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A Model for Predicting the Effects of Microclimate on Infection of Soybean by Phomopsis longicolla. J. C. Rupe, Department of Plant Pathology, University of Kentucky, Lexington, KY 40546, Current address: Department of Plant Pathology, University of Arkansas, Fayetteville, AR 72701; R. S. Ferriss, Department of Plant Pathology, University of Kentucky, Lexington, KY 40546. Phytopathology 77:1162-1166. Accepted for publication 24 February 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1162.

A model was developed relating infection of soybean tissue by Phomopsis longicolla to environmental conditions. The model was based on natural infection of soybean seedlings placed in a field at weekly intervals throughout the growing season in 1981, 1982, and 1983, as well as laboratory inoculation and in vitro growth and germination studies. Parameters used in the model were rain, surface wetness, temperature, and relative humidity. Based on its average hourly temperature, each surface wetness period was assigned a rating that was the equivalent period of surface wetness at optimum temperature for pathogen growth (2530 C). The relationship of in vitro growth rate to temperature was used to assign these ratings. An infection period was defined as beginning with rain (inoculum dispersal) and ending with surface drying. Factors were included in the model to account for exhaustion of inoculum and continuation of infection periods after interruption. When applied to the field infection results, little or no infection occurred at rating totals less than 13, but infection increased linearly with an increase in rating totals greater than 13. These results were similar to the results of the inoculation studies at 25 C. The coefficient of determination (r2) for the model was 0.73. The model could serve as the basis of a model predicting pod infection by P. longicolla.