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Diallel Analysis for Reaction of Eight Corn Inbreds to Helminthosporium maydis Race T. S. M. Lim, Assistant Professor of Plant Pathology, University of Illinois at Urbana-Champaign, Urbana, IL 61801; Phytopathology 65:10-15. Accepted for publication 18 June 1974. DOI: 10.1094/Phyto-65-10.

Eight inbred lines and 28 F1 hybrids from a set of diallel crosses among the lines, each in both cms-T and normal (N) cytoplasms, were evaluated for reaction to Helminthosporium maydis race T. Plants were artificially inoculated in the field and disease ratings, lesion length, and lesion area measured. All three disease reactions were highly intercorrelated. Disease reactions from cms-T plants were highly correlated with those from their N cytoplasm counterparts. Resistance in corn to race T was both cytoplasmic and nuclear-genic. All eight inbreds and F1 hybrids with N cytoplasm exhibited a higher degree of resistance to race T than their cms-T cytoplasm counterparts. Mo17T was the only inbred showing a relatively high level of resistance in cms-T cytoplasm. The most resistant crosses involved Mo17 as a parent. Estimations of combining ability indicated that genetic variation for disease resistance in corn was associated with highly significant general combining ability (GCA) effects. The (Vr, Wr) graphic analysis indicated that nuclear gene resistance to race T had additive gene effects and was partially dominant. Estimations of degree of dominance showed the resistant inbred Mo17 in both types of cytoplasm had the most dominant genes. The use of the restoration factor (Rf) is suggested to facilitate incorporation of high degrees of nuclear gene resistance into elite cms-T corn, allowing effective selfing, sib-crossing, or backcrossing. Recurrent selection in populations of cms-T cytoplasm having a degree of resistance is also suggested through the use of Rf.

Additional keywords: cms-T cytoplasm, N cytoplasm, combining ability.