First Report of Pythium myriotylum in a Tobacco Seedling Float System in South Carolina.

Float bay production of seedlings accounts for approximately 62% of tobacco (Nicotiana tabacum L.) transplant production in South Carolina. Float systems utilize styrofoam trays containing soilless media that float in plastic-lined bays filled with nutrient solution. Oomycete fungi with motile zoospores, such as Pythium spp., can devastate plants grown hydroponically (3). This poses a serious concern for growers because no fungicides are labeled for control of Pythium spp. in nutrient solutions for tobacco greenhouse float systems. In May 1996, Pythium myriotylum Drechs. was recovered from diseased seedlings produced in float bays in Walterboro, SC. The pathogen was isolated on PAR selective medium (2). Koch's postulates were fulfilled by inoculating plants in an experimental float bay and then reisolating the pathogen on PAR from symptomatic plants. Diseased plants appeared chlorotic and stunted, with water-soaked lesions on the stem. Root systems were severely necrotic and infected plants could not be used as transplants. Disease development was confined to plants sharing a common nutrient solution in individual bays. Within affected bays nearly 100% of plants were diseased, suggesting pathogen spread through the nutrient solution. At the time of disease onset, the grower was attempting a second crop of transplants. The high late-season temperatures were favorable for P. myriotylum, which has a growth optimum of 37°C. Primary inoculum could potentially arise from contaminated media or water and/or improper sanitation procedures in greenhouse maintenance. Pathogenic isolates of several Pythium spp. were found in commercial potting media in North Carolina in 1995 (1). References: (1) D. K. Cartwright et al. Plant Dis. 79:538, 1995. (2) S. N. Jeffers and S. B. Martin. Plant Dis. 70:1038, 1986. (3) M. E. Stanghellini and S. L. Rasmussen. Plant Dis. 78:1129, 1994.