Cadaver-Mimic Cultures Of Homoptera-Pathogenic Fungus Pandora Delphacis Grown On Solid Substrates And Their Biological Features And Functions

The entomophthoralean fungus, Pandora delphacis (Hori) Humber, (Zygomycotina: Entomophthorales) is an insect pathogen specific to homopteran insects such as planthoppers, leafhoppers, and aphids and frequently cause epizootics in insect populations. Problems with propagation of inocula at a low cost, but high efficiency, have been a technical obstacle to progress for entomophthoralean study and utilization for a long run. The present study was aimed to develop a solid-culture-based technology for easy and cheap propagation of P. delphacis F95129 inocula using small grains as substrate for fungal growth. Sporulation capacity and timing pattern were relied upon to evaluate culture quality and identify potential factors to affect the quality. Infectivity or virulence of inocula derived from the culture was assayed against the green peach aphid, Myzus persicae (Sulzer), based on time-dose-mortality modeling. The results are summarized as follows.Solid culture. In preliminary experiments, broomcorn millets (yielded by the crop Panicum miliaceum L.) and foxtail millets (yielded by the crop Setaria italica L.) were determined as optimal substrate for solid culture of P. delphacis. Steamed millets with water content of 45% (broomcorn) and 42% (foxtail) were inoculated with liquid culture of P. delphacis (containing mycelial mass of ~25 mg/ml) at a ratio of 20% (v/w) and then incubated at 25癈 and L:D 12:12. During a 17-day period of incubation, 12 millets of each species were sampled at 2-day intervals from day 3 on and individually assessed for their sporulation capacity and timing pattern using a self-designed device for spore collection. For the broomcorn millets, the 5-day-old culture sporulated most abundantly, discharging up to 17.111.3 x104 conidia/millet. The cultures incubated for 7-11 days also had a satisfactory sporulation capability, yielding 13.0-13.9 x104 conidia/millet. For the foxtail millets, the cultures incubated for 3-11 days produced 8.5-10.9 ×104 conidia/millet with the 7-day-old culture sporulating most abundantly. Sporulation on cultures of both milletspecies lasted for 6 days on non-nutritional substrate at 25C and L:D 12:12. Compared to 2.3±0.3 x104 conidia discharged from each cadaver of Myzus persicae adults killed by P. delphacis with a <60-h duration of sporulation, 5.6-1 A and 3.7-4.7 times more conidia were discharged from each of the broomcorn millets cultured for 5-11 days and from each of the foxtail millets cultured for 3-11 days, respectively, with an over doubled sporulation duration. In bioassays, conidia discharged from both millet cultures caused a mortality of 69.8% (broomcorn) and 68.7% (foxtail) in M. persicae within 7 days after exposure to conidial shower. The results indicate that the millet cultures of P. delphacis are biologically similar to aphid cadavers killed by its infection. This is the first success for solid culture of Pandora species on small grains.Infectivity and virulence of millet culture. The 7-day-old broomcorn millet culture was bioassayed against M. persicae nymphs by exposing aphids to conidial showers at 9 different concentrations (0.4-50.1 conidia/mm2). Based on modeling of the resultant time-dose-mortality data, the estimates of LC50 for the culture against the aphid species were 3.0, 1.7, and 1.2 cominia/mm2 on days 5-7 after exposure to conidial shower; the estimates of LT50 dropped from 4.6 days at the concentration of 4.5 conidia/mm2 to 3.2 days at 26.3 conidia/mm2. All aphid cadavers exhibited typical Pandora syndrome. Thus, the millet culture of P. delphacis was highly virulent to M. persicae, suggesting an applicability of the easy and convenient millet culture method to entomophthoralean fungi.Factors influential on sporulation of millet cultures. Three different batches of broomcorn millet cultures obtained under uniform conditions in millet processing, initial inocula level, and incubation were found not significantly differing in sporulation capacity. On non-nutritional agar plates, temperature significantly affected sp...