Plant pests (including pathogens) threaten forests, reduce carbon sequestration, disrupt trade, and are costly to manage. Traditionally, forest surveys involve the visual inspection of trees for symptoms of disease; however, this process is time consuming and by the time symptoms are observed, the disease may be widespread. New methods of surveillance are needed to help plant health authorities monitor and protect forests from disease. Previous research has shown that metabarcoding of environmental DNA (eDNA) can be used to identify plant pests. This study collected rainwater samples from five forest sites across Northern Ireland every month for a year to examine whether eDNA metabarcoding could help authorities detect plant diseases in forests. Metabarcoding of the internal transcribed spacer (ITS) region was used to determine the fungal and oomycete profile of rainwater samples that passed through the canopy of spruce, pine, oak, and ash trees, along with a non-tree field trap. In total, 65 known plant fungal and oomycete pests were detected; seven were regulated pests, and two were pests that had not been previously identified in Northern Ireland. This work demonstrates that metabarcoding of eDNA from rainwater can detect plant pests and could be used in forest surveillance programmes.

Ash dieback, caused by the fungus Hymenoscyphus fraxineus, continues to threaten European ash (Fraxinus excelsior), yet the contribution of the leaf microbiome to disease severity remains poorly understood. We surveyed 133 ash trees across nine sites in Northern Ireland, using canopy cover as a proxy for health, and characterised leaf-associated microbial communities using Pacific Biosciences (PacBio) long-read amplicon sequencing (full-length 16S and ITS) and QIIME2-based workflows. Many trees exhibited partial tolerance to ash dieback, with most maintaining stable canopy cover year-on-year, while fewer trees show a decline and a smaller portion showing improvement. Microbial communities were largely stable irrespective of ash health with little difference in alpha diversity (Shannon) or beta diversity (Bray–Curtis PERMANOVA) for either bacteria or fungi. Differential abundance and correlation analyses showed that H. fraxineus was, as expected, negatively associated with canopy cover. Only one fungal species, Papiliotrema flavescens, demonstrated a strong positive association with healthier trees, consistent with previous findings. These results indicate that Northern Ireland hosts a reservoir of ash trees displaying tolerance to ash dieback. While the leaf microbiome does not appear to drive this tolerance at the community level, one fungus, P. flavescens, was correlated with healthier ash.