Effectiveness and environmental stability of quantitative powdery mildew (Blumeria graminis) resistance among winter wheat cultivars

Powdery mildew in wheat (Blumeria graminis f. sp. tritici) is a major disease in Northern and Central Europe. The aim of the study was to analyse the effectiveness and environmental stability of quantitative powdery mildew resistance under high epidemic pressure in the field across years in the absence/presence of ineffective race-specific resistances. Cultivars with and without Pm (major) genes were inoculated in three experiments with a genetically broad mildew population with all matching virulences. Resistance was measured three times by assessing the percentage of leaf area covered by powdery mildew on a plot basis (0–100%). Mean powdery mildew severity of the highly susceptible cv. Kanzler varied across 10 years from 24% to 66% (Exp. 1). Means of three cultivars without Pm genes, Ramiro, Miras and Zentos, and several cultivars with ineffective Pm genes varied quantitatively from 4% to 13%. Environmental stability of the quantitative resistances was on average higher than that of susceptible genotypes, as revealed by a regression approach. In the second experiment, all groups of cultivars with ineffective Pm gene(s) contained a large proportion of entries with a similar low mildew rating as the quantitatively resistant standard Miras. Mildew severity of pairs of cultivars with the same Pm gene(s) was significantly different across 6 years (Exp. 3) indicating the presence of additional quantitative resistances in some of these cultivars. In the analysis of variance, genotypic variance had a high impact (P < 0.01) with low importance of genotype · environment interaction. Consequently, heritabilites were high (0.95–0.97). In conclusion, breeders have already accumulated effective minor genes for powdery mildew resistance in many of the released German winter wheat cultivars. These quantitative resistances are long lasting, environmentally stable and provide a high level of protection to powdery mildew.