Pathotypic diversity of Hyaloperonospora brassicae collected from Brassica oleracea

Downy mildew caused by Hyaloperonospora brassicae is an economically destructive disease of brassica crops in many growing regions throughout the world. Specialised pathogenicity of downy mildews from different Brassica species and closely related ornamental or wild relatives has been described from host range studies. Pathotypic variation amongst Hyaloperonospora brassicae isolates from Brassica oleracea has also been described; however, a standard set of B. oleracea lines that could enable reproducible classification of H. brassicae pathotypes was poorly developed. For this purpose, we examined the use of eight genetically refined host lines derived from our previous collaborative work on downy mildew resistance as a differential set to characterise pathotypes in the European population of H. brassicae. Interaction phenotypes for each combination of isolate and host line were assessed following drop inoculation of cotyledons and a spectrum of seven phenotypes was observed based on the level of sporulation on cotyledons and visible host responses. Two host lines were resistant or moderately resistant to the entire collection of isolates, and another was universally susceptible. Five lines showed differential responses to the H. brassicae isolates. A minimum of six pathotypes and five major effect resistance genes are proposed to explain all of the observed interaction phenotypes. The B. oleracea lines from this study can be useful for monitoring pathotype frequencies in H. brassicae populations in the same or other vegetable growing regions, and to assess the potential durability of disease control from different combinations of the predicted downy mildew resistance genes

Mapping of a locus for adult plant resistance to downy mildew in broccoli (Brassica oleracea convar. italica)

The identification of the gene Pp523, conferring downy mildew resistance to adult plants of broccoli (Brassica oleracea convar. italica), led to the construction of a genetic map that included this resistance locus, 301 amplified fragment length polymorphisms, 55 random amplified polymorphic DNAs, 46 inter-simple sequence repeats, three simple sequence repeats, four other PCR markers and a flower colour locus, all gathered into nine major linkage groups. Nineteen additional molecular markers were clustered into one group of four markers, one group of three markers and six pairs of markers. The map spans over 731.9 cM, corresponding to 89.5% of the 818 cM estimated to be the total genome length. A significant number of the mapped markers, 19.3%, showed distorted segregation. The average distance between mapped adjacent markers is 1.64 cM, which places this map among the densest published to date for this species. Using bulked segregant analysis, we identified a group of molecular markers flanking and closely linked in coupling to the resistance gene and included these in the map. Two markers linked in coupling, OPK17_980 and AT.CTA_133/134, are located at 3.1 cM and 3.6 cM, respectively, at each side from the resistance gene. These markers can be used for marker-assisted selection in breeding programs aiming at the introgression of this gene in susceptible B. oleracea genotypes. The fine mapping of the genomic region surrounding the Pp523 resistance gene is currently being carried out, a basic condition for its isolation via positional cloning