Influence of temperature and relative humidity on sporulation of Cercospora zeae-maydis and expansion of gray leaf spot lesions on maize leaves

Controlled environment studies were conducted to determine the effects of temperature on the expansion of lesions of gray leaf spot, and the effects of temperature and relative humidity on the sporulation of Cercospora zeae-maydis on maize (Zea mays). For the lesion expansion experiment, potted maize plants were spray inoculated at growth stage V6, bagged, and incubated at 25 to 28 C and 100% relative humidity for 36 to 40 h. Symptomatic plants were transferred to growth chambers and exposed to constant temperatures of 25, 30, and 35 C. Lesion area (length by width) was measured at 4-day intervals for 17 days. For sporulation studies, lesions were excised from naturally infected maize leaves, measured, and incubated at constant temperature (20, 25, 30, or 35 degrees C) and relative humidity (70, 80, 90, or 100%) for 72 h. Sporulation was estimated as the number of conidia per square centimeter of diseased leaf tissue. A quadratic function was used to model the relationship between log-transformed conidia per square centimeter at 100% relative humidity and temperature. Temperature had a significant effect on lesion expansion (P <= 0.05). At 25 and 30 degrees C, the rate of lesion expansion was significantly higher than at 35 degrees C (P <= 0.05). The largest lesions and the highest mean rate of lesion expansion were observed at 30 degrees C; however, the mean lesion expansion rate at this temperature was not significantly different from that at 25 degrees C. The interaction effect of temperature and relative humidity on the log of conidia per square centimeter of diseased tissue was significant (P <= 0.05). At 100% relative humidity, the effect of temperature on sporulation was significant (P <= 0.05), with maximum spore production occurring at 25 and 30 degrees C. The quadratic model explained between 49 and 80% of the variation in the log of conidia per square centimeter at 100% with variation in temperature. These results suggest that the rapid increase in gray leaf spot severity generally observed during mid- and late summer may be due to favorable conditions for lesion expansion during this period. When relative humidity is > 95%, expanding lesions may serve as a source of inoculum for secondary infections.