XVI International Botanical Congess

Polystigmina ochraceum Sacc. (conidial state of ascomycetous fungus Polystigma ochraceum (Wahlenb.) Sacc.), the causal agent of red spots of the bird-cherry leaves, is a host specific obligate pathogen which can be categorized as necrotroph. P.ochraceum produces branched, septate, red intercellular hyphae.Each hypha is surrounded by fibrillar extracellular martix. The plasmalemma of the hyphae is much convoluted and has numerous invaginations with plasmalemmasomes.The hyphal cell protoplast is packed with nucleus, typical organelles and vacuolar reserves. Large vacuoles containing electron-dense deposits of polyphosphates are common. The fungus kills bird-cherry cells to absorb their contents, therefore host epidermal and mesophyll cells associated with infection region become collapsed and necrotic. Several ultrastructural changes associated with host cell degradation are observed.