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Response to Selection for Resistance to Four Diseases in Two Corn Populations¹

Index selection designed to maximize gain in resistance to a single disease and selection for resistance based on disease score, per se, were evaluated. Responses to index and per se half-sib family 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 two corn (Zea mays L.) populations (RSL and RSSSC) were measured by comparing the performance of selected bulks with base populations in artificially inoculated evaluation trials in 2 years.

Modified ear-to-row index selection was effective in increasing resistance to northern corn leaf blight in both populations, and to Diplodia stalk rot and to anthracnose stalk rot in one population each. Half-sib selection on disease score per se in one population (with recombination of remnant seed) was effective in increasing resistance to northern corn leaf blight, anthracnose leaf blight and anthracnose stalk rot. No consistent correlated response in grain yield (measured in uninoculated evaluation trials) to selection for disease resistance was found.

Observed response to selection agreed well with predicted response based on variance and covariance estimates obtained from previous half-sib evaluation experiments. Selection on indexes constructed from data on several traits and designed to maximize gain in disease resistance was no more effective than selection on disease score per se.

Key Words: Zea mays L. • Colletotrichum graminicola • Diplodia maydis • Helminthosporium turcicum • Half-sib selection • Selection index • Correlated response

² Graduate research assistant (now Forage Breeder, CIAT, Apartado Aereo 6713, Call, Colombia), professor of plant genetics, associate professor, plant pathology, and professor of plant genetics, Univ. of Illinois, Urbana.

Received for publication January 29, 1981.