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Etiology and Epidemiology of Seedling Rot of Soybean by Pythium ultimum. Robert L. Schlub, Former graduate research assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824, Present address of senior author: Soilborne Diseases Laboratory, Plant Protection Institute, U. S. Department of Agriculture, Beltsville, MD 20705; J. L. Lockwood, professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Phytopathology 71:134-138. Accepted for publication 20 June 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-134.

Pythium ultimum and Fusarium spp. were regularly isolated from soybean seedlings with preemergence seedling rot. Seedling emergence was reduced from 93% in the control to 57, 29, and 0% in propylene oxide-treated soil artificially infested with 5, 10, and 25 sporangia of P. ultimum, respectively, per gram of soil. In soil infested with P. ultimum at seven propagules per gram increasing soil moisture from 1.8 to 0.018 bars matric potential at 28 C reduced soybean emergence from 66 to 15%, respectively. The influence of temperature on disease development varied with the soil used. In propylene oxide-treated soil infested with seven propagules of P. ultimum per gram, emergence at 16, 20, 24, and 28 C was 9, 44, 43, and 60%, respectively. In one of two different naturally infested soil samples, greater emergence was observed at 16 C (23%) than at 28 C (10%), while in the other, there was no significant difference between 16 C (28%) and 28 C (31%). Isolates of Fusarium spp. failed to inhibit emergence of soybean seedlings at matric potentials from 0.013 to 0.4 bar. None of three different Fusarium spp. increased the severity of Pythium damping-off of soybean under conditions suitable for rapid germination. Multiple regression analysis of field data taken from an area with preemergence seedling rot indicated that the number of days of continually wet soil (>0.5 bar) from the time of planting plus the number of days of low soil moisture (<3 bars) were positively correlated with disease incidence. Rain in the first 3 days was predicted to reduce emergence further if the soil matric potential was 0 to 3 bars. Weekly average soil temperatures ranging from 20.3 to 28.2 C had no significant effect on emergence.