Surface Morphology, Wall Structure, and Initial Adhesion of Conidia of the Powdery Mildew FungusUncinuliella australiana

A combination of light microscopy, low-voltage scanning electron microscopy, and transmission electron microscopy was used in this study of conidia of the powdery mildew fungus Uncinuliella australiana, a pathogen of crape-myrtle (Lagerstroemia indica). These conidia were ellipsoidcylindrical in shape and visually transparent when viewed with bright field light microscopy. Each conidium possessed a thin wall that stained moderately for polysaccharides. Results from wheat germ agglutinin-gold labeling procedures indicated that chitin was present in the wall. Conidia possessed minute longitudinal surface ridges that extended along most of the length of each conidium. These ridges were formed by thickened, slightly raised portions of the wall. The surface of each conidium was coated with a thin networklike layer of extracellular mucilage. When strips of moist dialysis membrane were touched to conidia, this material spread instantly from conidial surfaces onto the membrane 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 adhesion