Disease severity and pathotype dynamics of Puccinia striiformis f.sp tritici in Denmark

Disease severity of wheat yellow rust, Puccinia striiformis f.sp. tritici, was analysed in Denmark from 1985 to 1989 in relation to the effects of wea ther on winter survival, distribution of host cultivars and pathotype dynam ics. Below-average temperatures in January and February (midwinter) reduced yellow rust on the susceptible cv. Anja, and in three of four growth seaso ns following cold winters no yellow rust was observed on any cultivar under natural conditions. The agronomic consequences of dispersal of yellow rust urediniospores from external sources to Denmark, in a period during which large areas were planted with relatively few wheat cultivars, were demonstr ated in several cases, most evidently when the Yr9 and Yr17 resistance gene s became ineffective. Yr9 was overcome by the pathogen in a period with sev ere yellow rust epidemics on commercial cultivars, while virulence for Yr17 was first observed in a year with almost no yellow rust. In contrast, the resistance in cv. Kraka (Yr1, CV) was increasingly effective in controlling yellow rust, because pathotypes with the matching combination of virulence declined in the pathogen population. Pathotype frequency dynamics were thu s influenced by selection forces within the country and by selection forces in areas where spores were spread to Denmark from outside. The importance of a sufficient level of partial resistance in the wheat germplasm to preve nt too much damage by yellow rust epidemics, in the event that the resistan ce genes are overcome by the pathogen population, is emphasized.