Monitoring foliar symptom expression in beech leaf disease through time

Abstract

Abstract When studying a forest disease, understanding the phenological relationship of the host tree and its pathogens is essential for identifying optimal management strategies to help prevent future spread of the disease. Since beech leaf disease (BLD) is a recently discovered disease, information about the general epidemiology and symptom phenology is largely unavailable. This study sought to answer questions related to symptom progression by conducting two observational studies on 10 trees from Cleveland Metroparks during the 2019 and 2020 growing seasons. BLD symptoms are characterized by two distinct leaf symptom types: dark green interveinal banding pattern or completely dark green and thickened leaf. Since there is evidence that the exotic nematode Litylenchus crenatae ssp. mccannii is associated with symptom development after direct inoculation into the buds, we hypothesized that symptoms would be apparent on the leaves at bud break. In our study, we visually confirmed the presence of both BLD leaf symptom types at bud break in naturally infected trees. Along with visual confirmation, a generalized linear mixed model (GLMM) showed that symptoms do not change throughout the growing season as time was not a significant variable when comparing symptoms across a growing season. Using both a Fisher's exact test and GLMM, we also determined that BLD leaf symptoms from a single leaf–bud pair do not progress in a specific or predictable pattern through subsequent growing seasons. These results formally validate the timing of BLD symptom expression and patterns of severity between years which will assist in furthering our understanding of the BLD pathosystem.