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

Four isolates of the entomopathogenic fungus Zoophthora radicans were compa red in a laboratory study to evaluate the effect of relative humidity (RH) on duration of primary conidial viability. Primary conidia were showered on to agar-coated glass microscope slides within an enclosed chamber equilibra ted to one of five test RH levels (60, 75, 80, 95, or 100%). Target RH leve ls 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 simultan eous vital fluorochrome staining to determine percentage conidial viability . At 60% RH, isolates did not differ significantly and average viability dr opped to less than 10% within the first 60 min. At 75% RH, viability did no t change significantly over 4 h. However, average viabilities at 75% RH dif fered significantly for the four isolates and ranged from 24 to 63%. At 80% RH, viability differed significantly among isolates and declined slowly ov er 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 isol ates for biological control.