ABSTRACT Beech leaf disease (BLD) is a foliar disease of American beech ( Fagus grandifolia ) that manifests as gall symptoms in leaves caused by infestations from Litylenchus crenatae ssp. mccannii, a plant‐parasitic nematode. As the disease progresses, infestations lead to bud abortion, crown deterioration and branch dieback. While BLD is known to impair leaf function, the impacts of the disease on tree carbon allocation and growth are unresolved. Here, we sampled three forest stands across a gradient of disease severity to assess how BLD influences the storage of non‐structural carbohydrates (NSC) across different tissue pools (root, bole and twigs), used tree rings to estimate growth declines associated with the onset of BLD infestation and evaluated the fine root ectomycorrhizal communities. Our data show that BLD can negatively impact NSC storage across tissue pools in stands exhibiting both low‐ and high‐severity infestation. Diminished starch content with respect to the asymptomatic stand was shown to precede any reduction in growth, measured via post‐infestation basal area increment at the low‐severity stand (1–2 years of infestation). Growth reductions at the high‐severity stand (3+ years post infestation) were significant and coincided with low NSC content, though mortality was not observed among any mature trees. Root‐associated fungal community composition differed significantly among sites, with a trend towards reduced diversity in more severely impacted trees. Our results suggest that BLD disrupts carbon allocation and storage in beech trees, potentially compromising their resilience to future stressors. The complex interplay between nematode infestation, fungal community shifts and altered carbohydrate dynamics highlights the multifaceted nature of BLD and its potential long‐term consequences for beech forests.