Defence reactions of Hordeum chilense accessions to three formae specialesof cereal powdery mildew fungi

The histology of resistance to infection by Blumeria graminis DC Speer f.sp . hordei, Blumeria graminis f.sp. tritici, and Blumeria graminis f.sp. aven ae was studied in 15 accession lines of Hordeum chilense. All were highly r esistant to all formae speciales. There were small differences in spore ger mination rates and the morphological normality of germlings formed on the d ifferent lines. Relatively large differences between accessions were detect ed in terms of penetration resistance and the frequency with which attacked epidermal cells died. Greater penetration resistance was expressed by epid ermal cells of the seventh-formed leaves than by earlier leaves, indicating that penetration resistance is a component of adult plant resistance in H. chilense. Epidermal cells overlying vascular tissues (long epidermal cells ) were more resistant to penetration than cells adjacent or close to stomat al complexes. In a limited examination involving two accession lines, epide rmal cells on the abaxial leaf surface of one line were more penetration re sistant than those on the adaxial surface; the other line was highly resist ant on both surfaces. However, in the few cases where long cells and abaxia l cells were penetrated, the frequency of associated cell death was lower t han in short cells or on the adaxial leaf surface. This is consistent with the macroscopic observation that the few colonies formed on H. chilense wer e often close to the mid-rib and more frequent on the abaxial leaf surface. The data support the view that H. chilense offers a valuable source of pow dery mildew resistance for cultivated cereals.