EFFECT OF TEMPERATURE AND RELATIVE-HUMIDITY ON GERMINATION AND GERM TUBE DEVELOPMENT OF MYCOSPHAERELLA-FIJIENSIS VAR DIFFORMIS

Conidia (three isolates) and ascospores (one isolate) of Mycosphaerella fifiensis var. difformis germinated at 20-35 C. Germination followed a quadratic response function on temperature (R2 = 0.90-0.98), with an estimated optimum at 26.5 C. Maximum germination was observed in free-water and decreased at lower relative humidities (RH). Conidia germinated over a wider range of RH (92-100%) than ascospores (98-100%). Ascospores germinated earlier than conidia. The highest number of germ tubes per conidium was at 25 C for two isolates and at 30 C for the third isolate in free-water after 24 h. The number decreased at lower RH. Maximum length of germ tubes from ascospores occurred at 30 C (91-95-mu-m) followed by 25 C (68-75-mu-m), with an estimated optimum at 27.7 C. Germ tube length was similar at 100% RH and in free-water, but decreased at lower RH. Differences among the isolates and the possibility of some degree of natural variability in the response of the fungus to environmental factors are discussed.