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Light Intensity as a Factor in Field Evaluations of General Resistance of Potatoes to Phytophthora infestans. Gail Schumann, Graduate Research Assistant, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; H. David Thurston, Professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 67:1400-1402. Accepted for publication 19 April 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1400.

Field plots of potato cultivars Katahdin, Sebago, and Russet Rural were artificially shaded to provide different light intensities. Apparent infection rates (r) by Phytophthora infestans were higher for Katahdin and Sebago plants under 47% and 80% shade than under 0% shade. Shading, however, did not affect the apparent infection rate for Russet Rural plants which have little, if any, general resistance; plants of this cultivar died quickly in all treatments regardless of the amount of shade. Field light intensity should be considered when comparative evaluations of general resistance to P. infestans are made in different geographical areas.

Additional keywords: horizontal resistance, field resistance, Solanum tuberosum, late blight.