The combined effects of temperature and leaf wetness periods on soybean frogeye leaf spot intensity

The frogeye leaf spot, a disease caused by the fungus Cercospora sojina, affects soybean crops worldwide with enormous economic impact. In this study, we evaluated the combined effects of temperature and duration of leaf wetness periods on the intensity of frogeye leaf spot in soybean. Experiments were conducted in a growth chamber with cultivar Don Mario 7.0i at temperatures of 15, 20, 25, 30 and 35°C and leaf wetness periods of 12, 24, 36, 48 and 72 hours. The experimental design was completely randomized with five replications. When soybean plants were grown at 15°C, affected leaflet area, number of lesions per leaflet and diameter of lesions could only be measured after 60 hours of leaf wetness. At the temperatures of 20 and 25°C this period was reduced to 24 hours of leaf wetness, at 30oC, we found the need for 36 hours of leaf wetness and at a temperature of 35°C, 48 hours. The optimal temperatures for disease development were 27°C for diameter and affected leaflet area and 28°C for number of lesions per leaflet with 72 hours of leaf wetness.