S. P. Wraight

Temperature-dependent germination and host penetration of the entomophthoralean fungus Zoophthora radicans on the leafhopper Empoasca kraemeri

Zoophthora radicans primary and secondary ovoid conidia inoculated onto Empoasca kraemeri fifth-instar nymphs germinated in three alternative modes, forming either germ-tubes, capilliconidiophores or replicative conidiophores. Production of germ-tubes was positively correlated, and production of capilliconidiophores inversely correlated, to temperatures over 5–32°C. Significant numbers of replicative conidiophores were produced only at 25–30°. The temperatures inducing the fastest rates of germination as capilliconidiophores, germ-tubes, and replicative conidiophores were 16·5, 22, and 26°, successively. Germ-tube formation was the most rapid mode of germination, occurring in ca 0·7 h at 22°; germination as capilliconidiophores or replicative conidiophores required 1·9 and 3·2 h, respectively, under optimal temperature conditions. Total germination (all modes combined) was optimum at 22°. High-temperature inhibition of germination was evident at 28° and severe at 30°; however, most germ-tubes produced at these temperatures were able to complete the processes of appressorium formation and penetration of the host cuticle. At 32°, only 6% of conidia germinated and no appressoria were produced. AT 10, 20, and 25°, 40, 20, and 4%, of all penetrations, successively, originated from capilliconidia produced by the ovoid conidia. The optimum temperature for infection (23°) was found to be similar to the optimum for vegetative germination (22°). The median time from inoculation to infection of the leafhoppers at 23° was 5.6 h. At temperatures near the upper threshold, lack of germination of the inoculum was the most important factor limiting host infection.