Sweet chestnut: Are Britain's trees really in trouble?

BRITAIN is blessed with two types of chestnut – white-flowering horse chestnut (Aesculus hippocastanum) and sweet chestnut (Castanea sativa), not botanically related to each other and neither native species. The general consensus is that sweet chestnut arrived with the Romans while horse chestnut, introduced in the mid-16th century, has been on these islands for a considerably shorter time. Both trees led a charmed life until the dawn of the new millennium, when they quickly succumbed to alien insect pests and exotic diseases.

SWEET CHESTNUT ON THE SKIDS

This ancient sweet chestnut on a West Hertfordshire common was originally situated in wood pasture. The surrounding woodland grew up over the last 100 years after grazing was stopped in the 1920s. (Image: FJ)

Horse chestnut is essentially finished as an amenity tree, having succumbed to a combo of lethal bacterial bleeding canker disease and horse chestnut leaf miner, which ruins the leaf canopy. Neither white-flowering horse chestnut nor red-flowering horse chestnut are stocked by tree nurseries. Reading between the lines in a programme announced last year by the UK government and the Royal Horticultural Society (RHS), we could be well on the way to losing sweet chestnut as well. 

Castanea sativa is now feeling the heat as the government tries to pull the chestnuts from the pest and disease fire. During National Plant Health Week in May 2023, DEFRA, APHA and Forest Research, together with the RHS, launched a campaign called ‘Check a Sweet Chestnut’ to teach the general public how to recognise the tree, assess its health and report findings to Forest Research scientists. The reason given for this new project was protection of the species from the combined effects of two alien agents, the parasitic fungus Cryphonectria parasitica, causal pathogen of sweet chestnut blight, and an insect pest, the

oriental chestnut wasp – OCGW – Dryocosmus kuriphilus. 
But sweet chestnut is also plagued by two other potentially fatal diseases, both of which pre-date chestnut blight disease and OCGW in Britain. Neither was mentioned in the UK government/RHS press release despite presenting a clear and present danger.

Nicola Spence, the chief plant health officer at DEFRA, said: “Sweet chestnut trees – like many other plant and tree species – are increasingly vulnerable to pests and diseases. Encouraging the public to be good plant-health citizens and report disease sightings is absolutely crucial if we are to minimise risk to our cherished treescapes.”

Government reports and distribution maps have consistently shown how chestnut blight disease and OCGW are increasingly widespread in England. This new initiative hints at a situation that may be getting out of hand. 
Chestnut blight was first recorded in the UK during 2011 in a young orchard in Warwickshire which had been planted in 2007 using infected material imported from at least one nursery in France. Consignments of trees had been split up by distributors and sold in multiple lots to customers throughout the country.

Despite a ‘trace-forward exercise’ by UK plant health authorities, the disease was able to establish at a number sites across the country because documentation relating to some plant imports and subsequent sales was not available. 

Oriental chestnut gall wasp was first discovered in 2015 in woodland in North Kent and on mature street trees at St Albans in Hertfordshire. By 2016, the pest was being recorded at so many sites in London and its environs that plant health authorities had no option but to drop all plans for eradication in favour of pest containment. 

Dr Nicola Spence, centre

The most recent distribution map (2020) published by Forest Research shows sweet chestnut blight has been recorded on at least one site within 35 10 km grid squares across England, as far westwards as Cornwall and eastwards to the Suffolk/Norfolk border and East Kent. The most northerly finding was in Cheshire, with most disease found south of a line from the Bristol Channel to the Thames Estuary. Forest Research says the UK plant health service is currently dealing with sweet chestnut blight on 65 sites across England in London, Buckinghamshire, Nottinghamshire, West Sussex, Derbyshire, Kent, Devon and Cornwall.

Oriental chestnut gall wasp remains widely spread and increasingly entrenched in London and more recently the Home Counties. The latest location map (2020) for OCGW pinpoints pest findings as far south as coastal Sussex and northwards into the south Midlands.

Given the government’s stated aim is to create a national map of the health of sweet chestnut, I am surprised nothing was said about two other established and potentially damaging diseases (ramorum blight of sweet chestnut and ink disease), both of which pre-date chestnut blight and oriental chestnut gall wasp. 

Sweet chestnut was one of the first trees to be found infected by Phytophthora ramorum when this fungus-like pathogen reared its head in the UK during the first few years of the 21st century. As with other deciduous trees including common beech and sycamore, the pathogen was infecting bark and causing lesions.

The first scientific report of P. ramorum infecting the foliage of Castanea sativa was published in 2005, with authors commenting how this was the first report of P. ramorum causing disease on the foliage of a native European tree species and how laboratory tests showed the pathogen sporulates on the leaves of sweet chestnut trees. 

Castanea sativa turned out to be a terminal host through stem canker disease but also a sporulation host with foliar blight and leaf necrosis disease dimensions. Twenty years have passed since P. ramorum was first discovered in the UK. Since then, and up to 2020, P. ramorum had been found on sweet chestnut at around 50 sites. Most were confined to England’s south-west peninsula (Cornwall and Devon) with the Somerset/Wiltshire border being the farthest east the disease was found. An outlier outbreak was identified just south of Birmingham in the County of Warwickshire. The distribution pattern of P. ramorum on sweet chestnut appears to mirror that of P. ramorum on Japanese larch with infection confined to the wetter western areas of southern England.

Last but not least, there is ink disease, caused by a pair of different Phyophthora species (Phytophthora cambivora and Phytophthora cinnamomi), set to become a spreading blot on the landscape. The pathogens were recorded as causing disease in sweet chestnut by the Forestry Commission way back in the 1930s, almost 100 years ago. At the time, FC scientists commented how Phytophthora cinnamomi in particular is more commonly associated with warmer climes and could become a real problem if the UK climate became warmer in the future. I wonder if they knew. Indeed, as the name suggests, Phytophthora cinnamomi was first isolated on cinnamon on the tropical island of Sumatra in Indonesia. I have experience of the pathogen on the wet, humid tropical island of Trinidad where it affects the bark of avocado trees to cause a disease called ‘avocado decline’. The overall effect of ink disease on the chestnut tree canopy is easy to confuse with that caused by Cryphonectria parasitica.

DISSECTING THE THREATS TO SWEET CHESTNUT

Castanea sativa bears profusely in southern England but there is often little in the way of kernel inside the nuts. (Image: FJ)

CABI (the Centre for Agriculture and Bioscience International) lists Cryphonectria parasitica (chestnut blight) and oriental chestnut gall wasp as the world’s worst disease and insect pest, respectively, of the Castanea genus. Chestnut blight has the capacity to essentially wipe out sweet chestnut in the same way that it destroyed American chestnut (Castanea dentata) down the eastern seaboard of the United States. The disease arrived on diseased plant material from China in 1904 and subsequently spread at such a rate that by 1940 the American chestnut was essentially finished as a forest and cultivated tree with some 3–4 billion trees killed during that 36-year period. 

OCGW is deceptively damaging. The FC says that on its own, OCGW is a low-impact pest, but that is not the picture portrayed for other countries. Of OCGW CABI says: “By attacking the vegetative buds and forming a gall, D. kuriphilus disrupts twig growth and reduces fruiting. Commercial nut growers may expect yield reductions of 50–70 per cent. Severe infestations may result in the decline and death of chestnut trees. D. kuriphilus is the most severe insect pest worldwide on chestnuts, wiping out nut production and even killing trees.”

Compounding this insect pest problem is long-established evidence that gall damage can make sweet chestnut trees more susceptible to infection and development of other insect pests and diseases. UK plant health authorities recognise this potential problem, with DEFRA saying this is especially so with chestnut blight caused by the fungus Cryphonectria parasitica. Chestnut blight has been infecting sweet chestnut in the UK from at least 2011 (and probably 2007), getting progressively worse and clearly one of the main planks of the ‘Check a Sweet Chestnut’ programme. The FC made an abortive attempt in 2015 to declare chestnut blight as eradicated from UK when it was clearly still thriving. It’s interesting that the announcement came shortly after OCGW was found in England.  

Indeed, the FC has never acknowledged that Cryphonectria parasitica entered the UK on infected sweet chestnut planting material imported from nurseries in France, even though that scenario is patently obvious to all. Just to confuse us even more, an FC contingency plan for Cryphonectria parasitica, dated 2022, says: “It [Cryphonectria parasitica] is officially absent from the UK, although there have been 10 findings of the pathogen, all associated with new planting stock.” Really?

PRACTICALITIES AND PITFALLS

Disfiguring and damaging leaf galls caused by oriental chestnut gall wasp. (Image: Patrick Mannix)

The government says the findings of the ‘Check a Sweet Chestnut’ project will help Forest Research scientists who are studying the country’s sweet chestnuts to pinpoint problem areas. The data will then be used to create a national map of the health of sweet chestnut trees across Britain. Forest Research already has detailed location maps for chestnut blight, OCGW and ramorum disease on sweet chestnut, but the problem is they do not appear to have been updated for nearly four years, during which time these problems are likely to have intensified and spread. And judgung by the numbers of foresters and landowners relaying fears to me about symptoms they are currently observing on their sweet chestnut, there would appear to be a potentially serious pest and disease threat out there for this tree.

I still can’t get my head around why the project is confined to chestnut blight and OCGW, when ramorum disease on chestnut and ink disease are clearly just as potentially serious problems for the species. Chestnut blight, ramorum disease and ink disease all affect the bark of trees and therefore tissues responsible for translocation of soluble food (phloem), transport of water and minerals (living xylem) and, of course, the lateral meristem tissue (cambium). While symptoms on the bark are distinctive, overall debilitating effects on the tree canopy will be difficult to distinguish.

Organisers of the project were clearly relying on widespread publicity to get the public initially interested, but this is perhaps something which should always be approached with caution. On several occasions I have seen articles on sweet chestnut in the national press which have been illustrated with superb pictures of horse chestnut. These were not daily rags, but quality broadsheets which should have known better.

I have been monitoring and writing about pests and diseases of forest and amenity trees in the UK for the last 25 years. The UK government, via DEFRA, the FC and other departments collectively comprising the UK plant health authorities, has been consistent on two counts throughout. It has invariably downplayed the severity of an ever-lengthening list of pests and diseases that have proceeded to damage and destroy our trees and forests over the last quarter-century, while always keeping its cards very close to the chest.

Sourcing real-time information is usually like trying to extract blood from a stone, so this latest request for public participation tells me that sweet chestnut trees and UK plant health authorities could both be in deep trouble.