Spotting the rings of Mountain ash virus: citizen scientists are revealing new UK hosts

One of the priority diseases that Observatree volunteers are asked to report via TreeAlert is European mountain ash ringspot-associated virus. Rather a mouthful, but more easily referred to by its acronym (EMARAV), or its binomial (Emaravirus sorbi), or more colloquially as ‘mountain ash virus’ or simply ‘rowan virus’. As the names suggest, it’s a virus affecting mountain ash or rowan (Sorbus aucuparia). The virus is likely to have been present in the UK for many decades but was first confirmed by molecular tests in 2011 from rowan in Scotland. It is now widespread on rowan in Scotland and, as a volunteer living in the Highlands, I see it a lot locally (Figures 1-2). Although understanding its Scottish distribution and spread is still important, Observatree reporting is probably of most value in determining its spread across other areas of the UK. For example, there are increasing reports of EMARAV in northern, central, and even southern England from TreeAlerts and from online biological recording platforms (Figures 3-4).

Like all viruses, EMARAV is unculturable in a laboratory and its identification on rowan is based on a combination of characteristic symptoms (chlorotic ringspots, speckling, mottling or line-patterning) and the exclusion of look-alike causes like rowan rust (Gymnosporangium cornutum), Eriophyid gall mites, or insect feeding damage. The Observatree Identification Guide for EMARAV is a great resource in this respect. Although identification of EMARAV from symptoms can be reliable, absolute confirmation requires laboratory testing using molecular methods. This is also the case with many other viruses and with pathogens like phytoplasmas (causing symptoms such as witches’ brooms or ‘yellows’ diseases).

Forest Research and Fera have recently teamed up with Observatree volunteers to better understand the presence and distribution of tree viruses and phytoplasmas in the UK. This has been part of a research project, called Virnotree, with other European partners in the EUPHRESCO Network. In the case of phytoplasmas (which are like wall-less bacteria), citizen reporting has been globally important in unveiling hidden phytoplasmas when linked to laboratory testing. Whilst Virnotree focused on common tree species, volunteers were invited to include other tree species showing similar virus-like symptoms. This resulted in my observations of virus-like symptoms on two types of shrubs or small trees. Firstly, chlorotic ringspotting on a serviceberry (Amelanchier, possibly Amelanchier x lamarckii) from a flower bed in a small town in the Great Glen (Figures 5-6). Then similar looking symptoms on a chokeberry (Aronia, possibly Aronia x prunifolia) in a garden in the Cairngorms (Figures 7-8). Symptoms on both plants looked like those of EMARAV on rowan, which were also observed nearby, and Virnotree requested samples for testing. In both cases, diagnostic testing of the leaf samples revealed the presence of EMARAV genetic material, confirming these as new UK hosts for EMARAV. Until now, EMARAV had only been officially recorded in the UK from Sorbus aucuparia, although there was an unconfirmed observation on at least one other Sorbus species, Sorbus vilmorinii, from a walled-garden on a Scottish west-coast estate, reported from a local naturalist group’s field outing (Figures 9-10).

Although serviceberry and chokeberry are new UK hosts of EMARAV, they aren’t necessarily a surprise as Amelanchier and Aronia are known as hosts in continental Europe and, like rowan, they are in the rose family. However, the findings posed new questions, most notably whether there was a risk to Aronia berry crops (Aronia melanocarpa) currently grown in the UK at a rather small scale in orchards? Another question related to how the virus spreads between rowan and these new hosts? Although little is understood about how the virus is transmitted within and between host species, it is thought that Eriophyid gall mites could be involved in spreading the virus, e.g. Phytoptus pyri, the pear leaf blister mite. Defra and the Scottish Government have reviewed and updated their plant health risk assessments and, based on current evidence, EMARAV on these new UK hosts is not considered to represent a significant or escalating threat. Nevertheless, this is perhaps a small demonstration of how citizen observers can help discover potential new risks when directly connected to laboratories and to research projects. Perhaps the next citizen detection of an unusual pest or disease symptom on a tree might prove more significant - and it could be yours!

Co-authors and Acknowledgements: This article is co-authored with Lisa Ward (Forest Research), Aimee Fowkes (Fera) and Adrian Fox (Fera). Thanks to Matt Parratt (Forest Research) for identifying plant species and Michael Crow for TreeAlert data; Maps dated December 2025. Fera and Forest Research participation in Virnotree was done with Defra funding. Photographs by Alan Inman (CC0, available from iNaturalist).