Biotic and abiotic regulation of resting spore formation in vivo of obligate aphid pathogen Pandora nouryi: Modeling analysis and biological implication

Entomophthoralean fungus Pandora nouryi is an obligate aphid pathogen that enables to produce resting spores (azygospores) for surviving host absence. To explore possible mechanisms involved in the regulation of resting spore formation in vivo, host cohorts consisting of 40–60 nymphs of green peach aphid Myzus persicae produced within 24 h on cabbage leaf discs in petri dishes were exposed to spore showers of P. nouryi at the concentrations (C) from a very few to nearly 2000 conidia/mm2 and then reared for 7–11 days at the regimes of 10–25 °C (T) and 8–16 h daylight (HL) or ambient (17.5 ± 3.1 °C, 13:11 L:D). Aphid mortalities observed from 35–83 cohorts (showered separately) at each regime showed typical sigmoid trend and fit well a general logistic equation (0.79 ⩽ r2 ⩽ 0.88), yielding similar LC50 estimates of 1.7–6.1 conidia/mm2. The proportions (P) of cadavers forming resting spores in the cohorts also fit the same equation (0.73 ⩽ r2 ⩽ 0.85) at all tested regimes except at 10 °C, a low temperature for the host-pathogen interaction. This indicates the dependence of resting spore formation on the spore concentration. The effects of T and HL on P over C were well elucidated by the fitted modified logistic equations P = 0.578/{1 + exp[1.710 − (0.136 − 0.0053T)C]} and P = 0.534/{1 + exp[1.639 + (0.034 − 0.0053HL)C]} (both r2 = 0.79). Our results highlight that the resting spore formation in vivo of P. nouryi is regulated primarily by the concentration of host-infecting conidia discharged from cadavers and facilitated by lower temperature and longer daylight.