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Infection of Aboveground Parts of Bean by Pythium aphanidermatum. S. H. Kim, Former Graduate Assistant, Department of Botany, University of Maryland, College Park 20742, Present address of senior author: Bureau of Plant Industry, Pennsylvania Department of Agriculture, Harrisburg 17120; J. G. Kantzes(2), and L. O. Weaver(3). (2)(3)Professors, Department of Botany, University of Maryland, College Park 20742. Phytopathology 64:373-380. Accepted for publication 23 September 1973. DOI: 10.1094/Phyto-64-373.

Zoospores of Pythium aphanidermatum were shown to aggregate on leaves and stems of Phaseolus vulgaris at the bases of trichomes, against glandular trichomes, around trichome sockets, at stomata, at the junction of epidermal cells, and at wounds. Upon aggregation, the zoospores encysted, and the germ tubes made contact with host tissue, within 30 min after inoculation at 24 C. One-day-old mycelial fragments, however, required 10 h to establish appressoria. Once hyphae became established in plant cells, pathogenesis proceeded at the same rate, regardless of whether the inoculum had been zoospores or mycelial fragments. The first aerial hyphae on colonized suscepts usually arose from cystospores and subcuticular or intracellular hyphal bulges.