Effects of environment on the in vitro growth and development of Colletotrichum gloeosporioides isolates from the Philippines

The environmental conditions that influence conidial germination, appressorium development and melanization, and appressorium germination in Colletotrichum gloeosporioides were investigated in vitro. The effect of relative humidities (RHs) between 39–100% and of temperatures of 20, 25 and 30°C were determined. Results differed when two isolates of the anthracnose pathogen isolated from Philippine mangoes were compared. The optimum temperature for production of appressoria by isolates I2 and I4 were 25 and 20°C, respectively. Both isolates however had a similar temperature requirement (25°C) for optimum cellulolytic activity. At 30°C, more appressoria became melanized than at lower temperatures but frequency of penetration was highest at 25°C. Under optimum conditions (25°C, 100% RH) only 25% of appressoria produced infection hyphae on a cellulose membrane after 3 days incubation. The effect of periods of low RH which preceded a 24 h period of high RH were also investigated. Conidia held at 63 and 86% RH for 1–2 weeks retained viability, and still formed appressoria when placed at 100% RH. The implication of these results in the use of a forecasting model developed previously are discussed.