APPRESSORIUM FORMATION BY HEMILEIA-VASTATRIX

At germination, germ tubes extrude through germ pores of the urediniospore wall of Hemileia vastatrix. As many as five germ tubes can be formed from a single urediniospore, but only one elongates and extends randomly over the epidermis. Exploratory branches form along its length. Upon reaching the raised lips of the subsidiary cells of coffee leaves, or the guard cells of french bean, a non-host, the germ tube tip, of either a side branch or a main axis of the germ tube, swells and a near-spherical appressorium is formed. A wedge-shaped structure, the appressorial wedge, develops from the base of the appressorium into the vestibule of the stoma. Upon reaching the stomatal pore, the appressorial wedge tapers to form an infection wedge. A protuberance is often found on the appressorium opposite the collapsed germ tube. Remnants of adhesive material are absent from the lower surfaces of the urediniospore, germ tube and appressorium, thus indicating that the germ tube must utilize another mechanism of attachment to the leaf surface.