Abstract:In order to explicit the correlation between biological characteristic and pathogenicity of Isaria fumosorosea ( =Paecilomyces fumosoroseus) to Plutella xylostella, five I.fumosorosea strains, including original father strain PFCF-O, degenerated father strain PFCF-D, single-spore isolates PFCF-D58, PFCF-D20, and PFCF-D50, which isolated from PFCF-D, were assayed on sporulation, growth rate, spore germinate rate, extracellular protease and their pathogenicity to the 2nd instar larva of P.xylostella. The results showed that the growth rate of strain PFCF-O was fast at initial stage after inoculated to medial but slowed down with the time elapsed. The strain PFCF-O appeared an obviously higher sporulation (2.7×107 conidial/mL) after 20 days culture, spore germinate rate (62.59%), extracellular protease level (1.16) and corrected mortality (97.74%) to the 2nd instar larva compared to other strains. However, among the strains PFCF-D, PFCF-D20, PFCF-D50 and PFCF-D58, there were no significant differences on the growth rate and sporulation germinate rate. Sporulation of these strains was 1.05×107-2.34×107 conidial/mL and extracellular protease level was 1.02-1.07. The corrected mortality of these four strains to the 2nd instar larva was 19.86%-51.44%, significantly lower than that of strain PFCF-O. The pathogenicity of I.fumosorosea against P.xylostella significantly correlated to the sporulation, spore germinate rate, extracellular protease level and strains growth rate of the initial stage after inoculated to medial, but negatively to the growth rate of the later stage after inoculated to the medial.