‘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. For December we’re looking at common ash, chalara ash dieback, and emerald ash borer.

Common or European ash (Fraxinus excelsior) belongs to the olive family (Oleaceae) along with privet, lilac, and olives. Ash leaves are compound, that is they’re made up of smaller leaflets attached to a mid-rib called a rachis. In winter it is readily identified by the stout black buds, reminiscent of a Bishops mitre or deer hooves. The winged seeds are known as keys and hang in bunches on the trees, often remaining attached well into winter.

Mountain ash (Sorbus aucuparia) is not related to common ash and is not affected by either chalara ash dieback or emerald ash borer.

Priority disease – chalara ash dieback

Ash dieback hit the headlines when it was identified in the UK in 2012 and is now present in most parts of the United Kingdom.  The fungus that causes chalara ash dieback (Chalara fraxinea) originated in eastern Asia where native ash species such as Chinese ash show only minor symptoms on their foliage. Unfortunately common ash is highly susceptible and once infected there is no cure, but tolerance is variable. The fungus is spread via wind-blown spores produced from late spring which settle on ash leaflets. The fungus then invades the leaflet tissues and grows into the leaf mid-rib, through the leaf stalk, into the shoot and then into the larger branches and the main stem.


Once in the woody stems and branches the fungus causes diamond-shaped, dark coloured lesions that are always at the insertion points of leaf stalks, shoots, and branches. Where girdling occurs the foliage above the lesion wilts and turns black but is retained on the tree. Below lesions the infected trees often produce abundant epicormic shoots which are most easily seen (along with the lesions) following leaf fall in the autumn.

Priority pest – emerald ash borer

Emerald ash borer (EAB) is native to Asia but was identified as the cause of ash decline and death around Detroit USA in 2002 and is now found across 22 states and into Canada. EAB has also been spreading eastwards across Eurasia and is now established in Ukraine. It is not currently known to be present in the UK.

Adult beetles appear in early summer and are 7.5 and 13.5mm long, slender bullet-shaped and metallic emerald-green. Females lay their eggs in bark crevices and once hatched the whitish flattened larvae burrow into the tree and begin feeding. Over the course of one to two years they chew out sinuous tunnels.


Early symptoms of EAB infestation are related to the girdling effect of larval tunnelling. Foliage discoloration is soon followed by crown and branch dieback, usually starting at the top of the tree and moving downwards as infestation progresses. These symptoms might also be accompanied by abundant epicormic growth throughout the tree and sometimes heavy seed production. After pupation the adults emerge via characteristic D-shaped exit holes in the bark of infested ash trees.

For more information check the Observatree resource pages for Chalara ash dieback and emerald ash borer, or the Host of the Month for December. You can also test your knowledge with the Host of the month Quiz 


December is an ideal time to seek out ash trees and see if you can identify any signs and symptoms of ash dieback or EAB. Both are priority pests and pathogens so please report possible sightings via TreeAlert. Healthy tree data is equally important so please do report those too.