GENETIC-ANALYSIS OF RESISTANCE TO SCAB IN SPRING WHEAT CULTIVAR FRONTANA

The Brazilian spring wheat (Triticum aestivum) cv. Frontana is resista nt to scab (caused by Fusarium graminearum) in Mexico and various othe r countries. The number of genes involved in resistance to this diseas e was estimated. This was done by evaluating random inbred F-6 lines a nd their parents for scab resistance. The lines were derived from cros ses of Frontana with susceptible, or moderately susceptible, cvs. Inia 66, Opata 85, and Pavon 76. These evaluations took place in the field with a mixture of Mexican isolates of F. graminearum. Spikes were ino culated by placing a tiny tuft of cotton soaked with the inoculum in t he middle spikelet close to the anthers just prior to anthesis. Glassi ne bags were placed over the inoculated spikes. These spikes were harv ested 45 days after inoculation. Scab severity was determined by recor ding the number of infected and healthy spikelets. Analyses of varianc e showed significant differences in scab severities of the F-6 lines. The distribution of F-6 lines was continuous in each cross. The narrow -sense heritability estimates for the crosses evaluated during 1991 an d 1993 were 0.66 and 0.93, respectively. Both quantitative and qualita tive models were applied to estimate the number of segregating genes. The resistance of Frontana is controlled by the additive interaction o f a minimum of three minor genes. Transgressive segregants were identi fied in each cross, indicating that the susceptible (or moderately sus ceptible) parents also carry one (or two) minor genes. The combination s of these genes with the genes in Frontana have given F-6 lines with significantly better scab resistance than that of Frontana.