Effect of relative humidity on viability of primary conidia of Zoophthora radicans

Four isolates of the entomopathogenic fungus Zoophthora radicans were compared in a laboratory study to evaluate the effect of relative humidity (RH) on duration of primary conidial viability. Primary conidia were showered onto agar-coated glass microscope slides within an enclosed chamber equilibrated to one of five test RH levels (60, 75, 80, 95, or 100%). Target RH levels were achieved by recirculating air through a glycerin/water solution, of controlled specific gravity, contained in a reservoir within the chamber. Conidial samples of each isolate incubated for 5, 10, 30, 60, 120, 180, or 240 min at each RH were removed and inspected using a technique of simultaneous vital fluorochrome staining to determine percentage conidial viability. At 60% RH, isolates did not differ significantly and average viability dropped to less than 10% within the first 60 min. At 75% RH, viability did not change significantly over 4 h. However, average viabilities at 75% RH differed significantly for the four isolates and ranged from 24 to 63%. At 80% RH, viability differed significantly among isolates and declined slowly over time, remaining above 80% for 2 h and above 50% for 4 h. At 95 and 100% RH, average viability was near 95% and did not vary significantly with time or isolate. These data can be used to assist selection of appropriate isolates for biological control.