Ultrastructure of the Ascocarps, Asci, And Ascospores of Mycosphaerella Populorum

Mycosphaerella populorum ascocarps, developing in overwintering poplar leaves, exhibit characteristics typical of the Dothideales of the Loculoascomycetidae. Each pseudothecium develops a locule containing a fascicle of bitunicate asci which arise asynchronously from basal ascogenous cells, an ostiole without periphyses, and heavily melanized outer wall cells. Ascospore-delimiting membranes seem to arise from the ascus plasmalemma and to assist in spore wall formation. Mature ascospores are one-septate with a central pore. The ascospores usually germinate by a germ tube from each cell. The germ tubes seem to penetrate the leaf only through stornata.