Evolution and distribution of virulence characteristics of Belgian Bremia lactucae populations between 2008 and 2013

Downy mildew, caused by the oomycete Bremia lactucae, is a devastating disease of lettuce, which is controlled by applying fungicides and/or planting resistant lettuce cultivars. As resistance of lettuce cultivars is based on gene-for-gene interaction and offers mostly short-term protection, a lot of effort is put in breeding new resistant lettuce cultivars. For efficient breeding, it is important to know the virulence characteristics of B. lactucae populations in order to select for lettuce cultivars carrying the most effective resistance genes for production. In this study, 55 isolates of B. lactucae were collected from Belgian lettuce culture and their virulence characteristics were analysed. Results showed that the resistance genes with the highest resistance efficacy to the B. lactucae isolates are Dm 15, Dm 17 and the resistance factors of Balesta, Bedford and Bellissimo, while Dm 1, Dm 4, Dm 5/8, Dm 10 and Dm 12 were least effective. There was a lot of variability in virulence within the B. lactucae populations, with 41 of the 55 characterized isolates having a unique virulence-phenotype. This virulence variability may result from sexual reproduction or asexual genetic variation. Sexual reproduction, however, does not seem to be an important cue for B. lactucae variation in Belgium since oospore formation did not occur frequently and attempts for oospore germination were not successful. In addition, mating type B1 was predominant in the B. lactucae population. Our results provide important information for selection and future breeding of resistant lettuce cultivars, especially for Belgian lettuce producers.