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 fijiensis var. difformis germinated at 20-35 C. Germination followed a quadratic response function on temperature (R(2) = 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 micrometers) followed by 25 C (68-75 micrometers), 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.