SURFACE-MORPHOLOGY, WALL STRUCTURE, AND INITIAL ADHESION OF CONIDIA OF THE POWDERY MILDEW FUNGUS UNCINULIELLA-AUSTRALIANA

A combination of light microscopy, low-voltage scanning electron micro scopy, and transmission electron microscopy was used in this study of conidia of the powdery mildew fungus Uncinuliella australiana, a patho gen of crape-myrtle (Lagerstroemia indica). These conidia were ellipso id-cylindrical in shape and virtually transparent when viewed with bri ght field light microscopy. Each conidium possessed a thin wall that s tained moderately for polysaccharides. Results from wheat germ aggluti nin-gold labeling procedures indicated that chitin was present in the wall. Conidia possessed minute longitudinal surface ridges that extend ed along most of the length of each conidium. These ridges were formed by thickened, slightly raised portions of the wall. The surface of ea ch conidium was coated with a thin networklike layer of extracellular mucilage. When strips of moist dialysis membrane were touched to conid ia, this material spread instantly from conidial surfaces onto the mem brane forming deposits referred to here as adhesion pads. Formation of an adhesion pad between the underside of a conidium and the membrane surface appeared to be the initial step in the process of conidial adh esion.