Genetic Analysis of Resistance to Scab in Spring Wheat Cultivar Frontana. RAVI P. SINGH, International Maize and Wheat Improvement Center (CIMMYT), Lisboa 27, Apdo. Postal 6-641, 06600 Mexico, D.F., Mexico. HONG MA, and SANJAYA RAJARAM, International Maize and Wheat Improvement Center (CIMMYT), Lisboa 27, Apdo. Postal 6-641, 06600 Mexico, D.F., Mexico. Plant Dis. 79:238-240. Accepted for publication 2 November 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0238.

The Brazilian spring wheat (Triticum aestivum) cv. Frontana is resistant to scab (caused by Fusarium graminearum) in Mexico and various other countries. The number of genes involved in resistance to this disease was estimated. This was done by evaluating random inbred F6 lines and their parents for scab resistance. The lines were derived from crosses 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 inoculated by placing a tiny tuft of cotton soaked with the inoculum in the middle spikelet close to the anthers just prior to anthesis. Glassine bags were placed over the inoculated spikes. These spikes were harvested 45 days after inoculation. Scab severity was determined by recording the number of infected and healthy spikelets. Analyses of variance showed significant differences in scab severities of the F6 lines. The distribution of F6 lines was continuous in each cross. The narrow-sense heritability estimates for the crosses evaluated during 1991 and 1993 were 0.66 and 0.93, respectively. Both quantitative and qualitative models were applied to estimate the number of segregating genes. The resistance of Frontana is controlled by the additive interaction of a minimum of three minor genes. Transgressive segregants were identified in each cross, indicating that the susceptible (or moderately susceptible) parents also carry one (or two) minor genes. The combinations of these genes with the genes in Frontana have given F6 lines with significantly better scab resistance than that of Frontana.