Institute of Crop Science and Resource Conservation, Phytomedicine, University of Bonn, Nussallee 9, D-53115 Bonn, Germany.
Adhesion to the host surface is the first step for successful plant pathogen development and has been reported to be associated with both passive and active processes. For conidia of Venturia inaequalis, which depend on leaf wetness for germination, this process has not yet been described. Conidia of V. inaequalis adhered to wet hydrophobic surfaces immediately after contact to the surface, hours before initiation of germination. Attachment of nongerminated conidia was much better on hydrophobic surfaces, such as apple leaves and polystyrene, than on hydrophilic glass. Conidia released adhesive material localized in a droplet named spore tip glue (STG) at the spore apex which interacted with a contact surface only when water was present. Histochemical investigations indicated the presence of proteins and carbohydrates in STG, lectin labeling the presence of β-galactose and N-acetylglucosaminyl residues. Transmission electron microscopy revealed two phases in the STG at the tip of dry mature conidia; as STG was present on the outer side of the intact fungal cell wall its formation should be associated with the secretion of glue through pores of the conidial wall. Surface-active substances affected the adhesion of conidia to hydrophobic surfaces stressing the importance of hydrophobic interactions. The use of protein biosynthesis inhibitors did not affect adhesion of conidia indicating that the adhesive material was preformed. It is concluded that the coincidence of STG, contact to a hydrophobic surface, and free water are essential for the adhesion of V. inaequalis conidia.