Ultrastructural and histochemical studies on interactions between Vitis vinifera L. and Uncinula necator (Schw.) Burr.

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Abstract
Summary: Host‐parasite interactions between the grapevine Vitis vinifera L. and the powdery mildew Uncinula necator (Schw.) Burr, were studied by light microscopy and histochemistry, scanning and transmission EM. The lobed appressoria (10 μm diam) of the fungus adhered to the host cuticle because of their shape and the production of a soluble adhesive substance. Each formed a penetration peg (0–3 μm diam) which, after penetration into the epidermal cell, developed into a globular haustorium consisting of a nucleated central body with fingerlike lobes, embedded in a matrix. The surrounding extra haustorial membrane was derived from the transformed plasmalemma of the host cell. As a defence mechanism the walls of the host cell and neighbouring cells were encrusted with silica and phenolic substances associated with cell wall bound peroxidase activity. Penetration may have been blocked by the formation of a papilla composed of different layers containing carbohydrates, silica and phenolic substances. Cells forming a thick papilla were not infected. In cells with old infections, callose deposits could be observed in walls and/or around the haustorial neck and papilla, and sometimes around the whole haustorium.