ULTRASTRUCTURE OF CONIDIA, CONIDIUM GERMINATION, AND APPRESSORIUM DEVELOPMENT IN THE PLANT PATHOGENIC FUNGUS COLLETOTRICHUM-TRUNCATUM

Nongerminating conidia of Colletotrichum truncatum were coated with copious amounts of a finely fibrillar extracellular matrix. This matrix spread out onto the dialysis membrane used as a substrate in this study. Each thin-walled conidium contained a single nucleus that underwent mitosis 1-2 h following placement of aqueous suspensions of conidia on membranes. A septum subsequently developed near the middle of the conidium, creating two uninucleate cells. Just prior to or during septum development a germ tube emerged laterally, usually near one end of the conidium. The nucleus moved into the germ tube and underwent mitosis. One daughter nucleus remained in the germ tube, the other moved back into the conidium. Developing germ tubes appeared to produce large amounts of electron-dense, fibrillar material that coated their surfaces. This material blended into the remnants of the matrix initially coating conidia and could not be clearly differentiated from the latter material. Germ tubes grew to various lengths before forming appressoria. Appressorium differentiation began shortly after the germ-tube tip curved sharply. A septum developed to delimit the tip that differentiated into a swollen appressorium. By 6 h following initial hydration of conidia, appressoria were melanized and the surrounding extracellular material had condensed onto their surfaces, forming an electron-dense coating that appeared to stick appressoria to dialysis membranes. A tiny penetration peg developed from an apparently wall-less region on the underside of the mature appressorium and, in some instances, grew a short distance into the dialysis membrane.