Contribution of microbial core-collections and exploitation of polygenic resistances to the development of Phytophthora control strategies in solanaceous crops

Oomycetes form a diverse group of fungus-like eukaryotic microorganisms that include saprophytes as weil as pathogens of animais and some of the most devastating pathogens of dicotyledonous plants. Among them, few species of the genus Phytophthora, such as P. capsici and P. infestons cause enormous economic da mage on Solanaceous crops. Major genes have been identified in tomato to confer resistance against P. infestons, but were rapidly overcome by virulent populations: Pepper resistance towards P. capsici is provided by several quantitative trait loci (QTls). Efficient exploitatio-n of such polygenic resistance sources in plant breeding programs requires a good evaluation of their spectrum and durability. This requires a precise knowledge of the diversity of pathogen populations (with a focus on virulence, aggressiveness and host range) and some of their adaptive traits, such as changes in virulence or adaptation to prevalent cultivars. Theoretically, the use of Phytophthora corecollections reflecting the overall intraspecific diversity would help to assess the a priori durability of resistance sources.[br/][br/] We initiated a study to evaluate the potential sustainability of polygenic pepper and tomato resistances to Phytophthora. It implied i) to estimate the diversity of P. infestans and P. caps ici collections and their evolution over time; ii) to develop diverse quantitative and qualitative pathogenic assays to identify the various components underlying polygenic resistances. Collections included isolates from various geographic origins. Genetic diversity was investigated using a combination of neutral markers and genes under positive selection. First results on P. capsici reveal significant variations in the structure and dynamics of regional populations, and suggest that P. caps ici core-collections may be of local interest. We also showed that P. infestons populations are diverse but structured. Hence, strains isolated on tomato or potato differ at the genotypic and phenotypic levels. The use of P. infestons core-collections in breeding programs therefore requires gaining knowledge on the structure and epidemiological characteristics of natural pathogen populations collected on tomato