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Influence of Soil Water Potential on Performance of Soybean Seeds Infected by Phomopsis sp.. Mark L. Gleason, Department of Plant Pathology, University of Kentucky, Lexington 40546-0091; Richard S. Ferriss, Department of Plant Pathology, University of Kentucky, Lexington 40546-0091. Phytopathology 75:1236-1241. Accepted for publication 1 July 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1236.

Effects of soil water potential on emergence and establishment of seedlings from soybean seeds infected by Phomopsis sp. were examined in growth chamber and greenhouse experiments. Seeds were incubated for 3 days in untreated or pasteurized soil at soil matric potentials of approximately 0, - 0.008, - 0.01, - 0.1, - 4, - 17, or - 51 bars, then for 16- 18 days at - 0.008 to - 2.2 bars. Seed lots with high proportions of seeds infected by Phomopsis sp. (either naturally or by inoculation) had lower percent emergence and establishment than lightly infected lots, particularly when the initial incubation period was in soil at φm≤ - 17 bars. Growth of the fungus from infected seed coats to cotyledons (as indicated by increases in percent incidence in cotyledons after incubation in pasteurized soil) was much greater in soil at φm≤- 0.1 bar than at φm≥- 0.01 bar. Phomopsis sp. was the only seedborne fungus whose incidence in seeds of five seed lots was significantly negatively correlated with emergence in unsaturated soil. The results suggest that seedborne Phomopsis sp. reduces emergence and establishment most when seeds are incubated in dry soil. Evidence that low soil water potential inhibits seedling growth more than growth of Phomopsis sp. may help explain this phenomenon.

Additional keywords: Diaporthe phaseolorum, Glycine max, pod and stem blight, seed vigor, seedborne pathogens.