Observatree Pest and Disease Review 2024

In the UK, the threat from plant pests and diseases is assessed by pest risk analysis (PRA) and managed through the UK Plant Health Risk Register. Each year about a hundred new pests and pathogens are added and currently there are over 1400 entries. Each organism is allocated a score, with and without mitigation, on a scale from 1-125 dependent on a wide-ranging list of factors that includes: likelihood of entry or establishment, rate of spread, impact (economic, environmental, social) and value of hosts or industries risk. The greater the risk the higher the score.

Observatree volunteers carry out surveillance for named priority pests and pathogens including organisms such as Oak processionary moth, Asian longhorn beetle, and Ash dieback. At the start of the project in 2013, working with plant health policy advisers and pest risk analysts a list of 20 high-profile pests and diseases were selected. Great care was taken when considering these stakeholder suggestions to ensure that the targets selected were suitable for citizen science surveillance and suitable for Observatree. The list includes a mix of organisms not known to occur in the UK such as Emerald ash borer; but several that had just arrived such as Ash dieback. Including a readily found and identifiable pest or disease gives volunteers an opportunity to practice and develop their skills such as tree identification, geographical location recording, and reporting via Tree Alert.

Every year the Observatree priority list is reviewed due to changes in the UK Risk Register. Although the temptation would be to continuously add several new organisms a year, we are mindful of the volunteers and projects’ capacity to cope with too many organisms (30 priority pest is probably the upper limit). The priority list has now expanded to include emerging threats such as Cedar blight (Sirococcus tsugae), European Mountain Ash Ringspot associated Virus, Elm zigzag sawfly (Aproceros leucopoda), Beech leaf disease and most recently in 2024 Agrilus fleischeri.

Working with colleagues from Defra’s horizon scanning team we have drawn up a list of all the additions in 2024 to the UK risk register and have checked other horizon scanning data sources such as the European Plant Protection Organisation (EPPO) Pest Risk Alert website as well as speaking to a wide range of colleagues across the UK Plant Health service to identify potential targets.

For Observatree, we use set criteria to prioritise this list, this includes the pest or pathogen must be on a tree species (focussed on forestry and woodland species), an unmitigated risk register score of 30 or more, and the target must be suitable for citizen science surveillance. The first criterion can be a little subjective and requires consideration as some pests have a broad host range including amenity and orchard species. The third criterion needs to consider whether the pest is ‘conspicuous’ - can it be seen at ground level and not require any invasive sampling, does it have features that allows for recognition in the field and sufficiently different from common native species, could it be confused with a rare or red listed species?

To this end, during our latest review we identified five potential targets of interest:

Chestnut trunk borer (Neocerambyx raddei)

A longhorn beetle affecting mainly oak species in North-East Asia, with the potential to be introduced to the UK via trees for planting (including bonsais) or firewood. Host susceptibility of European species of oak and sweet chestnut should continue to be monitored through existing sentinel trees projects. Hosts significant and the damage and beetle morphology should be recognisably different from native species.

Hornbeam decline (Cryphonectria carpinicola)

Cryphonectria carpinicola is a fungus that affects hornbeam that is present in parts of Europe and was found for the first time in the UK in 2024. Impacts of C. carpinicola alone in the UK are uncertain, as there is conflicting information around its pathogenicity, but when it infects hornbeam in combination with a fungus which has long been established in the UK (Anthostoma decipiens), impacts are likely to be higher. 

Monema flavescens

This pest is a highly polyphagous species that feeds on broadleaf trees (reported on 51 species from 24 families) and caterpillars are very distinctive so suitable for citizen science surveillance. It is an EPPO Alert organism as it may be a damaging pest but has not been formally risk assessed in the UK risk register.  It’s not recorded been in the UK and hasn’t been intercepted here but has been intercepted on imports to Europe.

Australian tortoise beetle (Trachymela sloanei)

Eucalyptus leaf feeder originating from Australia, which has been introduced into other parts of the world during the last decades. It is easy to recognise the tortoise beetles on eucalyptus.

Asian holly moth (Plesiomorpha falviceps)

Recent reports from Europe suggest this species could become established in the UK similarly to another recent Asian import, the now widespread Box-tree moth Cydalima perspectalis. A new adventive to Britain, first see London in 2016.  The caterpillar is likely recognisable on the foliage. It only affects Japanese holly which makes it only a potential pest to ornamental hedges and bushes and not native holly.

After consultation with UK plant health policy, we concluded that all the existing organisms on our Priority Pest and Disease are still relevant and the work of the Observatree volunteers is highly valued.  Of the additions to the UK risk register in 2024 and through additional horizon scanning work we identified 5 potential pests from a list of 23 targets.  Of these 5, none were of sufficient impact to warrant escalation to the current Priority P&D list.  However, we will be raising awareness with our volunteers about these organisms and keep a watching brief in consultation with UK plan health policy if the situation changes.  Any suspicious reports should be reported through Tree Alert.