Sweet Chestnut Blight: An Overview
(C) Crown Copyright Forestry Commission
Scientific Name of the Causal Agent
Cryphonectria parasitica is the scientific name of the fungus responsible for sweet chestnut blight.
Disease Description
Sweet chestnut blight is a highly destructive disease affecting sweet chestnut trees, particularly those in the Castanea genus. The disease is caused by the ascomycete fungus Cryphonectria parasitica, which was previously known as Endothia parasitica. Under certain conditions, the fungus may also infect other tree species, including various types of oak (Quercus spp.).
Distribution
The Cryphonectria parasitica fungus has triggered severe epidemics of sweet chestnut blight across extensive regions of North America. It has also impacted European sweet chestnut (Castanea sativa) throughout much of continental Europe. Since 2011, the disease has been identified in several locations within the United Kingdom, mainly in central and southern England.
Maps are available that display confirmed cases in Great Britain using 10 km × 10 km grid squares, and list European countries and regions where the disease has been observed.
The Threat
Infection by C. parasitica is typically fatal for both European (Castanea sativa) and North American (Castanea dentata) sweet chestnut trees, nearly eradicating the sweet chestnut population in North America. Although losses in Europe have not reached such severe levels, the disease has steadily spread, causing substantial regional losses and decline.
Sweet chestnut woodland in the UK is limited, with most located in England, amounting to roughly 2 per cent of England’s woodland cover. The majority of sweet chestnut woodland is found in southern England, so the impact of the disease is mainly regional and local.
Sweet chestnut trees are cultivated commercially for timber and nuts. While these industries are relatively small, they are locally significant, particularly in Kent, where chestnut coppicing for timber has seen a revival in the early 21st century.
Susceptible Species
Cryphonectria parasitica poses a serious threat predominantly to American and European sweet chestnuts, with European chestnuts possibly being less susceptible. In the UK, the main species at risk are European sweet chestnut, grown for nuts, timber, and amenity purposes, and the less frequently planted American sweet chestnut.
The fungus can occasionally affect oak species standing near heavily infected chestnut trees. Susceptible oaks include native sessile and, less commonly, pedunculate or English oaks (Quercus petraea and Quercus robur), Holm oak (Quercus ilex), and others. On oaks, the disease usually results in superficial bark infections that seldom kill branches or entire trees.
Additionally, the European & Mediterranean Plant Protection Organization (EPPO) lists Castanopsis species (chinquapins), Acer species (maples), Rhus typhina (staghorn sumac), and Carya ovata (shagbark hickory) as hosts.
The disease does not threaten humans, pets, or livestock. It does not affect horse chestnut or ‘conker’ trees (Aesculus hippocastanum), which are unrelated to sweet chestnut species, nor does it affect chestnut oak (Quercus prinus).
Identification and Symptoms
All symptoms occur above ground. The fungus attacks the bark of European sweet chestnut, entering through fissures or wounds. On grafted trees, infections are most commonly found near the graft site, where callusing occurs. In coppices or orchards, infections often appear at the base of the stem, although the bark death does not extend into the roots.
Cryphonectria parasitica can spread rapidly in infected bark, girdling stems or branches, and causing visible, sunken cankers. Orange fruiting bodies, which produce spores, are also visible on the bark, erupting through lenticels and exuding long, orange-yellow tendrils of spores in moist conditions.
Above the girdling canker, leaves wilt and turn brown but remain on the tree. Below the canker, branches retain healthy foliage, and new shoots may emerge below the dead bark. Multiple cankers are often found on a single tree.
On young, smooth-barked branches, cankered bark appears bright brown, contrasting with the greenish colour of healthy bark. On older stems, the discoloration or sunken appearance is less obvious. Rapid bark death results in girdling of the stem without callus formation.
Sometimes, disease progression is slower, with new bark layers forming beneath affected areas, causing swelling and cracking of the outer bark.
A distinctive symptom is the formation of pale-brown mycelial fans in the inner bark, which is only visible by removing the outer bark.
Some symptoms, such as crown dieback, can also be caused by other pathogens, including Phytophthora cambivora and Phytophthora cinnamomi. The latter is associated with ‘ink disease’, named for the blue-black stain around damaged roots. These pathogens are already present in the UK and have long caused disease in sweet chestnut.
Other organisms found alongside dieback and cankers include Gnomoniopsis smithogilvyi, Diplodina castaneae, Phytophthora ramorum, and forms of Amphiporthe castanea (formerly Cryptodiaporthe castanea).
Additional resources are available with images and information for identifying sweet chestnut blight.
(C) Crown Copyright Forestry Commission
Reporting and Regulation
Sweet chestnut blight is a regulated Quarantine Pest in the UK and must be reported if suspected. After reviewing symptom guides, suspected cases in Great Britain should be reported via TreeAlert. Suspected sightings in Northern Ireland should be reported using TreeCheck, the all-Ireland tree disease reporting tool. Both platforms require clear, well-lit, close-up photographs of symptoms.
More information on symptom checking is available through the Check a Sweet Chestnut campaign, delivered in collaboration with Forest Research, the RHS, Defra, APHA, Forestry Commission, Observatree, and Coventry University.
Spread of the Disease
Long-distance spread of Cryphonectria parasitica can occur via movement of infected sweet chestnut plants, wood, or bark in trade. Once bark is colonised, the pathogen’s mycelium can survive for up to 10 months, even if air-dried, with a minor risk of transmission via fruits or seeds.
Locally, spores are spread by wind and water, and may also be transmitted by insects and birds. Wounds caused by insects, such as the Oriental chestnut gall wasp (Dryocosmus kuriphilus), can facilitate entry, with evidence suggesting this may be happening at some sites in England.
The pathogen can also live as a saprotroph on broad-leaved trees outside its host range, allowing it to persist even after infected chestnut trees are removed. Infection can survive on cut timber, branches, and woody material.
Ongoing research aims to improve understanding of the disease in the UK. At most affected sites, only a few trees are infected, with very few fatalities.
Preventing and Minimising Spread
Preventing or minimising spread is best achieved by:
Reporting suspected cases immediately using TreeAlert or to a plant health inspector
Not moving infected plants, bark, or wood
Practising high standards of biosecurity, including washing and disinfecting tools, vehicles, equipment, footwear, and clothing after visiting or working at infection sites, and before visiting or working at new sites
Refer to the ‘Regulation’ and ‘Official action’ sections for details on regulatory measures designed to prevent or limit further introductions and spread.
Regulation
Cryphonectria parasitica is a regulated pest in the UK. Restrictions apply to the importation of sweet chestnut material—including plants for planting, wood, and bark—from all third countries. These regulations aim to minimise the risk of further disease introductions. UK legislation requires the use of plant passports for moving all Castanea species with bark and isolated bark.
Guidance on these regulations is available on the UK Government website for wood, timber, bark, wood products, plants, and other planting material.
A new demarcated area was announced in Devon on 26 March 2026, effective from 2 April 2026.
Official Action
Sweet chestnut blight was first confirmed in the UK in November 2011 at a nut orchard in Warwickshire, traced to imports from Europe. Since then, outbreak sites have been identified across England, with rigorous environmental surveillance following findings to determine disease extent.
Where infections are confirmed, statutory measures for eradication and containment are applied. A contingency plan was published in 2016 and updated in 2022. Statutory Plant Health Notices (SPHNs) are issued to owners of affected sites, detailing required biosecurity measures, which may include removing and destroying all sweet chestnut trees. Movement restrictions are imposed site-by-site based on the current policy approach.
Protections were enhanced following the 2011 findings, including requirements for plants to originate from pest-free areas, officially declared free of Cryphonectria parasitica.
Biology
The fungus enters trees through bark injuries, spreading to the vascular cambium and wood, killing these tissues as it advances. Nutrient flow to and from sections above the infection is eventually blocked, killing the upper parts of the tree. Spores, produced in orange fruiting bodies on infected bark, spread the disease to healthy trees.
Origins and Background
Cryphonectria parasitica originated in eastern Asia, where it is present on indigenous sweet chestnut species. These species have adapted to the fungus through long-term co-evolution, resulting in minimal damage.
The susceptibility of American sweet chestnut became evident shortly after the disease was first identified in New York in 1904. Within 50 years, sweet chestnut blight had spread throughout the native range of the American sweet chestnut, devastating much of the estimated 4 billion tree population.
The disease is now widespread in Europe, except for countries with EU Protected Zone status. In the UK, sweet chestnut blight was first identified in 2011 in Warwickshire and has since been found at several sites across England.
Long-Term Prospects
There is evidence that the pathogen’s virulence can weaken in Europe, allowing infected sweet chestnut plants to recover. This phenomenon, known as hypovirulence, occurs when the fungus is infected by Cryphonectria hypovirus 1 (CHV1), which limits its ability to grow in bark or produce spores. Hypovirulence is used as biological control in many affected European countries.
This method involves applying hypovirulent strains to trees with virulent cankers, converting the pathogen to a less aggressive form through CHV1, enabling recovery. There are European sites where the hypovirus is established, resulting in low blight severity decades after introduction.
The effectiveness of hypovirulence depends on limited genetic diversity in C. parasitica populations. High genetic variation, as seen in the USA, hinders the spread of the hypovirus and limits its impact.
Scientific research on this pathogen continues, including significant studies with the Government of Jersey to inform long-term strategies involving CHV1 biological control at sites where eradication is challenging.
(C) Forestry Commission and Howard Drury 2026
