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Influence of Temperature and Leaf Wetness Period on Conidial Germination In Vitro and Infection of Cercospora kikuchii on Soybean. W. Schuh, Assistant professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802.; Phytopathology 81:1315-1318. Accepted for publication 11 June 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-1315.

In vitro germination of conidia of Cercospora kikuchii was influenced by relative humidity. At 100% RH, 100% of the conidia germinated at all temperatures (15, 20, 25, 30, 35 C). No germination was observed below 92% RH. The temperature range tested did not alter the germination. The second fully developed trifoliate leaf of soybean (cv. Amsoy 71) was inoculated with a conidial suspension of C. kikuchii and then subjected to various temperatures (15, 20, 25, 30 C) and leaf wetness durations (18, 24, 30, 36 h). Disease severity was assessed 28 days later. No disease was observed at leaf wetness periods of less than 18 h or temperatures above 30 C. The optimum temperature was observed at 25 C. Disease severity increased with increasing leaf wetness period at all temperatures. The rate of increase in disease severity declined with longer leaf wetness periods. A regression model, using the arcsine-transformed disease severity values as a function of temperature (T), leaf wetness period (D), D2, T2, and T D, all statistically significant, described the relationship with an R2 value of 0.81.