Landscape and tree community effects on the infestation of an endemic fungal pathogen in riparian forests were mediated by leaf traits

Abstract

Background Plant pathogens are regarded as crucial agents shaping the dynamics of natural forest communities. Marssonina leaf spot of poplar is induced by an endemic pathogenic fungus Drepanopeziza populi, causing increased damage to riparian poplar stands in recent years. However, such endemic fungal diseases have received little attention at the landscape scale, despite the key role of landscape heterogeneity in the development and spread of emerging forest diseases. Moreover, most studies have insufficiently captured multiple ecological factors driving the infestation of an endemic pathogen acting at the landscape, community, and individual scales. Methods We measured pathogen load, disease prevalence, and disease severity of Marssonina leaf spot in poplars in riparian forests. We explored the direct and indirect effects of multiple ecological factors on pathogen infestation using a path analysis. Specifically, we first assessed the effects of landscape and community factors on leaf traits including leaf area, specific leaf area (SLA) and leaf dry matter content (LDMC), and then examined the role of these factors in shaping disease dynamics. Results Path analysis showed that landscape features had no direct impact on leaf traits and pathogen infestation, but directly affected tree community composition. Landscapes with higher forest cover resulted in higher host density and tree diversity. Host density was the most important factor of pathogen load, with higher host density resulting in more symptomatic leaves. Tree diversity had direct effects on disease prevalence, with poplars growing in mixed forest stands far less affected by pathogens than in pure stands. Moreover, disease prevalence was positively related to pathogen load. Tree diversity strongly reduced SLA, but increased LDMC. Higher SLA was found to increase pathogen load and disease severity, but higher LDMC was found to reduce both of them. Conclusions Our results show that the effects of landscape and tree community on Marssonina leaf spot disease are mediated by leaf traits. Disentangling the effects of biotic and abiotic factors affecting pathogen infestation contributes to reduce the overall impact of this disease, which can provide policy makers with sustainable management of endemic plant diseases in natural forests.