Scanning electron microscope observations of the development of sporophores ofColletotrichum atramentarium (B. et Br.) Taub. on infected potato periderm

Sections of potato periderm (1cm²), bearing sclerotia ofC. atramentarium were incubated for 16 days at 25°C, fixed in OsO₄ vapor at room temperature, dehydrated in a graded ethanol series and stored in 100% ethanol. Fixed specimens were coated with gold and examined with a Stereoscan electron microscope. Initial observations revealed numerous sclerotia and non-spore producing acervuli. After 16 days acervuli in various stages of development were observed. Initial development consisted of periderm swellings caused by a sclerotial body which enlarged to occupy an entire cork cell and eventually ruptured the periderm. A palisade layer formed from which setae developed, and apical ends of conidiophores were evident. Further development of the palisade layer was partially obscured by extrusion of a mucilaginous layer. Cylindrical, single-celled conidia (2.2–3.8u x 14.4–19u) were extruded through the mucilaginous matrix. Remnants of periderm wall were associated with acervuli and conidia lay in closely packed masses around the bases of mature acervuli. Conidia washed from spore producing acervuli onto PDA, germinated within 24 hours at room temperature. Electron scan is a rapid method for following acervulus development ofC. atramentarium on diseased potato periderm and demonstrates the feasibility of studying other potato periderm diseases with this technique.