PENETRATION AND INFECTION OF SUSCEPTIBLE AND RESISTANT WHEAT CULTIVARS BY A NECROSIS TOXIN-PRODUCING ISOLATE OF PYRENOPHORA-TRITICI-REPENTIS

Light microscopy and scanning electron microscopy were used to follow the infection process of a necrosis toxin-producing isolate (86-124) o f the tan spot pathogen Pyrenophora tritici-repentis on leaves of susc eptible and resistant wheat cultivars. Conidia germinated to produce g erm tubes that gave rise to appressoria above epidermal cells or over the stomatal complex on both susceptible and resistant leaves. Germ tu bes appeared to be attracted to stomatal complexes. The fungus penetra ted the host leaf by direct invasion of the epidermal cells below the appressorium. If penetration did not occur below an appressorium, the germ tube grew further along the leaf surface to produce a secondary a ppressorium. Successful penetration of epidermal cells occurred with t he formation of an intracellular vesicle which produced one or more se condary intracellular hyphae. The intracellular hyphae penetrated the lower wall of the infected epidermal cell prior to invasion of the mes ophyll tissue. Penetration prior to invasion of mesophyll tissue occur red in a similar manner on leaves of susceptible and resistant wheat p lants.