First Report of the Pathogenicity of Pythium torulosum on Soybean. B. Q. Zhang, Department of Plant Pathology, Iowa State University, Ames 50010 . X. B. Yang, Department of Plant Pathology, Iowa State University, Ames 50010; and W. D. Chen, Illinois Natural History Survey, Champaign 61820. Plant Dis. 80:710. Accepted for publication 10 April 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0710B.

Pythium torulosum Coker & F. Patterson has been reported to be pathogenic to corn (Zea mays L.) (1), but not to soybean (Glycine max (L.) Merr.) (2). P. torulosum was isolated from soil and diseased soybean seedlings during the 1994 and 1995 seasons in Iowa. Nine isolates were tested for pathogenicity on corn and soybean by blending inoculum produced on potato dextrose agar into steam-sterilized soil. Pots were placed at 10°C for 7 days and then 22°C for 4 days. All isolates of P. torulosum caused preemergence damping-off on both com and soybean. On corn, five isolates caused 60 to 87% and four isolates 50 to 59% preemergence damping-off. P. ultimum Trow that was isolated from a diseased soybean seedling and used as a pathogenic control caused 55% preemergence damping-off on corn. On soybean, two isolates of P. torulosum caused 52%, three isolates 40 to 50%, and four isolates 30 to 40% preemergence damping-off. P. ultimum caused 70% preemergence damping-off on soybean. Isolates that caused a greater incidence of damping-off on corn also caused a higher incidence on soybean. No preemergence damping-off was observed in noninoculated controls for either corn or soybean. Examination of diseased soybeans showed that P. torulosum either caused seed rot or death of germinated seeds. On 2-week-old or younger soybean seedlings, the isolates also caused root tip discoloration and yellow lesions on the roots and hypocotyl base. The pathogen was reisolated from diseased tissues and Koch's postulates were completed. Infection of P. torulosum on corn roots was reported by Rao et al. in 1978 (1). This is the first report of P. torulosum infection on soybeans