An electron microscopic investigation of barley roots infected in vitro by Bipolaris sorokiniana showed the existence of an extracellular sheath on germ tubes and appressoria attached to the root surface. Growth of the fungus in the epidermis and outer cortex was predominantly intracellular, whereas in the inner cortex the hyphae observed were mainly intercellular. Hyphae could not be detected in the stele 24 or 72 h after inoculation. Enzymatic activity in the apex of penetration hyphae is a possible explanation of the electron-dense areas seen in host cell walls 72 h after inoculation. Separation of plasmalemma from cell wall and degeneration of host nuclei and mitochondria were other infection-induced changes commonly seen. A host response to fungal infection involved the development of papillae between the plasma membrane and cell wall of the plant as well as around fungal hyphae. Key words: Bipolaris sorokiniana, Cochliobolus sativus, Helminthosporium sativum, Hordeum vulgare, barley, root, microscopy.

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