Virulence Gene Frequency Change in Erysiphe graminis f. sp. hordei Due to Selection by Non‐Corresponding Barley Mildew Resistance Genes and Hitchhiking

Abstract

AbstractIndividual isolates (in total 4040) of Erysiphe graminis f. sp. hordei were collected between 1989 and 1991 in fields of spring barley varieties with resistance genes Mla9, Mla12 and Ml(La) in the local air spora, i.e. remote from barley fields, in Hesse, Germany. Their virulence pattern was determined on a 13‐partite differential set. In the air spora. virulence complexity (the number of virulence genes per isolate) increased by 30% (4.2 vs. 5.5) between 1989 and 1991. This was mainly due to an increase of pathotypes with virulence genes Va7, Val3, Vk and V(La). In each mildew field population, frequencies of several non‐corresponding virulence genes, e.g. V(La) on Mla9 variety, greatly increased during the period. Such change can be caused by hitchhiking selection, i.e. indirect selection resulting from asexual reproduction, and direct selection by non‐corresponding resistance genes. To separate the two effects, the aerial barley mildew population of 1991 was sampled on cv. Pallas and near‐isogenic lines of Pallas. Frequencies of non‐corresponding virulence genes as observed in samples from the near‐isogenic lines were compared to expected non‐corresponding virulence frequencies which were obtained by subdividing the sample on Pallas into 12 subsamples, comprising all isolates virulent to line 1 through 12, respectively. Among 90 resistance/virulence gene combinations selection for non‐cor‐responding virulence was significant in five cases (e.g. Va9 on Mla7 host) and against non‐corresponding virulence in nine cases (e.g. Vk on Ml(La) host). Hitchhiking selection was significant in eight cases. In 11 cases the two types of selection were significant in combination but not individually. It is suggested to monitor the stability of selection against non‐corresponding virulence genes in race surveys and utilize it in diversified barley crops such as variety mixtures in order to retard the evolution of complex races.