Effect of temperature on vegetative growth of Beauveria bassiana isolates from different origins

Effects of temperature on growth on a semisynthetic medium of 65 isolates of Beauveria bassiana from different geoclimatic and host origins were determined. The isolates were grouped according to the climate of their geographic origin, distinguishing between cool and warm regions; according to their host origin, distinguishing between isolates from acridids or from soil associated with acridid populations; and according to host order. In general, B. bassiana grew at a wide temperature range from 8 to 35 C. A well defined maximum thermal threshold occurred at temperatures of >35-37 C for 50 isolates; >32-35 C for 12; and >30-32 C for 1 isolate. The lower temperature threshold for all isolates tested was below 8 C. Relative growth rate, calculated from the maximum growth rate for each isolate, was significantly affected by temperature and isolate. Optimal temperatures were generally between 25 and 28 C with several isolates exhibiting optimal growth at temperatures as low as 20 or as high as 30 C. Relative growth rates were not very useful in distinguishing differences among many of the isolates at the near-optimal temperatures. In contrast, the ability to distinguish differences in relative growth rates among isolates increased progressively as temperatures increased above 28 C or decreased below 20 C. There were no apparent relationships between relative growth rates and climatic origin. In comparisons of acridid-associated isolates, relative growth rates were higher for the insect isolates at 8 C and for the, African soil isolates at 35 C. At the other temperatures, no clear relationships were apparent. When comparisons were made according to host order, there were significant differences in relative growth rates according to host order at 8, 11, 30, and 32 C but not at the other temperatures. Since temperature growth responses varied considerably among isolates, with some having wide ranges of temperature optima while others were much more restricted, strain selection according to thermal requirements may be warranted when choosing a strain for development as a microbial control agent.