Pathogenic Variability in Mycosphaerella brassicicola, the Causal Agent of the Ringspot Disease of Crucifers

Twenty monoascosporic isolates of Mycosphaerella brassicicola from diverse origins were inoculated to 13 different accessions of cauliflower. No hypersensitive response was noticed and the isolates exhibited variable pathogenicity. Three isolates from cauliflower were isolated from the same lesion and had variable pathogenicity. This result suggests that different infection units e.g. ascospores contribute to the development of a single lesion. Furthermore, this finding also indicates that one should work with monoascosporic isolates. Pathogenicity results of this study showed that European isolates from cabbages had reduced pathogenicity compared to French cauliflower ones. Variable levels of pathogenicity were found in French cauliflower isolates. This group had also the most aggressive isolates. Our results suggest that Brittany may be a diversification area for the pathogen. In fact, the climate is mild and humid and crucifers (e.g. cauliflower and broccoli) are continuously grown over the year in this region. Pathotypes found in other growing areas may have moved from Brittany. All these findings supported by other studies suggest that resistance sources available in other countries may not be effective in Brittany. Consequently, screening for resistance sources should be done with cauliflower isolates from Brittany. No accession tested was susceptible or resistant to all isolates tested.