FAGUSTAT: Investigating Beech Leaf Disease, a threat to beech trees and forests in Europe

Beech Leaf Disease (BLD), reported from the northeastern United States and Canada (Ontario), is causing severe damage to mainly American beech (Fagus grandifolia) but also to European beech (F. sylvatica) [1]. The disease is characterised by dark interveinal bands on the leaves, leaf deformation and bud abortion in spring, leading to canopy thinning and tree mortality in advanced cases. So far, the etiology of the disease is not fully understood. Litylenchus crenatae subsp. mccannii, a nematode species isolated from beech leaves and buds in America, is involved in BLD, although other microorganisms probably contribute to its development [2]. In Japan, L. crenatae was found in leaves of F. crenata, but has not been associated with damage. It is not known if BLD or Litylenchus spp. occur in Europe, therefore, it should be considered as a threat to the European beech trees and forests at this moment. Within the Euphresco project 2020-A-334, FAGUSTAT, we are increasing awareness of BLD in Europe and making the first assessment of its status in the region. In 2021, we performed surveys on the health status of Fagus spp. in the six participating countries (Belgium, the Netherlands, Romania, Slovenia, United Kingdom and Ireland). We collected about 250 samples of beech leaves, buds and nuts in forests, parks, botanical gardens and nurseries and extracted the nematodes using the Baermann technique. Although look-alike BLD symptoms were observed, only very few nematodes(e.g., Aphelenchoidessp., Plectus sp., Panagrolaimus sp.) were found; none belonging to Litylenchus. We drafted BLD information sheets to further increase public awareness. As pathways of BLD transmission are still unknown, we also explore possible ways of entry and spread. To study the biology of L. crenatae under controlled conditions, we are testing different culturing methods (carrot discs, callus tissue, fungal cultures and beech sapling inoculation) to obtain a large population of the nematode. In addition, we took subsamples of (a)symptomatic leaf and bud samples throughout the survey to determine their microbial community using high-throughput sequencing (HTS) technology. This will provide insight into the main drivers of the communities living in/on beech leaves and buds. We recommend countries not involved in the FAGUSTAT project to look out for BLD symptoms, as knowledge about the presence (or absence) of Litylenchus spp. is important to assess its threat to European beech trees.