Media composition influences on growth, enzyme activity, and virulence of the entomopathogen hyphomycete Isaria fumosoroseus

Three isolates of Isaria fumosoroseus Wize (Hyphomycetes) were cultured on six media composed of differing amounts of chitin, carbon, and nitrogen. The effect of nutrition on growth and virulence was studied by measuring colony growth, spore yield, germination rate, spore-bound protease (Pr-1) and lipase activity, and virulence of inoculum produced by different media against second instars of diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). Chitin peptone nutrition media resulted in the highest colony growth but spore yields were lowest for this medium, whereas the osmotic stress medium resulted in the lowest colony growth with fewer spores. Highest lipase activity (4.14 mu mol/ml/min) was observed for spores produced on high C/N medium for isolate IF28.2, whereas highest Pr1 activity (2.64 mu mol/ml/min) was also observed on high C/N medium for isolate IF28.2. A higher rate of Pr1 and lipase activity for all three isolates was observed on low C/N, 2% peptone, and the osmotic stress media. Conidia from nutrient-poor media (2% peptone) proved to be the least virulent for all three isolates with median survival time values of 2.23, 2.05, and 1.79 days for IF32, IF28.2, and IF49, respectively. Median survival time values for the various nutrient media proved to be positively correlated with spore-bound Pr1 and lipase activity, having correlation coefficient values of 0.115 and 0.538, respectively.