UK: On the presence of ringspot virus

Symptoms consistent with European mountain ash ringspot-associated virus (EMARaV), genus Emaravirus, have been historically observed across the UK on Sorbus aucuparia (rowan or European mountain ash). The virus was recently confirmed across Scotland. EMARaV has also been detected by molecular methods in Scandinavia, Russia and the Czech Republic.

In England, there was an initial finding of EMARaV from rowan leaves exhibiting chlorotic ring and line-pattern symptoms (Fig 1.) at the Yorkshire Arboretum, North Yorkshire, in July 2013. A site-specific survey of Sorbus spp. was conducted by Fera in July 2014. The virus was also included as a 'priority pest' in the Observatree citizen science project, with further symptomatic samples of Sorbus spp. submitted from sites across England.

To confirm the presence of the virus in these samples, total RNA was extracted and tested by RT- PCR. Amplicons of the expected size for RNA 2 (300 bp) and RNA 3 (204 bp) were sent for sequencing (Table 1). These were used to construct maximum likelihood (1000 replicates) phylogenetic trees including a representative selection of other EMARaV sequences available on GenBank.

EMARaV was confirmed at the Yorkshire Arboretum in S. aucuparia subsp. sibirica, and S. glomerulata, S. rehderiana, S. sp. aff. scalaris and S. pteridophylla, representing first host records for the latter four species. The virus was additionally detected from S. aucuparia from Crawley, West Sussex, Amotherby and Coneythorpe Bank (both North Yorkshire), and in Sorbus spp. from southwest London.

A neighbour joining phylogenetic tree generated from the partial RNA2 sequences obtained from the UK findings using MEGA6 (Fig. 2) confirmed the viruses as EMARaV. It also showed that the main outbreak in the Yorkshire Arboretum and Amotherby, c. 5 miles away, may have originated from the same source but that the original 2013 outbreak at the Yorkshire Arboretum, Coneythorpe Bank and London may be distinct from this outbreak and each other (approximately 2% sequence divergence). No RNA2 sequence was obtained from the Crawley sample. Similar results were obtained for RNA3 (data not presented).

Following initial findings in Scotland, a UK pest risk analysis was conducted. These findings support the analysis's hypothesis that the virus is present in a broad range of Sorbus species and confirms that EMARaV is present in several different regions of England with the infections likely to be from multiple previous incursions.

Source: ndrs.org.uk

Publication date: Tue 17 Oct 2017