Thousand cankers disease: what the agencies record
What Forest Research, Natural Resources Canada and the Invasive Species Centre say about thousand cankers disease of walnut: the fungus and the walnut twig beetle that carries it, the hosts, the symptoms, where it is recorded and where it is not.
Checked against the sources at the bottom of this page on October 8, 2026. Rules, fees and pay change: the source has the last word.
This page reports what the official source says. It is not legal advice: ask the office named here. It lists what the sources record about thousand cankers disease and does not diagnose any tree. Forest Research lists the disease as notifiable in the United Kingdom, which is a reporting request for a disease it records as not present there. The list of other diseases is on the tree diseases page, and the reporting lines are in the United Kingdom and Canada report-a-pest guides.
At a glance
| Item | What the sources say |
|---|---|
| Pathogen | Geosmithia morbida, a fungus carried by the walnut twig beetle, Pityophthorus juglandis. Natural Resources Canada and the Invasive Species Centre both describe the disease as the work of the fungus and the beetle together. |
| Hosts | Walnut trees in the genus Juglans. Forest Research says all are thought susceptible, black walnut (Juglans nigra) particularly. It also says some wingnut species (Pterocarya) can be affected. |
| Where | Forest Research says several eastern and western states of the USA and the Lombardy region of Italy. Natural Resources Canada says the fungus is not found in Canada, and the Invasive Species Centre says the disease is not present in Canada but occurs in US states near it. |
| Status | Not present in the United Kingdom and notifiable there, per Forest Research. |
| Report to | Great Britain: TreeAlert. Northern Ireland: TreeCheck. See the United Kingdom guide and the Canada guide. |
How the sources say it spreads
Forest Research says the fungus is spread by the walnut twig beetle. Beetles pick up fungal material on their bodies at infected trees and deposit it in the inner bark as they bore galleries in healthy trees, and a canker forms around each gallery. It says the beetles typically move across the grain and the larvae along it. The Invasive Species Centre adds that adult beetles overwinter in galleries under the bark, resume activity in late April, and fly in large numbers from mid-July to late August. Natural Resources Canada says humans could carry the beetle and fungus in firewood, wood packaging, pallets, lumber and veneer, and nursery stock. Forest Research says movement of walnut wood or plants carrying infested beetles most likely took the disease to Italy and the eastern United States. See moving wood and firewood.
Symptoms as the sources state them
- Crown. Forest Research says external symptoms usually show only in the end stages. Leaf yellowing, at first on one branch, is often the first sign, followed by sudden wilting of large limbs. The Invasive Species Centre lists wilting and yellowing in mid-summer, then twig and branch dieback that thins the canopy. Natural Resources Canada says dieback starts in the upper crown and works down.
- Bark. Forest Research describes pin-hole entrance and exit holes made by the beetles, with dark, wet, inky, oval cankers near them that merge as more form. The Invasive Species Centre says the bark surface may show no symptoms, or a dark stain and cracking above a canker.
- Under the bark. Forest Research says removing bark from cankered areas can show the beetle galleries and dead inner-bark tissue.
- Time to death. Forest Research says infected trees almost always die two or three years after leaf yellowing is first seen. Natural Resources Canada says trees may die within a year of the first symptoms. The two pages differ on this.
- Season. The only seasonal detail is the beetle's flight period above and mid-summer yellowing in the Invasive Species Centre list.
Look-alikes and lab confirmation
Natural Resources Canada says the symptoms are similar to those linked to drought, and that a separate canker from another fungus, Fusarium solani, can also appear on the trunk. Forest Research says the disease was described only in the early 21st century and was not fully understood at the page read on October 8, 2026. It asks for reports with clear close-up photographs of symptoms or of a suspected beetle. None of the three pages states a laboratory test.
What the agencies publish for management
Forest Research says no chemical, cultural or biological control and no tolerant black walnut varieties are available, that infected trees in the United States are removed and destroyed by grinding or burning, and that restrictions on moving walnut wood and plants protect states still free of the disease. The Invasive Species Centre says there is no approved fungicide for walnuts and that spraying insecticide against the beetle is not feasible because of its size. It says some US states impose sanitation and quarantines on untreated walnut lumber from infested areas. It adds that the disease does not affect the nuts. Who may apply pesticides is covered on the pesticide licences page.
Credit
The Forest Research material was read under the Open Government Licence v3.0 and is restated here in our own words. The Natural Resources Canada page is marked as possibly out of date and was last modified August 4, 2015.