Abstract: Mycelium of Ustilago esculenia P. Henn. is perennial which passes the cold winter seasons in rhizome of Zizania caduciflora Handl-Mazz. and invades the shoots developing from buds of the rhizome the next spring. Mycelium is septated, binucleate and profusely branched. Invasion of hyphae is intercellular and intracellular. No appressoria or haustoria are to be found. Cells of the parenchymatous tissue of infected portion of host plant divide rapidly with newly formed cells in great enlargement and high vacuolization which causes the shoot swelling. Part of hyphal cells with their wall transforms into mucous substance at the late stage of shoot swelling. Protoplasm of such mucoid hyphal cells con-tract into round cells, secreting thick wall and change into teliospores (chlamydospores).

Key words: Zizania caduciflora