Observatree is a Constant in 2020

General surveillance for priority pests and diseases in the wider environment is a significant role of Observatree volunteers. We have a list of 22 priority pests and diseases for the project, some already present and established in particular areas of the country, some recently arrived where we need to determine their spread, and finally, some that are not present in the UK and early detection would be critical for their management and control. For the last seven years Observatree volunteers have been contributing with their reports of suspected priority pests and diseases including reports of quarantine or regulated organisms such as chestnut blight (Cryphonectria parasitica) or the Oriental chestnut gall wasp (Dryocosmus kuriphilus).

We could say that this year has been unusual, and we had to adapt to new ways. However, nothing has stopped Observatree volunteers keeping their eyes open and reporting pests and diseases to the Tree Health Diagnostic and Advisory Service (THDAS). We never thought that this year will be the busiest year for THDAS since 2013. THDAS had over 2000 reports in the six months from the 1^st of April till the 30 of September 2020 and Observatree volunteers have contributed to these with 142 reports of suspected priority pests and diseases (a small part of the total survey work undertaken by Observatree volunteers who also monitor and report on healthy trees). Over 90% of the reports, submitted to THDAS, were using TreeAlert and in total eight priority pests and diseases were confirmed by THDAS from their submitted reports, four of those were pests and four were pathogens.

Eleven cases were reported of elm zigzag sawfly. This is a pest of elms that can potentially cause defoliation on elm trees. The first detection of this pest in Great Britain was in 2018 and reports from Observatree volunteers are helping to discover its distribution. The second pest reported by Observatree volunteers was oak processionary moth with nine reports. This pest can defoliate oaks and it also poses a health risk for humans and animals as older caterpillars develop tiny hairs containing an irritating protein called thaumetopoein. This pest was accidentally introduced in England in 2005 and now is established in most of Greater London. Oriental chestnut gall wasp was the third reported priority pest. This is a pest of sweet chestnut that was first detected in UK in 2015 and Observatree volunteers played a big role discovering the second outbreak area back then. Since its discovery, new reports have been submitted helping us to get a better idea of the distribution of this pest. Finally, we had two reports of horse chestnut leaf miner. This pest was introduced in 2002 and we are interested in its distribution in the north of England and Scotland and Observatree volunteers are encouraged to report from these areas. Observatree volunteers also reported other non-priority pests such a mite (Phyllocoptes sorbeus) which causes galls on leaves of rowan and a mite (Aceria erinea) which causes galls on leaves of walnut.

Regarding diseases, Chalara ash dieback was again the top priority disease reported with 33 reports. This disease was discovered in Britain in 2012 and Observatree volunteers have played an important role in recent years, reporting the disease in areas with no previous reports. Data submitted by Observatree volunteers contribute to the generation of update maps of the disease. European mountain ash ringspot associated virus (EMARAV) was the second most reported disease with four cases. This is often reported from Scotland, this year three of the four reports were from the south of England and one from Scotland. Three cases were confirmed Acute Oak Decline (AOD) and Observatree volunteers are working in close collaboration with the team at Forest Research working on AOD and led by Dr Sandra Denman. One case of Sirococcus blight of cedar (Sirococcus tsugae) was reported for this period. This is a pathogen of cedars and hemlocks and was detected in UK for the first time in 2013, and since then, we have been monitoring its spread. This year we also had reports of regulated pathogens such as Phytophthora ramorum on sweet chestnut or P. kernoviae on Rhododendron; or other pathogens such as honey fugus, needle cast of pine, Verticillium wilt on bay laurel or bleeding canker caused by Phytophthora pseudosyringae on beech.

It is true that it has been a different year but on the brighter side, we have had many opportunities where we met through webinars, we shared presentations from the partners of the project or from volunteers, we had different discussions on samples and interesting findings. For me, Observatree volunteers have been a constant through the year as highlighted on their report activity, and personally, I would like to say thank you to all of them for their commitment and enthusiasm.