Germination and infection processes of the entomophthoralean fungus Erynia radicans on the potato leafhopper, Empoasca fabae

Abstract

Listen

Translate article

The germination and penetration processes of Erynia radicans on fifth instar Empoasca fabae nymphs at 20°C, 100% relative humidity, were investigated by scanning electron and fluorescence microscopy. Oval primary and secondary conidia attached to the host cuticle and germinated rapidly (60% within 2 hr). While percentage germination did not differ on the three body regions of the host, significant differences were observed in the modes of germination. The percentage of spores that produced germ tubes increased from 10% on the leafhopper head to 13% on the thorax and 22% on the abdomen, while capilliconidiophore production decreased from 88 to 84 to 71% on the respective body regions. Hyphae from spores deposited on sclerites displayed strong directional growth toward interscleral membranes, especially on the host abdomen. Penetration of the cuticle was effected by formation of appressoria. Small, globular appressoria were formed immediately adjacent to spores or on short, nonseptate germ tubes, while large, elongate appressoria were produced usually on long, but occasionally on short, septate hyphae. Approximately 3% of penetrations occurred on the leafhopper head, 16% on the thorax, and 74% on the abdomen; ca. 61% of all penetrations occurred through membranes, especially in the intersegmental folds of the abdomen. Earliest penetrations were observed 3–4 hr after inoculation, but significant numbers of penetrations were not observed until after 6 hr. The number of penetrations on individual leafhoppers was largely independent of dose (mean r 2 = 0.174), indicating substantial variability in susceptibility of individual leafhoppers. The percentage of viable oval conidia which ultimately gave rise to penetrations (either directly or indirectly via secondary spore production) was ca. 5% after 10–12 hr and 20% after 48 hr. The percentage of leafhoppers infected (percentage penetrated by one or more hyphae from a mean dose of 41 spores per insect) increased rapidly from only 2.4% at 4 hr postinoculation to ca. 60% after 8 hr. The LD50 was estimated at 4.1 spores per leafhopper.