Epidemiology of Phomopsis Seed Decay of Soybean in Illinois. B. J. Shortt, Graduate Research Fellow, Department of Plant Pathology, University of Illinois, Urbana 61801. A. P. Grybauskas and F. D. Tenne, Former Graduate Research Assistants, and J. B. Sinclair, Professor, Department of Plant Pathology, University of Illinois, Urbana 61801. Plant Dis. 65:62-64. Copyright 1981 American Phytopathological Society. DOI: 10.1094/PD-65-62.

In a 3-yr study of seed decay of soybean (Glycine max) caused by Phomopsis spp. in Illinois, disease incidence was highest in 1977, lowest in 1976, and intermediate in 1975. A low positive correlation was found between temperature and disease incidence, but no consistent continuum of disease from north to south within the state was apparent. The highest incidence of Phomopsis seed decay occurred along major waterways in the wet years of 1975 and 1977. A high positive correlation was found between disease incidence and rainfall during pod fill, indicating that moisture, rather than temperature or geographic area, is the dominant environmental factor in disease development. Maturity dates of cultivars interacted with changing weather conditions to affect disease incidence. In our studies, cultivars in maturity group II had the highest level of Phomopsis seed decay. Cultivars used in seed production in Illinois should be grown at latitudes where they will mature late in the season and escape conditions conducive to high incidence of seed decay.