Effects of temperature and atmospheric moisture on the early growth of apple powdery mildew (Podosphaera leucotricha) colonies

Effects of four temperatures and nine water vapour pressure deficits on the early growth of apple powdery mildew colonies on young leaves inoculated with conidia were studied on potted apple rootstocks in controlled environment cabinets. The number of hyphae (germ tubes + primary and secondary hyphae) per colony, total hyphal length per colony and the length of time from inoculation to the first observation of secondary hyphae were recorded on the upper surfaces of stained preparations of leaves sampled at three intervals up to 60 h after inoculation. Analyses of variance of these variables describing colony growth revealed significant differences between treatments (temperature × vpd), with most (c. 61–79%) of the treatment effects due to temperature, as shown by regression analysis. The response to temperature (13–28 °C) was non-linear, with the optimum c. 22 °C. In contrast, there was no detectable trend in the response of colony growth to vapour pressure deficit (1.6–10.4 mmHg). The results suggest that the rate of development of young colonies depends more on temperature than on moisture stress.