Genetical studies of spontaneous sources of resistance to powdery mildew in barley

Digenic inheritance of resistance to barley powdery mildew is common. Thus Weihenstephan CP 127422 has a second gene, linked to Ml-g (p = 0.277–0.379), conditioning resistance to race K1. Several varieties (e.g. Gopal, Mian wali and Monte Cristo) with resistance at Ml-a locus carry a second closely linked (p = 0.06–0.08) gene with a spectrum of resistance similar to that of HOR 1063. Concerning Monte Cristo the additional factor is weakly linked to Ml-at. In other varieties such as Sv 57/510–44 and C.I. 8503 the distance is greater between the main Ml-a allele and the corresponding additional factor. Almost similar expressions but different spectra of resistance characterize the two genes present in HOR 1104, with indicated locations at Ml-at and Ml-h, respectively. No indications of linkage were obtained between potential alleles in the following pairs of loci: Ml-a — Ml-at, Ml-a — Ml-p, Ml-a — Ml-h, Ml-a — Ml-n, Ml-at — Ml-p, Ml-at — Ml-h, and Ml-at — Ml-n. On chromosome 5 there is no, or only weak, linkage between translocations and markers, and Ml-genes. This fits a proximal position of the translocation breakpoints and a distal localization of markers and Ml-loci. Strong linkage in the crosses T2–5a x Cebada Negra (BB) and T2–5e x Atat indicates a distal position of the breakpoints versus the marker genes.