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 resistance. However, little is known about the inheritance and diversity of this resistance. Combining abilities and gene effects for APR to powdery mildew were studied in a diallel cross involving one susceptible and six APR cultivars. Parents, F-1, and F-2 populations were evaluated in replicated field trials under natural epidemics of powdery mildew. Area under the disease progress curve (AUDPC) was used to measure disease severity over time. Significant differences in AUDPC were observed among parents, F-1 crosses, and F-2 populations. Comparisons of AUDPC values of parents, midparents, and F-1 crosses involving susceptible and APR parents indicated that resistance is partially dominant. Non-additive effects of the resistance genes were also evident from significant 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. General combining ability (GCA) and specific combining ability (SCA) for mildew 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 SCA 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 Huntsman, Massey, and Coker 983 had high negative GCA effects and should be promising parents in breeding programs for enhancement of powdery mildew resistance.