Population diversity and epidemiology of Bremia lactucae the cause of lettuce downy mildew

Abstract

Bremia lactucae is an obligate biotroph that causes Lettuce Downy Mildew (LDM), a foliar disease of lettuce which negatively affects crop value though reduced yield and quality. Emergence of new strains of B. lactucae that can overcome host resistance or result in reduced sensitivity to fungicide active ingredients is a consistent risk to LDM management and crop production.

From the 254 UK samples of B. lactucae collected and genotyped successfully using ten SSR loci, 135 multilocus genotypes were identified. Evidence was found of widespread incidence of heterokaryosis and overwintering clonal lineages. At least one UK isolate was able to overcome each differential line in IBEB differential set-C, of which cv Dandie containing Dm3 was least overcome by isolates tested. Of 15 B. lactucae samples tested with the fungicide active ingredients axoystrobin, mandipropamid and dimethomorph, no fungicide insensitivity was observed. No conclusive evidence of association between genotype to fungicide insensitivity nor virulence was obtained. When trials were inoculated with isolates of various pathogen genotypes, cultivar choice was found to greatly influence the population diversity of B. lactucae, with specific MLLs associated with some of the trial cultivars.

The study of aerial dispersal of sporangia using the LAMP assay and aerial samplers found it was predominantly local to infected plants (<5m). DNA of B. lactucae was detected up to 100m from an inoculum source. In commercial environments detection using the LAMP assay was as early as two days before LDM was reported in field, with symptomatic plants observed ~50-80m away from samplers.

The UK population of B. lactucae, based on samples collected, is diverse in genotype and phenotype. LDM management should account for the genetic flexibility of B. lactucae heterokaryons, which may contribute to overwintering soil-borne inoculum, and aerial dispersion of sporangia, which transmits the pathogen and can contribute to gene flow.