Initiation of the infection process by Erysiphe graminis: Conversion of the conidial surface from hydrophobicity to hydrophilicity and influence of the conidial exudate on the hydrophobicity of the barley leaf surface

When conidia of Erysiphe graminis are produced, they are hydrophobic and float when placed on the surface of a water droplet. In the hydrophobic state, a conidium may form one or more short germ tubes but fails to form an appressorial germ tube. Formation of an appressorial germ tube, from which the appressorium develops, is required for the establishment of the fungus as a pathogen on leaves of its barley host. Appressorial germ tube formation appears to require conversion of the conidium surface from a hydrophobic to a hydrophilic state. The present investigation shows that this conversion coincides with the contact-stimulated release of a liquid exudate from the conidium. Once the exudate is released, a conidium sinks when placed on water, indicating that it has become relatively more hydrophilic. When hydrophilic conidia are placed on cellophane or the surface of barley leaves, they undergo morphogenesis leading to the formation of appressoria. Incubation of the conidial liquid exudate on the hydrophobic surface of a barley leaf caused the leaf surface to become more hydrophilic. It is suggested that this phenomenon promotes the contact of the conidium with the leaf surface and establishes conditions required for a conidium to germinate by the formation of an appressorial germ tube.