Host genetics and environment shape fungal pathogen incidence on a foundation forest tree species, Populus tremuloides

Diseases can markedly alter the ecological and economic value of poplars. To better understand poplar-pathogen interactions, we investigated the independent and interactive effects of tree genotype, soil nutrient limitation, and interspecific competition on incidence of powdery mildew (caused by the fungal obligate pathogen Erysiphe adunca (Wallr.) Fr., 1829) in a foundation tree species, trembling aspen (Populus tremuloides Michx.). We established a common garden of potted aspen saplings, incorporating five tree genotypes, two levels of soil nutrients (low and high), and two levels of competition (with and without grass). We then surveyed natural incidence of powdery mildew and aspen vigor (i.e., growth). Incidence of powdery mildew varied among aspen genotypes, and variance in incidence shifted among environments in which the trees were grown. Added soil nutrients increased powdery mildew incidence on aspen, whereas grass competition had the opposite effect. Interestingly, grass competition either enhanced or dampened the variance in incidence of powdery mildew among tree genotypes, depending on soil nutrient levels. In addition, powdery mildew incidence was positively related to tree vigor. Our findings reveal strong genetic, environmental, and geneticxenvironmental effects of disease on a foundation tree species and that particular environments can either enhance or diminish variation in responses among tree genotypes.