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Improving Corn Population for Grain Yield and Resistance to Leaf Blight and Stalk Rot¹

Estimates of among and within half-sib family varl. ance components for disease scores in each of two corn (Zea mays L.) populations were obtained from artificially inoculated experiments in 1, 2, or 3 years. These estimates were used to predict response to mass, half-sib, and S₁ selection for resistance to northern corn leaf blight (caused by Helminthosporium turcicum Pass.), Diplodia stalk rot [caused by Diplodia maydis (Berk.) Sacc.], and anthracnose leaf blight and stalk rot [caused by Colletotrichum graminicola (Ces.) Wils.]. In addition, among half-sib family variance component estimates for yield were obtained from unlnoculated experiments for the same families. Genetic correlations among yield and reactions to the four diseases were estimated.

Per-year predicted gains in ieaf blight resistance for mass selection with pollen control were greater than for either S₁ or half-sib family selection (with 20 plants per family). Per-year predicted gain for mass selection-for stalk rot resistance was greater than that for half-sib family selection (20 plants per family), For a given selection intensity and effective population size, predicted gain in stalk rot resistance for mass selection with pollen control is about equal to that for S₁ selection at a great reduction in the number of plants rated for disease reaction and at no increase in the required number of hand pollinations.

Genetic correlations between grain yield in the absence of disease and disease reactions were all near zero and nonsignificant. All significant genetic correlations among disease reactions were positive. These results suggest that mass selection would be the most efficient method of improving disease resistance in corn populations and that such improvement would not affect yield potential of the populations if adequate effective population size is maintained.

Key Words: Zea mays L. • Diplodia maydis • Helminthosporium turcicum • Colletotrichum grandnicola • Genetic correlations • Mass selection • Half-sib selection • S₁ selection

² Graduate research assistant, professor of plant genetics, associate professor, plant pathology, and professor of plant genetics, Univ. of Illinois, Urbana.

Received for publication September 10, 1979.