What attacks walnut: pests and diseases the sources name
A register of the canker disease and its twig beetle, the bacterial blight and the anthracnose that Forest Research, Natural Resources Canada, the Invasive Species Centre, the University of California and the Woodland Trust tie to walnut: which walnut each row names, where it is recorded, the status each source gives, and where the full profile is.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page reports what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. Each row names its source, all read on October 9, 2026. Where this site has a full profile, the row links it, and the status details are on the linked page. The tree is on the black walnut page, and the plant-growth chemical it makes is on the juglone page. The English or common walnut (Juglans regia) is the species the Woodland Trust and Forest Research write about for Britain.
The register
| Agent | What the sources record |
|---|---|
| Thousand cankers disease, Geosmithia morbida, and its vector the walnut twig beetle, Pityophthorus juglandis. Full profile | Kind: Fungus carried by a bark beetle Hosts: Forest Research says it is believed to be able to affect all species of walnut, with black walnut particularly susceptible, and that some wingnut (Pterocarya) species can also be affected. It says little is known about the susceptibility of common walnut, though it is reported among the trees affected in Italy. Some North American walnuts, particularly western ones, seem able to tolerate the fungus with only minor cankers on small branches. Where and status: Forest Research lists several eastern and western states of the United States and the Lombardy region of Italy, with the first eastern finding in Tennessee in 2010. It lists the disease as not present in the United Kingdom and notifiable. Natural Resources Canada says it is not found in Canada. Season: The Invasive Species Centre says adult beetles overwinter in galleries under the bark, resume activity in late April, and fly most in the weeks from mid-July to late August. |
| Walnut blight, Xanthomonas | Kind: Bacterial disease Hosts: The University of California IPM program describes black lesions on catkins, shoots and leaves and on young nuts, saying all green tissue is susceptible and that economically significant damage occurs when the developing nut is infected. It says the bacterium overwinters mainly in dormant buds, rain spreads it and early-leafing varieties are hit hardest. The Woodland Trust says English walnut is susceptible to walnut blight, which causes small black spots on leaflets and can lead to new shoots dying back and damage to the fruit. Where and status: California orchards in the University of California text; Britain in the Woodland Trust text. No regulated status is given. Season: The University of California text ties infection to rain in spring. |
| Walnut anthracnose, Marssonina species | Kind: Leaf and fruit fungus Hosts: The University of California IPM program describes small necrotic specks on leaves that grow into blotches, with similar lesions on leaf stalks, stems, shoots and fruit, each carrying dark fruiting structures. It says excess rain in spring makes early infection more likely. Where and status: California orchards. No regulated status is given. |
| Walnut leaf blotch | Kind: Leaf disease Hosts: The Woodland Trust says it can cause leaves to fall prematurely but does not cause lasting damage to the tree. It names no fungus. Where and status: Britain. No regulated status is given. |
How the sources describe the effects
Natural Resources Canada says the beetle excavates numerous galleries in the inner bark and that the fungus it introduces causes multiple cankers, and that the pairing can kill a large black walnut in less than a year. It says affected trees are generally doomed once symptoms show, because the tree is already heavily infested. Forest Research says trees show readily visible symptoms usually only at the end stages, starting with yellowing leaves on the outside of the crown, and later sudden wilting of large limbs. It says the nut and timber industries in the United States have lost production, with job losses and business failures. It adds that timber from diseased trees is usable and that the disease is behind a declining walnut timber supply, especially in the west.
Who the sources say to report to
Forest Research asks for suspected thousand cankers disease in Great Britain to be reported through TreeAlert, and in Northern Ireland through TreeCheck, with clear close-up photographs of the symptoms or the beetle. The offices for each country are on the report a pest in the United Kingdom and report a pest in Canada pages. The pages read for blight, anthracnose and leaf blotch name no report line.
Sources
- Forest Research, thousand cankers disease page, adapted under the Open Government Licence v3.0, read October 9, 2026
- Natural Resources Canada, thousand canker disease profile, read October 9, 2026
- Invasive Species Centre (Ontario), thousand cankers disease page, read October 9, 2026
- University of California Statewide IPM Program, walnut blight page, read October 9, 2026
- University of California Statewide IPM Program, walnut anthracnose page, read October 9, 2026
- Woodland Trust, British trees, walnut, read October 9, 2026