Evolution and distribution of virulence in the German population of Bremia lactucae

Abstract

Changes in the genetic structure of the lettuce crop (Lactuca sativa) and populations of the lettuce downy mildew pathogen (Bremia lactucae) in Germany from 1974 to 1997 were studied and the genetic and population genetic results of race surveys of B. lactucae were reinterpreted. Data on 176 B. lactucae isolates collected from L. sativa cultivars at 48 sites in 11 German federal states were analysed. The host population contained 12 well‐defined race‐specific resistance genes (Dm1, Dm2, Dm3, Dm4, Dm5/8, Dm6, Dm7, Dm11, R12, Dm13, Dm16 and R18). However, only some of these genes (e.g. Dm3, Dm7, Dm11, Dm16 and R18) played an important role in management of resistance in the host crop. Substantial changes in the frequencies of these R‐genes were recorded over the period as a reaction to the occurrence of new virulent phenotypes in the pathogen population. Generally, the durability of resistance was very short. However, distinct regional patterns were observed for the frequencies of different virulence (v) factors. The data on changes in the virulence of B. lactucae population exemplified well the process of host–pathogen coevolution. During the period studied, there was a considerable increase in the frequency of some v‐factors (v2, v3, v5/8, v6, v7, v11 and v16) and in the complexity of v‐phenotypes, while the diversity in the pathogen population in general decreased. In 1996/97, 12 of the 14 v‐factors evaluated reached frequencies higher than 0·90 in the pathogen population. Relatively low frequencies were observed only for factors v14 (0·17) and v18 (0·24). The presence of 23 v‐factors was studied in six German isolates virulent on lettuce cultivar Titan, possessing the resistance gene R36 derived from Lactuca saligna and introduced in German lettuce crops during the mid‐1990s. Isolate DEG2 showed the highest complexity of v‐phenotype; it overcame the resistance in cv. Mariska (R18) and line CS‐RL (R18+?), but some sporulation occurred also on cv. Titan (Dm6 + R36). Conversely, the isolates SR2 and SAW1 overcame resistance based on R36, but were avirulent to R18. Future utilization of R‐genes in commercial lettuce cultivars is proposed; R‐genes R18 and R36 could be considered most efficient for this purpose. However, they may be effective only in certain regions.