Plant health status of Fagus spp.(FAGUSTAT)

Beech Leaf Disease (BLD) was detected in Ohio, USA, in 2012, on American beech (F. grandifolia). Since then it has spread to 12 northeastern states and Canada (Ontario), in forests and landscaped areas. The symptoms of BLD include interveinal dark bands on the leaves, crinkling and irregularly thickened leaves and twig dieback; ultimately leading to decline and death of young trees within years. Beech Leaf Disease causes severe damage to stands of F. grandifolia, but also to European beech, F. sylvatica, the main beech species in Europe. A new nematode, Litylenchus crenatae subsp. mccannii (Carta et al., 2020), is now considered as the causal agent of BLD (Vieira et al., 2023) but it was suggested that other microorganisms also play a role in the disease (Burke et al., 2020; Ewing et al., 2021). Pathways of transmission between trees have not been elucidated yet; they might include windborne water, mites, insects, or birds. BLD has never been reported from Europe and could be a severe threat to Fagus spp. stands. In this project, we made a first assessment of the presence/absence of BLD in Europe. Surveys were organised in Belgium, Ireland, the Netherlands, Romania, Slovenia and the UK. A fact sheet about BLD was made to inform people in the field and samples were collected using a common sampling strategy. Samples were taken during summer and fall (July-November) in 2021 and 2022, from beech trees showing symptoms similar to those caused by BLD. This resulted in 658 samples taken at 561 sites. More sites were visually inspected, but not all had trees showing BLD-like symptoms. Nematodes were extracted from leaves, buds and beechnuts, using the Baermann technique, with or without mistifier. Occasionally some non-plant parasitic nematodes were observed, but Litylenchus crenatae subsp. mccannii was never found. This project also aimed to increase public awareness of BLD and encourage the reporting of symptoms. Therefore, people in the field (inspectors, foresters, botanic gardens, parks, arboreta, NPPO…), as well as the general public, were informed via leaflets, posters, presentations, newsletters, publications in specialized magazines, institutional websites or observation platforms. Although the nematode was not found, the microbiome of symptomatic and healthy leaves was studied in Belgium and UK, using RNA-sequencing (metatranscriptomics) and metabarcoding. There were no clear differences compared with genera of bacteria and fungi reported from America. The drivers for the fungal and bacterial leaf microbiome, studied using barcoding, were mainly “location” and “environment” (nursery vs forest). Attempts were made to obtain inoculum of L. crenatae for the evaluation of the host status of European beech cultivars and to study the nematode’s behaviour under European climatic conditions (in confined conditions), but acquiring the nematode was very difficult. Moreover, attempts to culture the nematode on carrot disks, fungal mycelium, in beech plantlets and on beech callus failed. We engaged with BLD researchers in the USA and Canada through regular online meetings with BLD researchers, receiving valuable and updated information. To assess the possibilities of entry from America or Asia, information was gathered on the origin of beech tree materials. As current legislation does not allow import, except for some cases which require phytosanitary certificates, entry of infected leaves is theoretically impossible. Imports with BLD, could have occurred some decades ago however, so vigilance for the presence of BLD is still appropriate. The obtained information will contribute to pest risk assessments for Europe and the development of appropriate measures for protecting Fagus spp. in Europe.

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