettuce plants) was removed and the root system of the plant was placed in a beaker containing approximately 1,000 zoospores of P. myriotylum. After a 30-min incubation period, the artificially inoculated plant was replanted in the hydroponic chamber. Within 5 to 7 days all plants were severely wilted and exhibited extensive root rot. P. myriotylum was consistently reisolated from symptomatic plants. No wilt occurred on respective noninoculated plants. The above study was conducted three times with similar results. Although P. myriotylum has previously been isolated from hydroponically grown lettuce in experimental systems (1), this report demonstrates the destructiveness of this zoosporic pathogen in a commercial hydroponic lettuce production facility. Factors that contributed to the epidemic were an abundance of a susceptible host, a temperature regime optimum for pathogen growth and reproduction, and a mechanism for the rapid dispersal of the pathogen via the recirculating nutrient solution. Although the source of pathogen introduction into the facility is not known, the hydroponic system was located in an open field. Thus, the pathogen could have been introduced aerially via wind-blown dust, rain, or insects. We do know, however, that the transplants were pathogen-free.
Reference: (1) A. C. Schuerger and K. G. Pategas. Phytopathology 74:796, 1984.