Host of the month - Rowan/ Mountain Ash

‘Host of the month’ is a series of Blogs and PDF’s that highlight a tree host and their associated priority pests and diseases that are best seen and recorded in that month. This June we’re looking at Rowan/Mountain ash and European Mountain ash Ringspot Associated Virus

Rowans are members of the rose family (Rosaceae), which also includes cherries (Prunus) apples (Malus) and hawthorns (Crataegus). Recent taxonomic changes to the rowans and whitebeams mean that only Rowans are now in the genus Sorbus; Whitebeam and Wild service tree are now Aria and Torminalis respectively. There are around 70 Sorbus species worldwide including UK native Rowan (Sorbus aucuparia) and several species which are widely planted in parks and gardens. Rowan leaves are compound pinnate (made up of small leaflets attached to a central rachis) which are arranged alternately on the shoots. The clusters of small white flowers are followed in autumn by brightly coloured berries in the autumn, in the UK native (S. aucuparia) these are brilliant orange-red and provide a valuable food source for birds and small mammals in early autumn.

 Despite also being known as Mountain ash this species isn’t closely related to Common ash (Fraxinus excelsior) which is a member of the olive family (Oleaceae) and isn’t affected by pathogens like Chalara ash dieback or Emerald ash borer.

European Mountain ash Ringspot Associated Virus (EMARAV)

This virus has been reported on Wild service tree (Torminalis glaberrima), Whitebeam (Aria edulis), Chokeberry (Aronia spp.) and Service berry (Amelanchier spp.), but the main host is Rowan. It has been reported across the UK but is likely to be much more common than current records suggest. Infected trees appear to grow more slowly and may suffer gradual dieback over several years. Alone it is unlikely to kill an affected tree but the reduction in healthy leaves will lead to a decline in food production, stressing the tree and reducing it’s ability to fend off other threats.

Identification

Infection by this virus results in a mottled discolouration of the leaves, initially a pale speckling which gradually morphs into ringspots and eventually a distinctive ‘oak-leaf’ pattern akin to a Rorschach ink blot.

Lookalikes

Several other pathogens and pests affecting Rowan can present similar looking signs and symptoms. Some of the more common ones (Rowan gall mite, Pear leaf blister mite, and Juniper rust) produce raised areas of discolouration whereas those produced by EMARAV are flush with the leaflet surfaces.

For more information on the symptoms of EMARAV and lookalikes see the Observatree resources page and Host of the Month for July You can also test your knowledge with the Host of the Month Quiz