Relationship Between Weather and Soybean Seed Infection byPhomopsissp.

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Abstract
Five soybean (G. max) cultivars were planted at 3 dates in the field for 4 yr to investigate the relationship between seed infection by Phomopsis sp. and environmental conditions. Weather data were summarized during 2 growth periods: seed development (from the beginning of seed filling to physiological maturity) and seed maturation (physiological maturity to harvest maturity). Seed infection by Phomopsis sp. ranged from 0-68% across years, cultivars and planting dates. The incidence of Phomopsis sp. was significantly correlated with air temperature and minimum relative humidity but not with total precipitation or precipitation per day. A regression model was developed that included minimum temperature and minimum relative humidity during both growth periods, precipitation per day during the seed development period, and interactions between growth periods (9 terms, r2 = 0.70). It accurately predicted Phomopsis sp. infection when tested with data independent of those used to develop the model. The model predicted low levels (3%) of Phomopsis sp. seed infection from long-term average weather data for Lexington. Kentucky [USA] for an early maturing cultivar planted early (mid-May). Predicted levels increased to 39% when all weather variables were increased to 20% above average. Similar increases occurred when only the moisture variables were increased to 20% above average, but litte increase occurred if temperature alone was increased by 20%, suggesting that infection of soybean seed by Phomopsis sp. depends more on moisture than on temperature at Lexington.