Surveillance for virus and other non-culturable pathogens in forestry tree species (VirnoTree)

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Effective phytosanitary measures are essential to enable safe and sustainable trade while protecting plant health and forest ecosystems. The development and implementation of such measures depend on robust scientific evidence to ensure that regulatory responses are proportionate to the risks posed by pests and pathogens. At the same time, maintaining and improving the resilience of forests requires recognition that trees function as a holobiont—an ecological unit comprising the host tree and its associated microbiota, including fungi, bacteria, and viruses (Vainio et al., 2024; Rumbou et al., 2020).While viruses and other non-culturable (nonC) pathogens, such as phytoplasmas, are well recognised as significant threats in agricultural and horticultural crops, their prevalence, diversity, and ecological roles in forest trees (commercial, natural, and amenity) remain comparatively underexplored (Cooper, 1993; Marcone et al., 2021). Consequently, their potential impacts—including their role as infection reservoirs or contributors to tree decline—are poorly understood. Some phytoplasma diseases affecting forest trees are known to have substantial economic and ecological impacts due to the severe symptoms they cause, such as elm yellows and ash yellows (Marcone et al., 2021). However, for many viruses and other nonC organisms associated with trees, the extent of infection, host range, and biological significance remain largely unknown.Recent advances in high-throughput sequencing (HTS) technologies have greatly accelerated the discovery of plant viruses and other microbial associates. These approaches allow the detection of pathogen sequences without prior knowledge of their presence, revealing a far greater diversity of viruses in forest ecosystems than previously recognised. For example, Rumbou et al. (2021) reported 38 plant virus species infecting 19 tree and shrub species in temperate forests and urban environments in Germany, many of which have only been described within the last five years. Several of these viruses produce symptoms similar to those previously reported but uncharacterised by Cooper (1979), highlighting the likelihood that additional, currently undescribed viruses remain widespread in tree populations.Improving our understanding of the diversity, distribution, and biological relevance of tree-associated viruses and other nonC organisms is therefore essential for strengthening forest health surveillance, improving diagnostic capacity, and informing phytosanitary risk assessment.

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