Check a Sweet Chestnut

We need you! Be a good plant health citizen and help report on the health of our sweet chestnut trees

Launching in National Plant Health Week and continuing over the summer, we’re conducting a national check-up on the health of our sweet chestnut trees. We need as many people as possible to help out and report their findings through Forest Research’s TreeAlert system – the official online tool for reporting tree pests and diseases of concern in Britain.

What is the sweet chestnut?

The sweet chestnut (Castanea sativa) is a large tree, laden with yellow catkins in early summer. Its long, glossy green leaves have pointed tips, toothed edges, and turn golden before falling in autumn. Mature height is 17–22m and trees can live for up to 700 years. They have attractive bark, distinctively jagged-edged leaves and edible nuts.

Sweet chestnut is native to southern Europe, western Asia and North Africa. The first written records of them growing in Britain date to the 12th century, and they have long been naturalised here. Today they can be found commonly throughout the UK as urban trees, in parks, and in woodland.

How do I get involved?

1. Get prepared by reading about identifying sweet chestnut trees, sweet chestnut blight and the Oriental chestnut gall wasp on this page

2. Find a sweet chestnut tree when out on a walk, or by looking up where they have been mapped already on TreeZilla

3. Check the tree’s health – look for Oriental chestnut gall wasp leaf galls and disfigured patches of bark with a sparse crown of leaves (symptoms of sweet chestnut blight)

4. Report your findings to TreeAlert. If you find a healthy sweet chestnut tree, please register and submit a ‘healthy trees’ report from your dashboard. If you find suspected Oriental chestnut gall wasp or sweet chestnut blight, submit a ‘general report’. Make sure to select ‘2023 Check a Sweet Chestnut’ as the project.

Sweet chestnut (Castanea sativa)

What am I looking for?

The ‘Check a Sweet Chestnut’ project

We want to know where there are healthy and unhealthy sweet chestnuts across the UK so we can understand how far sweet chestnut blight and oriental chestnut gall wasp have spread since they were first reported. The information you provide will help us to produce an up-to-date map of healthy and unhealthy sweet chestnut trees and tell us whether our actions to control the spread of blight and gall wasp are working.

This year, between National Plant Health Week in May and National Tree Week in November, the RHS, Defra, APHA, Forest Research and Observatree invite you to submit data on sweet chestnut health to the UK tree health-reporting platform, TreeAlert.

We will add your data to two previous years of records collected between 2020 and 2022 for Project HOMED. Our plant health scientists will then analyse the data and create articles for a scientific journal and the RHS members’ magazine, The Garden, about the health of sweet chestnut trees in the UK and how we can better support communities to participate in tree health citizen science.

Please note: The Oriental chestnut gall wasp does not sting, and poses no threat to human health.

Further information

How do I participate – detailed explanation

To start, you will need to 

1. Learn how to identify sweet chestnuts, sweet chestnut blight and oriental gall wasp using the training materials linked to from this webpage

2. Create an account on TreeAlert

3. Explore your local area and locate sweet chestnut trees. Look in TreeZilla for any that are already logged but are in need of a check-up. Make a note of its location (a GPS logger app may be useful for this)

4. Check a Sweet Chestnut!

  • Look at the leaf canopy (if present) and determine if the canopy is full and healthy
  • Look at the leaf and twig shapes and determine if any are swollen and misshapen. Compare any symptoms or galls to Oriental Chestnut Gall Wasp
  • Look at the bark and see if you can see any patches that are sunken, cracking or peeling, red-brown in colour or featuring orange spots. The branches above may have few leaves. Compare the appearance to the symptoms of sweet chestnut blight using Observatree’s Cryphonectria symptom guide (6.94MB pdf)
  • Take photos of the whole tree, its trunk, the leaves and any flowers/nuts or symptoms present
  • Log in to your account on TreeAlert. Submit a ‘healthy tree’ report if your tree looked healthy, or a ‘general report’ if you found a plant health problem such as OCGW or sweet chestnut blight. Select ‘2023 Check a Sweet Chestnut’ from the project list.
How can I be sure I’m looking at a sweet chestnut tree (compared to a horse chestnut tree)?

This project is looking at sweet chestnut (Castanea sativa) - not to be confused with horse chestnut (Aesculus hippocastanum). 

  • Sweet chestnut bark is dull silvery purple when young, horizontally banded, which then turns to a distinctive vertically-cracked ridged bark which may appear to spiral up the tree after 60 years. (Horse chestnut bark is smooth and pink-grey when young, and brown and coarsely scaly by 80 years)
  • Sweet chestnut nuts are found in cases that look like burrs with many soft spikes. (Horse chestnuts have conkers found in stiffly spiny husks) 
  • Sweet chestnut leaves are long and shiny, and the edges have spined teeth 1cm apart. (Horse chestnut are not shiny, and palmate with 5–7 leaflets attached to each leaf stalk)
  • Sweet chestnut buds are hairless and alternate on the shoots. (Horse chestnut buds are sticky and found in opposite pairs)
  • Sweet chestnuts have a stiff spray of male catkins which smell like frying mushrooms. (Horse chestnut flowers are like showy candelabras)
What do I need to bring with me?

Bring a smartphone or digital camera and something to take note of the tree location and condition.

Dress appropriately for the weather conditions (e.g. warm, waterproof clothing or hat and sunscreen if hot) and wear clean, sturdy footwear. Please read the Health & Safety guidelines (pdf, 377KB).

If you find what looks like sweet chestnut blight, prevent spreading it to other trees by cleaning your hands and boots without contacting other sweet chestnut trees. Carrying hand sanitiser or a spray bottle of disinfectant or ethanol can help you stay clean when out and about. Please see the biosecurity guidance below for further information.

How is tree health citizen science helpful?

Citizen science has the potential to survey trees much more widely than is possible with directed inspections by the plant health authorities alone. By calling on the public to look out for new insects and diseases that can harm trees, there is a greater chance that our plant health authorities can act to control outbreaks early, before problems become widespread. These new insects and diseases may arrive undetected on internationally traded shipments and then move around the country. 

While this year we are focussing on sweet chestnuts, the understanding gained about how we support tree health in the UK applies to all of our tree species. We hope that by engaging with tree health citizen science, members of the public will feel more connected to their local trees. Citizen scientists also often enjoy other benefits, including spending time outdoors and learning about the natural environment.

What is TreeAlert?
TreeAlert was developed by the Forestry Commission, and is now managed by Forest Research on behalf of Defra, Forestry Commission, Scottish Forestry and the Welsh Government. Tree Alert is used to gather information about tree health issues across Great Britain. This information supports important tree health monitoring and surveillance work, contributes to ongoing scientific research, and helps to protect the nation's trees.

This information will support important tree health monitoring and surveillance work, contribute to ongoing scientific research in this field and, ultimately, support efforts to protect the nation's trees. Citizens can provide information about the health of the Great Britain's trees, woodlands and forests by reporting signs of unwelcome tree pests and diseases using TreeAlert.

You will need to make an account to submit records to TreeAlert.
What is Oriental chestnut gall wasp?

The Oriental chestnut gall waspDryocosmus kuriphilus, is a microscopic insect that infects sweet chestnut trees and forms green or pink swellings (galls) on buds, leaves and petioles. The galls can be up to 4 cm long. During summer, the galls are woody and brown-coloured.  If the galls are on the leaves, then leaves can appear warped and misshapen. Leaves with galls can fall prematurely or remain on the tree during the summer. 

Female wasps lay their eggs during the summer months in developing buds and approximately 30-40 days later, the larvae emerge. The larvae overwinter in the buds then become active in spring and cause galls to form. Within the galls, the larvae feed and develop into pupae, then female adults, which are able to lay eggs straight away. Adult wasps form a hole in the gall and emerge in late spring and summer, often between late May and July, when they go in search of new trees in which to lay their eggs. The wasps produce a single generation per year and male wasps are not needed for reproduction.

The first record of the oriental chestnut gall wasp in England was in June 2015 in Kent, assumed to have arrived through the international plant trade, and has since spread to other areas in the southeast of England. It is a regulated notifiable pest, so imported plant material is regulated and inspected to ensure more new wasps don’t  enter into the country. Sightings of this pest should be reported using TreeAlert.

Oriental chestnut gall wasp infestation can harm the health and vigour of the tree and make it an easier target for other pests and diseases. There is a potential association between Oriental chestnut gall wasp and the regulated and notifiable disease sweet chestnut blight.

More information about the Oriental chestnut gall wasp and pictures of the symptoms it causes can be found at:

What is sweet chestnut blight?

Chestnut blight is a disease caused by the fungus Cryphonectria parasitica. This disease originates from Asia and was introduced during the first half of the 20th century in the European continent and Northern America, where it has caused significant losses of sweet chestnut trees. The disease poses a significant threat for the UK’s sweet chestnuts. Currently the disease is limited to the SE of England. Sweet chestnut blight is a regulated and notifiable disease, first found in the UK in 2011 in Kent.

During wet and humid weather, the fungus produces spores which can disperse and infect new trees via fresh wounds on the bark. The fungus will then start killing the tissue of the tree. The disease causes a range of characteristic symptoms.

On the stem you will be able to see brown-orange lesions (patches) on younger stems cankers (sunken areas), cracks and girdling on the bark. The areas above the cankers will start to decline and symptoms of wilt and dieback will be seen in the canopy, with the dead leaves remaining attached on the tree. Epicormic growth (the production on multiple small shoots) may also be visible below the cankers on the stem.

Characteristic signs of the fungus can also be used to identify an infected tree. Several masses of yellow-orange to reddish-brown pustules appear around the trunk cankers through the trunk lenticels (pores). When the weather is wet and humid, they exude yellow-orange tendrils of spores. Furthermore, upon removal of the outer bark on the infected areas, the pale-brown mycelial fans might also be visible.

Chestnut blight attacks not only sweet chestnut trees but also other tree species such as oak. In order to prevent the spread of the pathogen and protect our trees, the plant health authorities have specific surveillance and control plans in place, which may include calling on landowners to remove infected trees to prevent further spread of the disease.

Sweet chestnut blight not only attacks sweet chestnut trees but also other tree species such as oak, so your contribution supports other trees too.

More information about the disease and pictures of the symptoms can be found at: 

How do I help with biosecurity?

Biosecurity measures are the practical steps designed to minimise the risk of introducing or spreading pests and diseases.

  • Wear footwear and outerwear that can easily be kept clean
  • Clean footwear and outerwear to ensure they are visually free from soil and organic debris
  • If you are revisiting an infected tree, plan to visit highest-risk sites last
  • Clean your hands with sanitising gel (or soap and water) and tools (eg tape measure) with bacterial wipes or via a spray bottle with  a bleach solution (1: 9 bleach/water) between touching trees,  to prevent spreading pests and diseases between trees
  • If taking samples, for example a leaf to examine for signs of a gall on the leaf closely later, clean and disinfect cutting tools and hands after each sample with antibacterial wipes or gel
  • Keep any samples in sealed containers
Further resources about tree health and citizen science

Citizen science tree projects

  • Ancient Tree Hunt – A living database mapping ancient and special trees
  • AshTag – Helping scientists understand how ash dieback spreads and identifying resistant trees by ‘tagging’ their progress
  • BioBlitz – Identifying as many different species of birds, bugs, plants and beasties to document and feed into local and national wildlife databases
  • Conker Tree Science – Taking part in activities to gather information on horse chestnut trees and their enemies
  • International Plant Sentinel Network – A global network providing plant health data through the use of sentinel plants to act as an early warning system for new and emerging pests and diseases
  • i-Tree Eco – Using study area data along with local hourly air pollution and meteorological data to quantify forest structure, environmental effects and value to communities
  • Leafsnap – Using visual recognition software to identify tree species from photos of their leaves
  • Living Ash Project – Aiming to secure ash trees for the future that show resistance to ash dieback
  • National Plant Monitoring Scheme – Data collected will provide an annual indication changes in plant abundance and diversity
  • Nature’s Calendar – Recording signs of each season in your local area providing evidence of how wildlife is responding to our changing climate
  • Observatree – UK tree health citizen science project website
  • Opal –  A UK-wide initiative allowing anyone to get hands-on with nature whatever their age, background or ability
  • Plantlife Scotland –  Management trials with Forest Research at five Scottish sites testing ways to encourage natural regeneration of populations
  • Plymouth Woodland Project –  Collecting vital information on woodlands in the Plymouth area to help in their conservation
  • RHS Plant Pests – Surveying the spread of non-native garden insects
  • Splash – Monitoring the impact of ash dieback on ground flora and epiphytes associated with ash in woodlands and other semi-natural habitats
  • teamCooper Fraxinus – A Facebook puzzle game using real genetic data that could help discover why certain ash trees have natural resistance to ash dieback
  • Track A Tree – Collecting information on the phenology of woodland trees and flowering plants in order to monitor woodlands over future springs
  • Traditional Orchard Survey –  Finding and studying traditional orchards to help preserve them and the animals they support
  • TreeType – Help increase knowledge of tree phenotypes – every tree is important!
  • Woodland Trust Tree ID app – Identify native and common non-native trees in the UK whatever the season in just a few steps


Read more on plant health in gardens from the RHS.

More on Science with the RHS

Get involved

The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.