Landscape and stand-scale factors drive the infestation of an endemic fungal pathogen: The role of leaf traits

Endemic plant pathogens are important agents of many forest processes and contribute to shaping forest ecosystem dynamics and diversity. However, Marssonina leaf spot of poplar is induced by a pathogenic fungus Drepanopeziza populi, causing increased damage to riparian poplars in recent years. And such endemic diseases have received little attention at the landscape scale, despite the key role of landscape features in the development and spread of certain emerging diseases. Moreover, few studies have sufficiently captured multiple ecological factors driving the infestation of an endemic pathogen acting at the landscape, stand, and individual scales. Here, we measured pathogen load, disease prevalence, and disease severity of Marssonina leaf spot in bitter poplars (Populus laurifolia) in riparian forests, and explored the direct and indirect effects of multiple ecological factors on pathogen infestation using structural equation models. Specifically, we first assessed the effects of landscape and stand-scale 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. We found that forest cover had no direct impact on host leaf traits and pathogen infestation, but directly affected stand characteristics, with higher forest cover in a landscape increasing host density and tree diversity. Host density was the most important factor of pathogen load, with a higher density of host poplars resulting in greater symptomatic leaf count. Tree diversity had direct effects on disease prevalence, with host poplars growing in mixed forest stands far less infected by pathogens than in pure stands. Disease prevalence was also positively related to pathogen load. Moreover, tree diversity strongly reduced SLA but increased LDMC. Leaves with higher SLA was found to increase pathogen load and disease severity, but leaves with higher LDMC was found to reduce them. Our results show that landscape-scale forest cover indirectly affects Marssonina leaf spot dynamics via stand characteristics and leaf traits. Disentangling the effects of biotic and abiotic factors on pathogen infestation can provide crucial insights into multi-scale ecological factors in driving plant disease expression, and contribute to reducing the overall impact of endemic forest diseases.