Effects of Soil Temperature on Microsclerotia of Calonectria ilicicola and Soybean Root Colonization by this Fungus

Field soil artificially infested with laboratory-produced microsclerotia of Calonectria ilicicola was incubated for 1, 2, 3, or 6 weeks at 20, 25, 30, 35, and 40°C. These temperatures approximate soil temperatures that were measured in soybean fields during the growing season in south Louisiana. Germinable microsclerotia were enumerated after incubation at different temperatures, and soybean seeds were planted in these soils. After 8 weeks, percent root colonization was determined as a measure of infectivity of microsclerotia. Results showed that soil temperature is a critical factor in survival of microsclerotia. The optimal soil temperature range for survival of microsclerotia was 20 to 30°C, and the maximum soil temperature limit was 35°C, above which microsclerotia did not survive. Effects of temperature on soybean root colonization were examined in growth chambers by growing soybean plants in soil infested with laboratory-grown microsclerotia for 4 weeks after seed germination. Maximum infection of young soybean roots by C. ilicicola occurred between 25 and 30°C but decreased with increasing temperatures and was negligible at 40°C. According to these results, soil temperature is a critical environmental factor controlling the development of red crown rot in soybeans in Louisiana. These findings suggest that, if red crown rot is a threat, soybean planting time should be based on soil temperature rather than calendar dates.