Gametic disequilibria between virulence genes in barley powdery mildew populations in relation to selection and recombination. II. Danish observations

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Abstract
Analyses of gametic disequilibria (linkage disequilibria) between virulence genes were carried out on 14 samples of Erysiphe graminis f.sp. hordei collected from the aerial population at six Danish localities from 1985 to 1988. In most samples, the virulence genotypes Va7 Va12, Va9 Va12, VarV(la). Vk V(la) and VhV(La) were significantly less frequent than the product of the corresponding gene frequencies (negative gametic disequilibrium), while the virulence genotypes VaVk,Va9VnVh, VvVn and VkVh and were significantly more frequent than the product of the corresponding gene frequencies (positive gametic disequilibrium). These results could largely be explained by the distribution of powdery mildew resistance genes in the barley varieties grown in Denmark. Selection forces induced by two resistance genes present mainly in different varieties were likely to generate negative gametic disequilibrium between the corresponding virulence genes, whereas selection forces induced by two resistance genes present mainly in the same variety were likely to generate positive gametic disequilibrium between the corresponding virulence genes. This pattern was in accordance with that predicted from two‐ and three‐locuS models comprising recombination and selection induced by host resistance genes (Øsstergård & Hovmøller, 1991). It was concluded that such a model system is necessary for designing an ideal strategy for the deployment of varieties (resistance genes), and that the signs of gametic disequilibria do not provide adequate information for that purpose.