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Sweet Chestnut Health Check: Help Fight a Double Threat

Sweet chestnut trees (Castanea sativa) are distinctive and popular, but they face a double threat.

By Peter Crow · April 17, 2024

Peter Crow, Manager of the multi-partner Observatree project, tells us why he needs your help to report healthy trees as well as those affected by Oriental chestnut gall wasp or Sweet chestnut blight.

Above: Oriental gall wasp. Picture Gyorgy Csoka ©Hungary Forest Research Institute Bugwood.org

With its long, glossy, pointed, serrated leaves, yellow catkins and edible nuts, Sweet chestnut trees are common in urban areas, parkland, and woodlands. They can be long-lived, becoming a valued landscape feature. They may only account for a small percentage of Britain’s tree cover, but they can be locally important, supporting a variety of traditional industries.

Sweet chestnut is native to southern Europe, western Asia and north Africa and may have arrived in Britain with the Romans. Historically, the tree was cultivated extensively in south-east England. Its strong, durable timber make it good for joinery and cabinet making. Sweet chestnut coppices produce long, straight stems. These have many traditional uses including chestnut paling fence stakes and hop poles. Their nuts are edible to humans and can be eaten raw, roasted or ground into flour. The flowers provide nectar for a particularly strongly flavoured, dark honey.

Unfortunately, a pest and a disease are affecting these trees and Observatree are keen to understand their distribution. Members of the public are therefore being asked to report any sightings of the Oriental chestnut gall wasp (OCGW) and chestnut blight (Cryphonectria parasitica) to TreeAlert 

Oriental Gall Wasp

Above: Galls formed by the larvae of Oriental chestnut gall wasp on Sweet chestnut leaves. Picture: Forestry Commission ©Matteo Maspero

Dryocosmus kuriphilus is a species of gall wasp native to China. It is commonly referred to as the Oriental chestnut gall wasp (OCGW) and produces galls on Sweet chestnuts. It 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. This prompted an increased surveillance by tree and plant health inspectors, altering our understanding of the distribution of the pest and how long it had probably been in the UK.

The wasp induces galls to develop on young twigs or leaves. Typically, this causes leaf distortion and deformity, potentially resulting in a reduced growth rate. The galls are usually 5 -20 mm in diameter. They are initially green or rose-pink in colour but turning red then brown as they age. Galls that have developed on twigs become woody and may remain on the tree for two years or more. No other galls have been reported on Sweet chestnut in Britain, but other leaf deformities do occur. Gall formation on the leading apical buds can result in lateral branching, potentially lowering the value of the coppiced timber. High numbers of galls may weaken trees, making them potentially more vulnerable to other pests and diseases, especially Sweet chestnut blight (see below).

The adult wasp is small, harmless to people and is unlikely to be noticed. The females lay eggs in the leaf buds of Sweet chestnut trees during the summer. Their eggs typically hatch within 30 to 40 days. The larval stage can also enter dormancy and overwinter in the buds before forming the galls the following spring. The larvae feed for 20-30 days within the galls. They then pupate and emerge as adult wasps during mid to late summer, creating holes in the galls as they exit. These wasps live for approximately 10 days, completing their life cycle as they lay more eggs.

In recent years, researchers have been studying another insect that predates on the OCGW and studies are ongoing to establish the effectiveness.

Sweet Chestnut Blight

Above: Bright orangy brown chestnut blight lesion. Picture courtesy of Dr. Daniel Rigling Eidg. Forschungsanstalt fur Wald Schnee und Landschaft WSL Switzerland

Sweet chestnut blight (Cryphonectria parasitica) is a notifiable fungal infection. It has caused severe epidemics of over large areas of North America, resulting in the death of many trees. It has also affected Sweet chestnut over a wide area of continental Europe and tree losses have been regionally significant. Since 2011 it has been found on a small number of sites in central and southern England.

The spores of C. parasitica are spread by wind and water, but might also be transmitted by insects and birds. Entry into suitable tissue for infection might be aided by wounds produced by insects, including the OCGW. The fungus can spread rapidly in infected stems or branches. If girdled, the dead bark becomes visible as a sunken, discoloured canker. The fungus has small orange fruiting bodies (visible on the bark) that erupt in moist weather to spread the blight. Above a girdling canker, leaves wilt and turn brown, but can remain hanging on the tree. Below the infected area, branches have healthy foliage, often associated with increase epicormic growth. It is common to find many cankers on a single tree, leading to a sparse crown.

Some of the symptoms caused by C. parasitica infection, such as crown dieback, can also be caused by other pathogens, including Phytophthoras such as Phytophthora ramorum, P cambivora and P. cinnamomi. The latter is frequently associated with ‘ink disease’, named after a blue-black stain found around damaged roots. These pathogens are already present in the UK, and are known to cause disease on Sweet chestnut.

How you can help

Above: Mycelial fans produced below the bark by Cryphonectria parasitica. Crown Copyright. Forest Research

The multi-partner Observatree project is led by Forest Research. We are asking members of the public to use TreeAlert to report:

  • Suspected cases of OCGW or chestnut blight
  • Healthy Sweet chestnut trees that show no signs or symptoms of pests or diseases (Healthy tree reporting is only available to users who register with TreeAlert)

Observatree is a tree health citizen science project that trains and manages a network of volunteers to identify and report priority tree pests and diseases. The project also produces many tree health information resources that are freely available for anyone to use.

Citizen Science is now recognised as an important source of information in the 2023 GB Plant Biosecurity Strategy. This is reflected in the recent investment in the redevelopment of TreeAlert, Britain’s online tool whereby any member of the public can submit a report of a suspected tree pest or disease. But additionally, anyone can now register on TreeAlert and record healthy trees.

These additional reports will help inform our understanding of where affected trees are, as well as the proportion being affected.  Please ensure that good biosecurity practice is followed when assessing the health of any trees to prevent the accidental spreading of any pest or disease.

Further information on OCGW, Sweet chestnut blight and further tree pests and diseases can be found at the Observatree website.

Peter Crow

Peter Crow has a background in forest related environmental science. He helped to pioneer the use of lidar to map archaeological sites beneath forest canopies. Observatree has proved a successful tree health citizen science project. It is now in its tenth year. Observatree volunteers have submitted more than 20,000 tree health reports (many of which are on healthy trees).