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Variation in Expression of Monogenic Resistance in Corn to Exserohilum turcicum Race 3 Under Different Temperature and Light Regimes. S. Leath, Research plant pathologist, USDA-ARS, Department of Plant Pathology, Box 7616, North Carolina State University, Raleigh, 27695-7616; R. P. Thakur(2), and K. J. Leonard(3). (2)Senior plant pathologist, Cereals Program, International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, Andhra Pradesh 502324, India; (3)Research plant pathologist, USDA-ARS, Cereal Rust Laboratory, University of Minnesota, St. Paul 55108. Phytopathology 80:309-313. Accepted for publication 22 September 1989. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1990. DOI: 10.1094/Phyto-80-309.

Expression of monogenic resistance in near-isogenic corn inbred lines H4460Ht1, H4460Ht2, and H4460Ht3 against isolates of races 1 and 3 of Exserohilum turcicum was determined under different temperature and light intensity regimes. These environmental conditions influenced lesion type, number, length, and sporulation of the fungus. In general, isolates of E. turcicum produced more lesions at 22/18 than at 26/22 C and more lesions at 162 or 324 than at 647 mol m2 s1 at 22/18 C. Lesions also were larger (P = 0.05) at the reduced light intensities. Virulence of three races of E. turcicum was clearly expressed and consistent with earlier reports at 22/18 C day/night temperature and low light intensity (324 or 162 mol m2 s1), but inconsistencies arose at 26/22 C day/night temperature and 647 mol m2 s1 light intensity. Resistance of Ht1, Ht2, and Ht3 often was incomplete at 22/18 C day/night temperature and low light intensity. Race 3 could be readily recognized at a 22/18 C day/night temperature regime with the differential reaction clearest at a light intensity of 324 mol m2 s1.