Effect of temperature on vegetative growth among isolates of Metarhizium anisopliae and M. flavoviride.

Effects of temperature on vegetative growth on a semi-synthetic medium of 22 isolates of Metarhizium anisopliae and 14 isolates of M. flavoviride were determined. The majority of isolates of both species grew between 11 and 32 degrees C; several isolates grew at 8 and 37 degrees C. None of the isolates grew at 40 degrees C. Relative growth rate, calculated from the maximum growth rate for each isolate, was significantly affected by temperature and isolate, with significant isolate * temperature interactions. The maximum absolute growth rates among the isolates ranged from 2.5 mm to 5.9 mm/day. Optimal temperatures were generally between 25 and 32 degrees C with several isolates exhibiting optimal growth at temperatures as high as 32 degrees C. Overall, relative growth rates were greater in isolates of M. anisopliae than M. flavoviride at temperatures of 25 degrees C or lower; conversely mean relative growth rates were greater in M. flavoviride than M. anisopliae at temperatures higher than 25 degrees C. However, the two most cold tolerant isolates at 8 degrees C were M. flavoviride and the three most heat tolerant at 35 degrees C were M. anisopliae. Since temperature growth responses varied considerably between isolates, strain selection according to thermal tolerance may be warranted when choosing a strain for development as a microbial control agent.