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 ( Dm 1, Dm 2, Dm 3, Dm 4, Dm 5/8, Dm 6, Dm 7, Dm 11, R12, Dm 13, Dm 16 and R18). However, only some of these genes (e.g. Dm 3, Dm 7, Dm 11, Dm 16 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 ( Dm 6 + 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.

Phenotypic and histological expression of different genetic backgrounds in interactions between lettuce, wild Lactuca spp., L. sativa × L. serriola hybrids and Bremia lactucae

“…Lactuca serriola and other wild Lactuca spp. are used in breeding programmes as donors of new resistance factors (Lebeda and Zinkernagel, 2003b). However, few data exist on histological aspects of the interactions and mechanisms of resistance.…”

“…To date, at least 20 Dm genes or R-factors have been introduced into lettuce cultivars (Pink, 2002;Lebeda and Zinkernagel, 2003a) and more than 40 Dm genes and/or R-factors are known Michelmore et al, 2003). The lack of durability of race-specific resistance (conferred mostly by single dominant genes) in commercial cultivars is caused by the rapid occurrence of new 'matching' virulence factors (v-factors), driven by the fact that the genetic structure of pathogen populations evolves in a tight micro-coevolutionary process with that of its host (Lebeda and Zinkernagel, 2003b). The high evolutionary potential of B. lactucae (sensu McDonald and Linde, 2002) for rapid adaptation to new Dm genes is determined by a number of factors: a short period of asexual reproduction (ca.…”

Structure and variation in the wild-plant pathosystem: Lactuca serriola–Bremia lactucae

“…Most of the resistance is considered to be race-specific (Lebeda et al , 2007b. This type of resistance has a big disadvantage as it does not provide durable protection against lettuce downy mildew and the introduction of new resistant cultivars is often followed by the appearance of new virulent pathogen races (Lebeda and Schwinn 1994;Lebeda and Zinkernagel 2003a). During the last few decades, lettuce resistance breeding has focused on searching for and utilizing novel sources of resistance to B. lactucae from wild Lactuca species (Lebeda et al , 2007b.…”

“…Crute 1987;Lebeda and Zinkernagel 2003a). So far, there have been no studies of interactions between Lactuca spp.-B.…”

“…On the lettuce crop (L. sativa), B. lactucae has a worldwide distribution (e.g. Achar 1996;Crute 1987;Datnoff et al 1994;Lebeda 1979;Lebeda and Zinkernagel 2003a;Marlatt 1974;Sharaf et al 2007;Trimboli and Crute 1983). It has also been recorded on more than 200 other Asteraceae species from about 40 genera of the tribes Lactuceae, Cynareae and Arctotideae (Crute and Dixon 1981;Koike and Ochoa 2007;Lebeda et al 2002).…”