Grafting transiently suppresses development of powdery mildew colonies, probably through a quantitative change in water relations of the host cucumber scions during graft healing

Abstract

Grafting can improve the resistance of scions to foliar diseases, but the exact mechanism is unknown. We hypothesized that responses to a quantitative change in water relations due to transient disconnection of vascular tissues were partly responsible. Cucumber (Cucumis sativus L.) scions were grafted onto squash (Cucurbita moschata Duch.) or cucumber (self-grafting) rootstocks and allowed to heal at 28°C and 95% relative humidity for 7 days. Spores of the cucurbit powdery mildew fungus (Podosphaera xanthii) were then inoculated onto the cotyledon leaves of the scions and of ungrafted (self-rooted) cucumber seedlings. Seven days after inoculation, colony density of P. xanthii was significantly smaller on the grafted and self-grafted scions than on the ungrafted seedlings. The adaxial epidermis of the cotyledon leaves thickened during graft healing. This is probably due to responses to water stress that protect against water loss, and may partly explain the smaller colony density. When inoculation was performed at the first-true-leaf stage 3 days after healing, the colony densities on the cotyledon and first true leaves of the grafted scions did not differ from those on ungrafted seedlings. These results suggest that the grafting transiently suppresses colony development of powdery mildew, probably through a quantitative change in water relations of host plants during graft healing.