Differentiation of infection structures of the powdery mildew fungusUncinula necatorand adhesion to the host cuticle

Abstract

Development and adhesion of infection structures of the grapevine powdery mildew fungus, Uncinula necator (Schw.) Burr., were investigated during the early stages of leaf colonization. Light microscopy showed that primary appressoria occurred 3.5 h post inoculation (p.i.) and that hyphae on the leaf surface, indicative of successful host colonization, appeared 14 h p.i. Low temperature scanning electron microscopy revealed deposits of extracellular material at the contact zone of fungal structures and plant cuticle, suggesting firm attachment of the pathogen. To investigate whether or not esterase or cutinase activity is involved in establishing the fungus on the host cuticle, histochemical assays and inhibitor studies were performed. Results indicated that esterase activity was associated with conidia and infection structures. A single fungal extracellular protein was identified as a cutinase by its ability to hydrolyze3H-cutin. Probing Southern blots of genomic DNA of U. necator, Magnaporthe grisea, and Fusarium solani f.sp. pisi with the cutinase gene of F. solani f.sp. pisi suggested that the cutinase gene of U. necator shares only limited sequence similarities with the cutinase genes of the other fungi investigated. Adhesion assays showed that the presence of esterase-cutinase inhibitors on the cuticle did not significantly affect adhesion. The role of the enzyme in fungal adhesion is discussed.Key words: grapevine powdery mildew, Vitis vinifera, cutinase, extracellular matrix, cryofixation, low temperature scanning electron microscopy.