Phenological Evaluation of Bud Retention, Leaf Production, and Nematode Abundance Associated With Beech Leaf Disease

Abstract

Beech leaf disease (BLD) is an emerging threat to American beech (Fagus grandifolia Ehrh.) throughout North American forests attributable to the plant-parasitic nematode Litylenchus crenatae ssp. mccannii (Lcm). This study describes the impacts of BLD on bud retention and subsequent leaf production, associated symptoms, and the abundance of Lcm throughout the course of a single growing season within two natural forest stands exposed to different levels of disease pressure. We found that trees with 2+ years of BLD infection exhibited reductions in bud retention that represented 52.3% fewer buds per branch (m⁻¹) relative to a control stand that did not exhibit BLD symptoms in the previous year. Diseased trees also produced less foliage following spring bud break, corresponding to 67.8% fewer leaves per branch (m⁻¹). Bud death among diseased trees resulted in a decoupling from the expected foliage production derived from winter bud inventories. Through the characterization of BLD symptom expression among emerging leaves, including the presence of second flush leaves, we measured casting of severely symptomatic leaves coordinated with a marked increase in second flush leaves in the late summer. Nematodes counted within infected trees ranged from 83 to 9800 individuals per bud, while the proportion of live motile stages, eggs, and dead Lcm varied significantly between the spring and late-summer sampling periods. Together, our data provide important insights regarding the impact of a multi-year BLD infection on bud and foliage production, which has the potential to adversely affect tree carbon assimilation and long-term survival.