Demographic and genetic impacts of powdery mildew in a young oak cohort

The demographic and genetic impacts of powdery mildew on the early stages of an oak population were studied in an ad hoc field design with two disease exposures. This enabled a detailed phenotypic monitoring of 1,733 emerging individuals from 15 progenies over nine years, and the genotyping of 68% of them. The pathogen induced high levels of seedling mortality several years after sowing, associated with reduced growth and capacity to overwinter. The probability of juvenile survival could be predicted from mean disease severity in early years and acorn weight. Fast-growing families showed the highest survival rate under both natural and protected disease exposure. Correlatively, no equalizing effect of increased powdery mildew pressure on the relative contribution of mother trees to the next generation could be detected. Contrary to a possible trade-off hypothesis between growth and defense, family height potential was not negatively related to disease resistance across the studied oak mother trees. Overall, our results suggest that in Quercus robur natural populations, infection levels (related to resistance sensu stricto) may be less determinant than growth-related tolerance to infection for the fate of seedlings. However, an equalizing effect of powdery mildew on relative oak genotype performances cannot be excluded at later stages since such an effect was already visible on height. Average genomic diversity was not significantly affected by mortality associated with powdery mildew. However, our study brings support to a deleterious effect of very low individual heterozygosity on the probability of survival across the different families. Finally, our study points to a few candidate genes for several fitness-related traits.