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Role of Pythium Species in the Seedling Disease Complex of Cotton in California. J. E. DeVay, Professor of Plant Pathology, University of California, Davis 95616. R. H. Garber, Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Shafter, CA 93263, and D. Matheron, Staff Research Associate, University of California, Davis. Plant Dis. 66:151-154. Accepted for publication 10 June 1981. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-151.

Survival of cotton seedlings (Gossypium hirsutum, cv. Acala SJ-2) grown from acid-delinted seed in six field tests during 1976 in the San Joaquin Valley of California was directly related to the concentration of soilborne propagules of Pythium ultimum. The viable propagules of P. ultimum recovered from soil were mainly oospores, but sporangia were occasionally found. Plant residues were the second most frequent source of viable propagules. Seedling survival ranged from 90 to 22% at 0 and 217 propagules of P. ultimum per gram of soil, respectively, with the death of 50% of the seedlings at approximately 100 propagules per gram of soil. Pythium species other than P. ultimum were present but at relatively low concentrations in soil samples. Pathogenicity tests indicated that P. ultimum and P. aphanidermatum were the most virulent species on cotton seedlings. Assays of Pythium species in samples of naturally infested field soils that were air-dried and stored at 23 C or kept moist and stored in sealed polyethylene bags at 4 or 23 C indicated that neither the period of storage (up to 5 mo) nor the conditions of storage caused any significant changes in populations of Pythium species.