Cellular Dynamics of Beech Leaf Disease on <i>Fagus sylvatica</i>

ABSTRACT European beech ( Fagus sylvatica ) forests are increasingly exposed to a range of environmental stressors, including the rise of plant pathogens. A concerning development is the rapid spread of beech leaf disease (BLD) in North America, prompting questions about the potential risk of this disease spreading to other non‐native areas. As a result, there is an urgent need to better understand the relationship between the causal agent of BLD, the foliar nematode Litylenchus crenatae subsp. mccannii (Lcm) and F. sylvatica . This study explores the impact of Lcm on European beech and aims to provide a more comprehensive assessment of the changes in bud development, leaf morphology and alterations to the leaf epidermis. Our results demonstrate a dynamic fluctuation in nematode populations within buds, highlighting the high reproductivity of Lcm in F. sylvatica . Nematode infestations in buds lead to hypertrophy and irregular cellular patterns of the bud scales. Furthermore, histological analyses of BLD‐affected leaves reveal significant alterations in leaf architecture, including an abnormal increase in the number of cell layers, particularly in the spongy mesophyll, and abnormal stomatal development. Notably, we observed irregular stomatal distributions and reduced stomatal density in symptomatic BLD leaf areas, suggesting disrupted gas exchange and reduced photosynthetic capacity of the leaf. Additionally, leaf fractures facilitate nematode dispersal onto the leaf surface, potentially contributing to the spread of Lcm. Given the impact of Lcm in beech trees, understanding how they infect their host will deliver routes to mitigate their threat to the forest.