Powdery Mildew Fungus Erysiphe Alphitoides Turns Oak Leaf Surface to the Highly Hydrophobic State

Abstract

Young leaves of the common oak Quercus robur are susceptible to the powdery mildew fungus Erysiphe alphitoides that can induce necrosis on the leaf surface. It is known that the fungal mycelium changes the wettability of the leaf surface. The present study performed a series of measurements of contact angles of water, ethylene glycol and diiodomethane on the oak leaf surface. Based on the obtained contact angle data, we calculated surface free energy on the adaxial surface of fresh healthy leaves and compared it with that of leaves infected by the fungus. Both types of leaf surfaces were also studied using cryo scanning electron microscopy, to examine a correlation between healthy/infected leaf surface structure and measured contact angles and surface free energy values. Our microscopical and experimental data clearly demonstrate that the powdery mildew turns the oak leaf surface to the highly hydrophobic state. The fungal mycelium shows three-dimensional hierarchical structure consisting of microscopic hyphae having cylindrical cross section with conidiomata (the first hierarchical level) and nanoscopic papillae and scales on the hyphal surface (the second hierarchical level). Such hierarchy in combination with the hydrophobic nature of the surface substances of the fungus leads to the highly hydrophobic properties (and low surface free energy) of the infected surface of the leaf.