Germination and effect of reduced humidity on expression of pathogenicity in Verticillium lecanii against the glasshouse whitefly Trialeurodes vaporariorum

SUMMARY Fifteen isolates of the entomopathogenic fungus Verticillium lecanii , from various hosts and locations, were bioassayed in the laboratory at 19 ± 1°C and > 95% r.h. against fourth instar scales of Trialeurodes vaporariorum. Highly pathogenic isolates generally originated from whitefly; however, there was no correlation between conidiospore size or germination rate on agar, and pathogenicity. LT 50 values ranged from 5.7 to > 9 days and LC 50 (7 days) from 1.3 × 10 5 to 4.2 × 10 7 spores/ml. The effect of periods in low humidity (70% r.h.) following 16 or 96 h at > 95% r.h. after immersion of whitefly scales on tomato leaf discs in a suspension of 1 × 10 6 spores/ml, was compared for five isolates. Progressively longer periods at 70% r.h. following 16 h at > 95% r.h., reduced significantly ( P < 0.001) the pathogenicity of all isolates. In these conditions isolate A was least affected by low‐humidity transfer. The high pathogenicity of isolate A was associated with its more rapid development on the host cuticle during the first 16 h in high humidity. When inoculated whitefly scales were transferred to 70% r.h. after a preliminary 96 h incubation in high humidity, all five isolates achieved a higher level of pathogenicity compared with their transfer to low humidity after 16 h high humidity. Thus the screening of V. lecanii isolates in limiting humidity conditions provided a more critical assessment of their pathogenicity and of their potential success in whitefly control relevant to the glasshouse environment.