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Effects of Pod Moisture on Soybean Seed Infection by Phomopsis sp.. J. C. Rupe, Department of Plant Pathology, University of Kentucky, Lexington 40546, Current address: Department of Plant Pathology, University of Arkansas, Fayetteville 72701; R. S. Ferriss, Department of Plant Pathology, University of Kentucky, Lexington 40546. Phytopathology 76:273-277. Accepted for publication 24 September 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-273.

Increases in soybean seed infection by Phomopsis sp. have been associated with wet weather after physiological maturity. To determine the relationship of pod and seed moisture to the rate of seed infection by Phomopsis sp., pods with high incidence of carpel infection, but low seed infection, were collected after physiological maturity, maintained at 25 C at various water contents, and sampled periodically. Seed infection rates were linearly related to water contents between 35 and 19%. At water contents below 19%, no seed infection was observed. At water contents above 35%, there was a great deal of variability in seed infection rates between experiments, suggesting antagonism by other microorganisms. Growth of Phomopsis sp. occurred at osmotic potentials as low as -186 bars and was optimal from -10 to -30 or -40 bars on potato-dextrose agar osmotically adjusted with KCl or sucrose, respectively. Growth was faster on media osmotically adjusted with sucrose than with KCl. The relationship of water content to water potential was similar for carpels and seeds and was used to express seed infection rates in terms of water potential. Below -45 bars (35% water content), seed infection rates responded to water potential in a manner similar to that of growth rate of Phomopsis sp. to osmotic potential on media.

Additional keywords: Diaporthe phaseolorum var. sojae, Glycine max, Phomopsis sojae, Phomopsis longicolla, Phomopsis phaseoli, pod and stem blight.