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A Comparative Analysis of Nuclear-determined Resistance to Helminthosporium maydis Races T and O in Maize¹

Nuclear gene resistance to Helminthosporium maydis races T and O was studied using diallel analysis. Plants were artificially inoculated in the field and ratings based on measurements of lesion length and lesion area were obtained. Resistance to both races was basically determined by the same set of nuclear genes. Deviations in the relative rankings of certain inbreds suggested that resistance genes functioned both additively and on a gene-for-gene basis with pathogenicity factors.

Gene action conditioning resistance was similar for both races. Additive gene action was primarily involved in determining resistance, but dominance effects were also significant. The results suggest that several, possibly the majority, of genes that condition resistance to race T of H. maydis in the resistant lines, Mol7 and C103, also contribute to resistance to race O.

Key Words: Diallel analysis • Gene action • Gene numbers • Zea mays L.

² Graduate assistant and associate professor, Dep. of Plant Breeding and Biometry, Cornell Univ., Ithaca, NY 14853 (present address for I. G. Caunter, School of Biological Sciences, Univ. Sains Malaysia, Penang, Malaysia).

Received for publication October 12, 1977.