Evolution of grapevine downy mildew (Plasmopara viticola) aggressiveness in response to host partial resistance

International audienceThe increasing use of plants resistant to fungal pathogens in agro-ecosystems may lead to the evolution of plant pathogen populations. Pathogen adaptation to partially resistant host varieties would imply the evolution of life history traits linked to aggressiveness (quantitative pathogenicity). Understanding how pathogen quantitative traits respond to the introduction of host partial resistance is a crucial step toward an accurate management of disease resistance to ensure its sustainability over time. Here, we estimated aggressiveness in grapevine downy mildew (Plasmopara viticola) in a cross-inoculation experiment using a broad range of quantitative traits [e.g. disease severity, sporangia production, sporangia size, latent period, rate of zoospore liberation]. Plasmopara viticola isolates were collected on 9 susceptible varieties (N = 55 isolates) and 14 resistant varieties (N = 55 isolates) and inoculated on the susceptible V. vinifera cv. Cabernet Sauvignon and on partially resistant genotypes (Regent, Prior and Bronner). Specifically, we addressed the following questions: Are pathogen isolates collected on resistant and susceptible varieties able to infect partially resistant genotypes? Do they exhibit similar levels of aggressiveness and which traits are involved? Do isolates from resistant and susceptible varieties exhibit similar aggressiveness on a susceptible variety? Is there any trade-off between aggressiveness components? The main results include: (i) most of the P. viticola isolates collected on different resistant varieties were able to infect both Regent and Prior varieties and exhibited significant higher sporulation than isolates from susceptible varieties; (ii) isolates from resistant and susceptible varieties exhibited similar level of aggressiveness in terms of sporulation when inoculated on the susceptible Cabernet Sauvignon variety; (iii) increase in sporangia number was correlated with a reduction in sporangia size illustrating how partial plant disease resistance can impact selection of pathogen's life-history traits. These results highlight that P. viticola aggressive isolates from various resistant varieties exhibit a high level of sporulation on partially resistant grapevine genotypes. Overall, there was no significant cost of virulence. Plant pathogen populations may rapidly evolve to adapt to partially resistant grapevine varieties and the quantity of sporangia produced and sporangia size were the key traits involved in the adaptation process