Pathogenicity of Paecilomyces fumosoroseus isolates to diamondback moth, Plutella xylostella: correlation with spore size, germination speed, and attachment to cuticle.

Infectivity to larvae of the diamondback moth, Plutella xylostella, was compared among eight Paecilomyces fumosoroseus isolates. Isolate infectivity was assessed for correlation with spore length and germination speed. Four isolates applied to P. xylostella cuticle were also compared for number of spores remaining on the cuticle after washing and for percentage germination after 36 h. Infection of larvae inoculated with the different isolates at an average dosage of 4000 conidia/cm2 ranged from 2 to 47%. The correlation of infectivity with spore length and germination speed in broth was highly significant. Fewer spores of the least infective isolate, ARSEF 1576, attached to larval cuticle compared to spores of the more infective isolates ARSEF 3682, 4461, and 4482 (P < 0.05). After 36 h on larval cuticle, the percentage of spores germinated for isolates 1576 and 3682 was 3 and 95%, respectively. Spores of isolate 1576 were smaller, germinated more slowly, and attached to cuticle in smaller numbers than spores of the more infective isolates. Further research will expand our understanding of the mechanisms of virulence among isolates of P. fumosoroseus. Copyright 1999 Academic Press.