New use of broomcorn millets for production of granular cultures of aphid-pathogenic fungus Pandora neoaphidis for high sporulation potential and infectivity to Myzus persicae

Abstract

Glutinous broomcorn millets from the crop Panicum miliaceum were first used as substrate to produce granular cultures of Pandora neoaphidis, an obligate fungal pathogen specific to aphids. Carrying a water content of 36.5% after being steamed in a regular autoclaving procedure, millet grains of each 15 g (dry weight) in a 100-ml flask were mixed with 3 ml modified Sabouraud dextrose broth containing half a mashed colony of P. neoaphidis grown on egg yolk milk agar and then incubated at 20°C and a light/dark cycle of 12 h/12 h for 21 days. Based on individually monitoring conidial production potential of 20 millet grains sampled from an arbitrarily taken flask at 3-day intervals, the millet cultures incubated for 6–15 days were capable of producing 16.8–23.4×10 4 conidia per millet grain with conidial ejection lasting for up to 6 days. The cultured millet grains individually produced significantly more conidia than apterous adults of Myzus persicae killed by P. neoaphidis (8.4×10 4 conidia per cadaver) and sporulated twice longer. The modeling of time–dose–mortality data from bioassays on M. persicae apterae exposed to conidial showers from the cultured millet grains and the mycelial mats produced in liquid culture resulted in similar estimates of LC 50 (millets: 21.4, 7.3, and 4.9 conidia mm −2 on days 5–7 after exposure; mycelial mats: 22.1, 10.6, and 7.7 conidia mm −2) although the LT 50 estimated at a given conidial concentration was slightly smaller for the millet cultures than for the mycelial mats. This indicates that the millet grains cultured with P. neoaphidis produced conidia as infective as or slightly more infective to M. persicae than those from the mycelial mats. Based on the sporulation potential, infectivity, and ease and cost of the millet cultures, the method developed in this study highly improved in vitro cultures of P. neoaphidis and may adapt to culturing other entomophthoralean fungi for microbial control of insect pests.