DIALLEL ANALYSIS OF ADULT-PLANT RESISTANCE TO POWDERY MILDEW IN WHEAT

Adult-plant resistance (APR) to powdery mildew [Blumeria graminis (DC. ) E. O. Speer f. sp. tritici Pm. Marchal] in wheat (Triticum aestivum L.) has been documented as being more durable than hypersensitive resi stance. However, little is known about the inheritance and diversity o f this resistance. Combining abilities and gene effects for APR to pow dery mildew were studied in a diallel cross involving one susceptible and six APR cultivars. Parents, F-1, and F-2 populations were evaluate d in replicated field trials under natural epidemics of powdery mildew . Area under the disease progress curve (AUDPC) was used to measure di sease severity over time. Significant differences in AUDPC were observ ed among parents, F-1 crosses, and F-2 populations. Comparisons of AUD PC values of parents, midparents, and F-1 crosses involving susceptibl e and APR parents indicated that resistance is partially dominant. Non -additive effects of the resistance genes were also evident from signi ficant contrasts between parents and K crosses. While nonadditive gene effects were involved, additive gene effects were predominant in the inheritance of APR to powdery mildew in the materials evaluated. Gener al combining ability (GCA) and specific combining ability (SCA) for mi ldew resistance were significant in both F-1 and F-2 generations. GCA was more important than SCA as suggested by large ratios of GCA and SC A variance components [2 sigma(g)(2)/(2 sigma(g)(2) + sigma(s)(2))] in F-1 (0.75) and F-2 (0.86) generations. The wheat cultivars Maris Hunt sman, Massey, and Coker 983 had high negative GCA effects and should b e promising parents in breeding programs for enhancement of powdery mi ldew resistance.