Effects of Environment and Nutrition on Conidial Germination and Appressorium Formation by Zoophthora Radicans (Zygomycetes: Entomophthorales): a Pathogen of the Potato Leafhopper (Homoptera: Cicadellidae)

Conidium germination and differentiation of Zoophthora radicans on water agar were investigated to identify requirements for the production of appressoria, the structures needed for virulence. Appressorium formation was more sensitive to environmental and nutritional changes than was conidium germination. Temperature affected both formation of germ tubes and appressoria, and the best temperature–nutrient interaction was found for appressorium formation at 25–30°C and 1% yeast extract. At 15°C, conidium germination was depressed independent of the nutrient concentration, and appressoria did not form. The volume of liquid overlaying the conidia and the yeast extract concentration individually affected appressorium formation. The best interaction between the volume of liquid and yeast extract concentration was the combination of a large volume of liquid medium (3 ml/962 mm 2 surface area) and 1% yeast extract. Of the 12 nitrogen sources tested, the best for germination and appressorium formation were yeast extract and Bacto-Soytone. Glucose, maltose, and starch were the best of the 12 carbon sources tested. Increasing osmotic pressure caused by different concentrations of maltose and polyethylene glycol adversely affected formation of germ tubes and appressoria. Subculturing did not affect formation of germ tubes, but the frequency of appressorium formation decreased after 12 transfers. The best pH for appressorium formation was between 7.0 and 7.2. Z. radicans produced appressoria on cuticles of dead Empoasca fabae (Harris) nymphs at 25°C in the dark