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Ecology and Epidemiology

Pseudoperonospora cubensis in Rust-Infected Bean. C. E. Yarwood, Professor Emeritus, Department of Plant Pathology, University of California, Berkeley, CA 94720; Phytopathology 67:1021-1022. Accepted for publication 7 February 1977. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1021.

When rust-infected bean leaves (uredinial stage of Uromyces phaseoli in Phaseolus vulgaris ‘Pinto’) were inoculated with sporangia of the cucumber downy mildew fungus (Pseudoperonospora cubensis in Cucumis sativa ‘National Pickling’) necrotic rings formed around isolated rust pustules, and heavily rusted areas of leaves were slowly killed, whereas no symptoms were caused by P. cubensis on nonrusted leaves. Pseudoperonospora cubensis formed sporangisphores and sporangia in such rusted tissues and the sporangia showed normal infectivity. The optimum time from inoculation with U. phaseoli until inoculation with P. cubensis was about 1 day. Infection with P. cubensis was exclusively through the lower surface of bean leaves, whereas infection with U. phaseoli was about equally successful through the upper or lower surfaces.