Variability in conidial thermotolerance of Metarhizium anisopliae isolates from different geographic origins.

Notable variability in thermotolerance was found among conidia of 16 isolates of the insect-pathogenic fungi Metarhizium anisopliae var. anisopliae and one M. anisopliae var. acridum isolated from latitudes 61 degrees N to 54 degrees S. Conidial suspensions were exposed to 40 or 45 degrees C for 2, 4, 8, and 12 h. Most of the isolates tolerated 40 degrees C very well, with relative germination (germination relative to unheated controls) above 90% after 12 h of exposure. Exceptions were three isolates originating from high latitude, viz., ARSEF 2038 (38 degrees N, South Korea), 4295 (54.4 degrees S, Australia), and 5626 (61.2 degrees N, Finland) that had approximately 80% germination. High variability, however, was observed among isolates at 45 degrees C; viz., after 2 h exposure, relative germination was above 80% for six isolates, between 50 and 70% for three isolates, and between 0 and 30% for eight isolates. After 8 and 12 h at 45 degrees C, only two M. anisopliae isolates pathogenic to grasshoppers, viz., ARSEF 324 (latitude 19 degrees S, Australia) and 3609 (15 degrees N, Thailand), had high relative germination (91.6 and 79.4%, respectively, for 8 h exposures; and 90 and 47.1%, respectively, for 12 h). These isolates also were the most tolerant to UV-B radiation [J. Invertebr. Pathol. 78 (2001) 98-108]. The median lethal dose (LD50) for isolate ARSEF 324 was 49.4 and 47.9 degrees C, for 2 and 4 h of exposures, respectively. Exposure of conidia to wet-heat greatly delayed germination of some isolates. In general, isolates from higher latitudes demonstrated greater heat susceptibility than isolates from nearer the equator. Dry conidia tolerated 50 degrees C better than 45 degrees C in aqueous suspension.