Threshold of Initial Infection and Incubation Period Model of Powdery Mildew on Heracleum moellendorffii Hance

Heracleum moellendorffii Hance is a significant wild vegetable with both medicinal and edible properties in Northeast China. Protected cultivation addresses issues such as declining wild populations, harvesting difficulties, and uneven market supply. High air temperature and high relative humidity coupled with poor management practices in greenhouses frequently result in outbreaks of powdery mildew (Erysiphe heraclei), severely compromising yield and quality. Traditional chemical controls are hindered by pesticide residues and delayed efficacy, highlighting the need for sustainable prevention strategies. In this study, controlled gradients of air temperature (10~35 °C) and relative humidity (40~100% RH) were established, and microscopic observation was combined with analysis of infection dynamics to determine the infection threshold and to describe the relationship between air temperature/RH and the incubation period. Within the tested ranges, the pathogen completed its infection cycle at 10~30 °C and 40~100% RH, although germination time and rate, hyphal formation duration, and conidiophore production varied among regimes. The most favorable environment consisted of 20~25 °C and 60~80% RH, under which the incubation period was minimized to 83 h and showed a nonlinear dependence on both factors. Under extreme conditions (10 °C or 90~100% RH), the incubation period was prolonged to 124–191 h. Single-factor quadratic models fitted to treatment means described the temperature and RH dependence of the incubation period; these are exploratory descriptions within the tested ranges, not validated predictive models. The results provide a basis for early warning and cultivation management in protected production, while the one-factor design and the absence of independent replicate chambers define the limits within which these parameters should be applied.