Thermotolerance of germlings and mycelium of the insect pathogenic fungus Metarhizium spp. and mycelial recovery after heat stress
The upper temperature limits for germination and growth were determined for Metarhizium acridum (ARSEF 324, 3341 and 3609), M. robertsii (ARSEF 2575), M. anisopliae (ARSEF 5749), and Aspergillus nidulans (ATCC 10074). Most of the Metarhizium species germinated well at 35 and 36 °C; however, compared to 28 °C, the growth was very slow (except ARSEF 5749 from Mexico, which germinated at 35 °C, but did not grow at 34 or 36 °C). Germination was severely im-paired at 38 and 40 °C for all Metarhizium species. ARSEF 324 conidia were the most thermo-tolerant. Based on radial growth measurements, however, none of the M. anisopliae or M. robertsii isolates grew (produced visible colonies) during 10 days at 38 °C. All Metarhizium species kept at 38 and 40 °C for 10 days resumed growth when transferred to 28 °C, and they all sporulated. When the plates were held 10 days at 42 °C, however, only the two M. acridum isolates (ARSEF 324 and 3609) grew after returning to 28 °C, but with some delay. Conidia germinated at restrictive temperatures, but the growth was impaired or discontinued soon after germination. Nevertheless, when transferred to a permissive temperature, they produced colonies and new conidia. In contrast, the thermophilic A. nidulans germinated and grew well at 42°C, and therefore was much more thermotolerant than any of the Metarhizium spp. isolates.