Zoophthora radicans (Zygomycetes: Entomophthorales) conidia production from naturally infected Empoasca kraemeri and dry-formulated mycelium under laboratory and field conditions

Abstract

Abstract Laboratory and field studies were conducted to assess the effects of temperature on sporulation of a dried-mycelium formulation of the entomophthoralean fungus Zoophthora radicans and to compare sporulation of laboratory-produced/formulated fungus versus fungus occurring on cadavers of naturally infected Empoasca leafhoppers. Conidia production by the formulation increased from 3.1×10 4 to a maximum of 13.7×10 4 conidia/mg (dry weight) over the temperature range from 5 to 20 °C and decreased to 10.7×10 4 conidia/mg at 25 °C and to nearly zero at 31 °C. A temperature-dependent development model estimated a sporulation optimum of 23.6 °C. Pieces of formulated mycelium (2×2×0.5 mm) placed on bean and cowpea foliage in the field showed a temporal pattern of nightly conidial discharge similar to the fungus on leafhopper cadavers; both fungi initiated sporulation within a few hours following dewset and ceased with the return of dry conditions after 08:00 h. However, sporulation of the fungus on cadavers peaked between 00:00 and 03:00 h, while peak sporulation of the formulated fungus usually occurred shortly after dawn. Fungus on adult leafhopper cadavers and the pieces of formulated fungus underwent multiple daytime desiccation/nighttime rehydration cycles, producing conidia for up to eight consecutive nights. Second-, third-, fourth-, and fifth-instar cadavers supported sporulation for only 5–6 nights. On a dry weight basis, the fungus on cadavers produced substantially more conidia than the formulated fungus; however, differences were less pronounced based on the surface area of the hymenium. In general, the dried-mycelium pieces generated conidia in a manner similar (both temporally and quantitatively) to the fungus on leafhopper cadavers. These results indicate that the dried-mycelium formulation is well suited as an inoculum source for initiation or augmentation of epizootics in leafhopper populations.