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Initial Penetration Process in Powdery Mildew Infection of Susceptible Barley Leaves. W. E. McKeen, Professor, Department of Plant Sciences, The University of Western Ontario, London N6A 3K7, Canada; S. R. Rimmer, Graduate student, Department of Plant Sciences, The University of Western Ontario, London N6A 3K7, Canada. Phytopathology 63:1049-1053. Accepted for publication 16 February 1973. DOI: 10.1094/Phyto-63-1049.

The chronological events in the primary penetration process of germinating conidia of powdery mildew (Erysiphe graminis f. sp. hordei) into epidermal cells of susceptible barley (Hordeum vulgare) are presented. The barley leaves induced the conidial germ tubes to grow longitudinally on the leaves and stimulated the production of appressoria. The papilla, a result of host reaction to the fungus, began to form in the epidermal cell below the appressorium arm as the appressorial infection pore developed. The tip of the blunt infection peg lacked a cell wall and the host epidermal wall was dissolved only in the region of the tip. Mechanical pressure was required to force the peg through the host cell wall and papilla, and to invaginate the cytoplasm.