Differences in partial resistance of barley to powdery mildew (Erysiphe graminis DC F.SP. hordei Marchal) after chemomutagenesis. II. Reaction of mutants to pathotypes

Summary Seedlings of 45 barley mutants with partial resistance in the field and their parental cultivars Asse, Bomi, and Vada were exposed to six local and two foreign races of powdery mildew, in climatically controlled cabinets. Infectability, pustule size, infection grade, and infection type were estimated. No mutant did demonstrate strong race-specific reactions although some race-specific interactions of moderate grade could be detected. The results of infection of mutants with single races confirmed the quantitative character of their change in resistance determined in the earlier field assessments. The 20 investigated mutants with increased resistance expressed a lower level of disease with each of the used races. Out of the 10 mutants with higher susceptibility in the field, 4 mutants again exhibited higher degrees of infection over all the different races, while the remaining 6 mutants were not distinguishable from the original cultivar under the given growing conditions. Out of the tested 14 developmentally resistant mutants, only the 5 genotypes with different seedling reaction could be analyzed for race-specificity in this study. In all the above cases as the result of one gene mutation, quantitative shifts in level of infection were recorded over all the 8 races.Qualitative estimates of infection type were supported by data on the frequency of chlorotic or necrotic lesions and green islands on the infected leaves. But quantitative methods of assessment, e.g. infection frequency and pustule size, were more effective in studying partial resistance of the mutants. Infection grade, estimated visually 14 days after inoculation, was in agreement with the quantitative parameters accurately measured 7 days after inoculation. Therefore, its careful use can be recommended to speed the screening by narrowing down the materials. Differences in virulence level of the races were observed and their influence on race-specificity studies was discussed.