An emerging disease of American beech (Fagus grandifolia), referred to as “Beech Leaf Disease” (BLD), has become common in the region around Lake Erie. The condition was first noticed in Lake County, OH, in 2012, but has since spread to include areas of throughout much of northern Ohio, southern Canada, and northwestern Pennsylvania. BLD involves characteristic interveinal thickening and darkening of leaf tissue followed by chlorosis-like yellowing of the leaf tissue and leaf curling. The cause of BLD is unknown but several microbial agents, including fungi, bacteria and nematodes have been suggested. In this study, we examine the microbiome of F. grandifolia, with varying degrees of symptoms, to determine whether known pathogenic microbes or insect pests could be associated with the condition. We collected leaves and buds from 14 beech trees, located within a common garden at the Holden Arboretum. Molecular methods were used to examine fungal, bacterial, and nematode community structure on leaves and dormant buds of affected trees. Additionally, leaf extractions were conducted and used to collect and identify any nematode species using morphological characteristics in addition to DNA sequencing.
Results/Conclusions
We found that bacterial and fungal communities differ significantly between leaf and bud tissue (P<0.001). Fungal community composition on both buds and leaves did not show a significant effect of BLD (P>0.05). Bacterial community composition on leaves did show a significant effect of BLD (P<0.001) using 1-way PERMANOVA, however there was no significant effect of treatment between buds. Analysis of TRFLP profiles and clone libraries suggest the genus Mucilaginibacter, which can produce exopolysaccharides including pectinase, is a significant indicator of bacterial communities on symptomatic leaves. We detected nematodes in a high proportion, on the leaf and bud samples we collected, with both asymptomatic and symptomatic tissues showing varying levels of infestation. Direct sequence analysis of extracted nematode ITS DNA identified the best match to the anguinid nematode Litylenchus crenatae. Though no official cause has been identified, understanding the microbiome found on F. grandifolia is an important first step in understanding this emerging threat. Continued spread of BLD could negatively impact beech survival and regeneration within hardwood forests, with additional ecosystem effects.