UK plant passports: The risk of thousand cankers disease

I spent the New Year holiday trying to work out why DEFRA and the FC suddenly included walnut (Juglans) and wingnut (Pterocarya) in the list of Plant Health Forestry Regulated Material (PHFRM), meaning these tree species now require UK plant passports for wood and timber movement, thus giving the registered authorised professional operator (RAPO) even more to worry about. Juglans and Pterocarya are in the subtribe Juglandinae of the family Juglandaceae and as such are very closely related.

Walnut is one of those exotic deciduous broadleaf species brought to Britain many centuries ago. Persian or common walnut (Juglans regia) was introduced by the Romans, who valued the nuts. Like sweet chestnut, also introduced by the Romans for its nuts, Persian walnut led a largely blight-free existence in Blighty – until now, that is.

Perhaps the clue is in the black walnut (Juglans nigra), a native of north-eastern North America and brought to Britain in 1639. That said the limited numbers of Persian and black walnut trees ‘growing in Britain’ have always appeared healthy enough to me, although North American forestry websites list a substantial number of diseases affecting Juglans nigra on its ‘home turf’.

One of these is thousand cankers disease, caused by the fungal pathogen called Geosmithia morbida, first documented as causing disease in Juglans (walnut) and Pterocarya (wingnut) in the western United States during the 1990s. It was found more recently (2014 onwards) on black walnut in the eastern United States. The first North American report of thousand cankers disease on English walnut (which is what the Americans call Juglans regia) was in California during 2014.

Mature, black walnut trees are few and far between, but climbing arborist Ashli Jenkins of Arborcare found one in the churchyard of St Peter’s Anglican Church at St Albans, Hertfordshire in summer 2019. The next scheduled visit to maintain the superb tree means Ashli will almost certainly need a UK plant passport in her pocket, unless the vicar decides to keep the arborist’s arisings from the tree for his wood-burning stove and compost bin.

However, if thousand cankers disease is the reason behind plant passport paranoia at DEFRA and the FC, then why did it take a tree malaise festering 5,500 km away in Pennsylvania to cause a panic? Further investigation shows why. The disease has also been found in North-Eastern Italy as follows:

• 2013 at Bressanvido, Vicenza Province, Veneto region on 80-year-old black walnut trees in a garden and nearby on 15-year-old trees in a timber plantation;
• 2016 again in Vicenza, but this time on a larger area (70,000 ha) plus two sites in Piemonte;
• 2019 on trees in Lombardia, Toscana and Emilia-Romagna.

The UK got back full control of its borders on 1 January, 2020 (so they say), but from this recent reaction to thousand cankers disease, UK plant health authorities appear no more confident about excluding notifiable and quarantine pests and diseases from trees and tree-related products originating in the European Union than they were on 31 December, 2020.

NOT ANOTHER DUTCH ELM DISEASE

It’s not only the proximity of this disease which is putting the fear of God into UK plant health authorities. There are biological and epidemiological reasons too, because the pathogen causing thousand cankers disease is carried, transmitted and spread by a specific insect – the walnut twig beetle (Pityophthorus juglandis).

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It is called thousand cankers disease because every fungus-carrying walnut twig beetle boring into the tree causes a canker around the hole as the fungus infects the surrounding plant tissue. Eventually, the tree is overwhelmed with a huge number of cankers.

There are echoes here of Dutch elm disease (DED) which is carried, transmitted and spread by elm bark beetles, although to be fair DED is a systemically acting disease affecting the elm tree’s vascular tissue, therefore acting more rapidly and ruthlessly than this disease of walnuts and wingnuts.

DISEASE IDENTIFICATION AND SYMPTOMS

The external symptoms of thousand cankers disease only become apparent when it is in the later, more advanced stages. The symptoms may include:

• Leaf yellowing in the outer canopy, initially confined to a single branch but with increasingly large areas affected as cankers coalesce to girdle the trunk and/or the branches
• Sudden leaf wilting eventually extending onto substantial tree branches.

Infected trees will invariably succumb and die within several years of showing the leaf-yellowing symptom.

Close examination of the affected bark may show pinhole-sized entrance and exit holes caused by the walnut twig beetles. Dark-coloured (inky), wet-necrotic and oval-shaped cankers may be located near the insect holes. Individual cankers are small, but sustained feeding and oviposition (egg-laying) by beetles on the same tree will produce large numbers of cankers which subsequently coalesce.

Removal of cankered bark may reveal the feeding and reproductive galleries excavated by the beetles as well as diseased and necrotic phloem tissue. As the boring beetles and fungal infection spread across the bark, coalescence of the cankers girdles the branches to effectively curtail movement of water and nutrients. Epicormic growth sprouting from the trunk in response to death of branches in the canopy is a common feature of trees affected by thousand cankers disease.

CLOSE ENCOUNTERS WITH WALNUT TREES

So how likely is a RAPO to encounter a walnut or wingnut tree during his or her daily work? Walnut timber plantations are few and far between. Most Persian walnut or black walnut trees are restricted to a few or even single trees planted in public parks and gardens for fruit or amenity purposes.

A small grove comprising common (Persian) walnut trees planted at Green Park in Central London, pictured in 2006. Another grove was planted around the same time in neighbouring Hyde Park, near the Hyde Park Corner entrance on Park Lane. Future work on these now mature and substantial trees will require UK plant passports to move the arborist’s arisings off site.

There are veteran if not ancient Persian walnut trees dotted around the country in parks and on estates which will have been planted as far back as Victorian times. I would expect arborists rather than foresters to more frequently work on walnut trees.

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I recall seeing what I now think were wingnuts as odd trees in central London parks. However, when I go back to check them out I will need my copy of Field Guide to the Trees of Britain and Northern Europe by Alan Mitchell (1974) to distinguish between the different species of Pterocarya, if not identify the genus itself.

FOREST RESEARCH SAYS:

Thousand cankers disease is not currently known to be present in the UK but we must remain vigilant to the possibility of accidental introductions. This is especially important since discovery of the disease in Italy, a near neighbour and trading partner in continental Europe.

We particularly encourage tree and plant professionals to be vigilant and report suspected sightings.

• Report sightings in Great Britain to us using Tree Alert
• Report sightings in Northern Ireland to the Irish forestry authorities using TreeCheck, the all-Ireland tree disease reporting tool.
• TreeAlert and TreeCheck require photographs to be uploaded. They should be clear, well-lit, close-up pictures of symptoms and/or the suspected walnut twig beetle
• Suspected sightings can also be made directly to the relevant plant health authority. This is the preferred route for suspected sightings made on trade premises, such as nurseries or garden centres.
• In England and Wales, contact your local APHA Plant Health and Seeds Inspector, or the PHSI headquarters in York: tel: 0300 1000 313; e-mail: planthealth.info@apha.gov.uk; Web: www.gov.uk/plant-health-controls
• In Scotland, contact the Scottish Government’s Horticulture and Marketing Unit: e-mail: hort.marketing@gov.scot; tel: 0131 244 8923; Web: www.gov.scot/PlantHealth/PlantDiseases
• In Northern Ireland, contact the DAERA Plant Health Inspection Branch: tel: 0300 200 7847; e-mail: planthealth@daera-ni.gov.uk; web: daera-ni.gov.uk/topics/plant-and-tree-health/about-plant-and-tree-health.

In all cases, provide precise details of the location and, if possible, clear photographs of the pest or symptoms.

AUTHOR’S NOTE

Forest Research says the fungus Geosmithia morbida is not currently present in the UK. However, this potential problem requires a relationship between an insect pest and a pathogenic fungus, and as such requires ‘two to tango’ and get fungal inoculum into trees to cause thousand canker disease.

Forest Research makes no comment about the status of Pityophthorus juglandis (walnut twig beetle) in the UK, although CABI identifies Italy as the only European country where the insect pest has been found. The relationship between Pityophthorus juglandis and Geosmithia morbida is symbiotic (mutually beneficial). The fungus allows itself to be eaten by the beetle in return for hitching a ride to the next tree host.

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