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°N to 54°S. Conidial suspensions were exposed to 40 or 45 °C for 2, 4, 8, and 12 h. Most of the isolates tolerated 40 °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°N, South Korea), 4295 (54.4°S, Australia), and 5626 (61.2°N, Finland) that had approximately 80% germination. High variability, however, was observed among isolates at 45 °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 °C, only two M. anisopliae isolates pathogenic to grasshoppers, viz., ARSEF 324 (latitude 19°S, Australia) and 3609 (15°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 °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 °C better than 45 °C in aqueous suspension.