Atmospheric moisture influences on conidia development in Podosphaera xanthii through host-plant morphological responses

This study investigated the effects of atmospheric moisture on conidia development in cucurbit powdery mildew fungus (Podosphaera xanthii) through host-plant responses. Cucumber (Cucumis sativus L.) seedlings were grown under a high vapour pressure deficit (VPD) (3.8 kPa) or a low VPD (0.4 kPa). When the cotyledons had expanded, spores of P. xanthii were inoculated onto the adaxial surface. Inoculated seedlings for both treatments were then placed in a growth chamber maintained at a VPD of 2.1 kPa. The density of visible P. xanthii colonies on the high-VPD-acclimated cotyledons was 0.46 to 0.85 times that of the low-VPD-acclimated cotyledons 7 days after inoculation. It is likely the post-germination behaviour of P. xanthii such as the infection and consequent hyphal development was affected because spore germination did not differ between the treatments. The percentage of adaxial epidermal leaf cells with haustoria was also lower in the seedlings which had acclimated to a higher VPD. The high-VPD-acclimated cotyledons were thicker and had greater dry mass per area. The water potential of cotyledons did not differ between the treatments, although the stomatal conductance of high-VPD-acclimated cotyledons was lower than that of low-VPD-acclimated cotyledons. From these results, we conclude that the inhibition of P. xanthii conidia development on high-VPD-acclimated cotyledons was mainly caused by changes in leaf morphological properties.