EARLY INTERACTIONS DURING POWDERY MILDEW INFECTION

Within minutes of spore deposition, enzymes including cutinase are rel eased by conidia of Erysiphe graminis; these may prepare the infection court and facilitate subsequent fungal development. Germination follo ws with emergence of the primary germ tube, which contacts the host le af. Extracellular material is secreted beneath the primary germ tube, which adheres to the leaf. The primary germ tube forms a penetration p eg that breaches the host surface and gains access to host cell compon ents including water. The primary germ tube also recognises factor(s) present in the host surface, and this stimulates elongation of the sec ond-formed germ tube. The elongated second tube in turn responds to ho st surface factor(s) by differentiating an appressorium. Extracellular material, secreted beneath the growing appressorial germ tube, is lai d down thickly around the appressorial lobe. An inner ring of extracel lular material can often be seen surrounding penetration pores reveale d by the removal of primary germ tubes and appressoria. The chemistry of the extracellular material is unknown, but in addition to adhesive properties recent studies show that a monoclonal antibody that recogni ses purified cutinase and, possibly, hemicellulases, binds to secretio ns from conidia, primary germ tubes, and appressoria. External factors can also influence germling development. Although light has little ef fect on the early stages of germling development, it has a profound ef fect in delaying the formation of haustoria by apparently mature appre ssoria. Understanding the control of early pathogen development could indicate avenues for genetic engineering and breeding for disease-resi stant plants.