‘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 November we’re looking at sweet chestnut, Oriental chestnut gall wasp, and sweet chestnut blight.

Thanks to a long history of cultivation the true native range of the sweet chestnut (Castanea sativa) is difficult to establish, but it is probably across the northern Mediterranean from north-west Spain, Italy, Greece and along the Black Sea coast into the Caucasus. The species is now found across Europe where it is grown for both the edible nuts and to produce timber. It is widespread in the UK but is not native and may have arrived with the Romans. Although the nuts are not reliably produced in the UK it is often grown and managed using coppicing to produce products such as chestnut paling and post other fencing types.

Sweet chestnut belongs to the Fagaceae family along with beeches and oaks and is characterised by nuts produced in an extremely spiny case, and catkin-like male flowers. The simple leaves are spearhead-shaped with large teeth along the margins, and the bark on mature trees is deeply furrowed and often spirals around the trunk.

Despite its name horse chestnut (Aesculus hippocastanum) is not related to sweet chestnut and is not affected by either Oriental chestnut gall wasp or sweet chestnut blight.

Priority pest - Oriental chestnut gall wasp
Oriental chestnut gall wasp is native to parts of Asia and was first recorded in the UK in 2015 by an amateur entomologist in Kent, a short time later it was also found in St Albans by an Observatree volunteer. Adult female wasps lay their eggs in the newly formed buds during July and August. The larvae feed for the remainder of the summer, go dormant over-winter then resume activity in the spring when they start to form new galls.

The most obvious sign of their presence are the galls which form on twigs or in the leaf stalk or mid-rib of leaves where they cause leaf distortion and deformity. Initially they are either green or pink, but as they age they turn first red and then brown. Galls on leaves fall from the tree in the autumn but those on twigs can remain on the tree for two years or more where they stand out in winter.

Priority disease - sweet chestnut blight
This destructive fungal pathogen is native to eastern Asia where it does limited damage to its host there, Chinese chestnut. Following introduction into North America in 1904 it soon spread across the entire range of American chestnut and killed an estimated 4 billion trees. It appeared in mainland Europe in the 1930’s and has caused extensive damage to sweet chestnut plantations. It was first confirmed in the UK after an outbreak in Warwickshire.

Spores enter host trees through fissures and wounds in the bark where they germinate and spread, rapidly forming lesions. Externally these lesions present as sunken and often brightly coloured cankers on younger material and roughened cracked cankers on older trees which eventually girdle stems and branches, cutting off the flow of water and nutrients to tissues above it. Leaves above cankers gradually wilt and die, whilst below the canker there are often many epicormic shoots. Fruit bodies are frequent and appear as yellow-orange pustules on infected bark.

For more information check the Observatree resource pages for OCGW and Sweet chestnut blight, or the Host of the Month for November. You can also test your knowledge with the Host of the month Quiz


November is an ideal time to seek out sweet chestnut trees and see if you can identify any signs and symptoms of OCGW and sweet chestnut blight. 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.