Disease Note.

Commercial Potting Medium as the Source of Pythium Causing a Disease on Tobacco Transplants. D. Kelly Cartwright, Department of Plant Pathology, North Carolina State University, Raleigh 27695. H. W. Spurr, Jr., and H. D. Shew, Department of Plant Pathology, North Carolina State University, Raleigh 27695. Plant Dis. 79:538. Accepted for publication 28 March 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0538A.

A root and stem disease of tobacco seedlings transplanted to a commercial potting medium that was a uniform blend of peat humus, sphagnum peat moss, and prewashed coarse sand resulted in about 25% losses annually in greenhouses. Symptoms included brown-to-black, soft, water-soaked areas extending along the soil line, wilting and yellowing of lower leaves, and lodging. Infected plants died within a few weeks after symptoms were first observed. Pythium aphanidermatum was isolated and identified from five out of six symptomatic plants sampled. Subsamples randomly collected from seven bags of the potting medium plated either directly or diluted in water and spread on a selective agar medium yielded P. aphanidermatum, P. irregulare, P. spinosum, P. oli-gandrum, and P. splendens. Inoculation of tobacco seedlings with mycelium of P. aphanidermatum caused symptoms identical to those described above. We conclude that the cause of this disease on tobacco transplants is P. aphanidermatum and the source of this pathogen is the commercial potting medium.