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Expression of Monogenic Chlorotic-Lesion Resistance to Helminthosporium maydis in Corn. D. R. Smith, Formerly Assistant Plant Pathologist, Department of Plant Pathology, University of Illinois, Urbana-Champaign 61801, Now Plant Pathologist, DEKALB AgResearch, Inc., Sycamore Road, DeKalb, Illinois 60115; Phytopathology 65:1160-1165. Accepted for publication 16 May 1975. DOI: 10.1094/Phyto-65-1160.

The recessive gene rhm conditions resistance in corn to Illinois isolates of Helminthosporium maydis races O and T. Its expression, in terms of lesion type, lesion size, and extent of sporulation, depended upon the level of resistance contributed by other genetic systems, plant cytoplasm, and race of H. maydis. Monogenic-resistant plants with normal or cms-T cytoplasm, infected with race O, formed small chlorotic lesions with reduced sporulation, in contrast to large necrotic lesions with abundant sporulation on susceptible plants. The increase in resistance to race O due to rhm was less in corn plants having other genetic systems for resistance, than in more susceptible plants. These effects of rhm against race O were consistent in both greenhouse-and field-grown plants. The expression of rhm against race T depended upon plant cytoplasm. Plants with normal cytoplasm exhibited chlorotic lesions. Fewer conidia of race T formed in necrotic lesions of normal cytoplasm plants than of race O. Seedling plants with cms-T cytoplasm homozygous for rhm, and infected with race T, expressed chlorotic-shot-hole-type lesions in the greenhouse. Gene rhm did not appear to affect sporulation of race T in lesions on plants having cms-T cytoplasm.