EARLY INTERACTIONS, ADHESION, AND ESTABLISHMENT OF THE INFECTION COURT BY ERYSIPHE-GRAMINIS

The establishment of a fungal pathogen on the surface of its host is e ssential to the success of the infection process. For many fungi, esta blishment on the host is an active process that may depend on recognit ion of the host surface through chemical or topographic signals. Event s that allow for establishment may be considered to represent the ''pr eparation of the infection court'' by the pathogen. This sometimes inv olves the adhesion of the pathogen to the host and possibly the altera tion of the host's surface topography or chemistry. Adhesion is often presumed to be a single, chemically mediated event associated with ger m tube or appressorium formation. However, adhesion of ungerminated pr opagules may also occur, and evidence suggests that it is mediated by the release of adhesive materials directly from the propagule upon con tact with a suitable substratum. Fungi may require either a hydrophobi c or a hydrophilic surface to initiate the infection process. The barl ey powdery mildew pathogen, Erysiphe graminis, requires a hydrophilic surface for appressorium formation, yet the barley leaf is extremely h ydrophobic. The problem is resolved by the release of an exudate from conidia that makes the hydrophobic leaf surface hydrophilic. In contra st, Colletotrichum graminicola requires a hydrophobic surface for the initiation of its infection process. Ungerminated conidia of this fung us release materials that allow for the rapid adhesion of conidia, whi ch ensures that germination and appressorium formation occur, initiati ng the infection process. For both fungi, these events happen well in advance of germination and establish the pathogen at the site of the i nfection court.