Partial resistance to powdery mildew in soft red winter wheat

Powdery mildew (caused by Erysiphe graminis f. sp. tritici) is a disease that can cause significant yield loss in soft red winter wheat (Triticum aestivum). In selecting for resistance, one strategy is to incorporate partial resistance into breeding populations. The objectives of this study were to (i) estimate heritability of partial resistance to powdery mildew, (ii) determine which growth stage is optimal for measuring powdery mildew in terms of predicting yield loss, and (iii) measure yield loss associated with powdery mildew. In 1991, we evaluated 94 F-3 lines from a single-cross population believed to be segregating for partial resistance in a replicated experiment near Lexington, KY. The bulked F-5 progeny were evaluated in a replicated experiment in 1993. Plants were rated according to leaf infected (LI), an index of powdery mildew on the flag leaf and the subtending two leaves at Feekes growth stages (GS) 9 and 10.5. Severity of infection was assessed only on the uppermost leaf on which powdery mildew was present. Broad sense heritability estimates ranged from 0.31 (LI, 1991) to 0.65 ( severity, 1991). Heritability of severity of infection was considerably higher at GS 9 than at GS 10.5 (0.57 versus 0.34). Severity of infection at GS 9 also had the strongest correlation with yield (r = -0.55; P < 0.01) of any powdery mildew rating. We observed an average yield loss of 20% associated with powdery mildew over the 2 years of the study. Our data indicate that GS 9 is better than GS 10.5 for evaluating powdery mildew in terms of likely yield loss and heritability.