Sooty bark disease: what the agencies record
What Forest Research, the California Department of Food and Agriculture and Forest Health Watch at Washington State University say about sooty bark disease of maple and sycamore: the fungus, the hosts, the heat and drought link, the black spore layer under the bark, the spore health notes for tree workers and where it has been found.
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 sooty bark disease and does not diagnose any tree. It also relays health notes the sources give for people who work around affected wood. The list of other diseases is on the tree diseases page, and the reporting lines are in the United Kingdom report-a-pest guide.
At a glance
| Item | What the sources say |
|---|---|
| Pathogen | Cryptostroma corticale, a fungus. Forest Health Watch gives the alternative genus Biscogniauxia. The California pest profile says it can live in a tree as an endophyte (a fungus living inside without harm) or act as a pathogen. |
| Hosts | Maples, with sycamore (Acer pseudoplatanus) the most commonly reported host in the United Kingdom per Forest Research. It also lists field, Norway, box elder, red, bigleaf and Japanese maple, horse chestnut and Pacific dogwood. The California profile adds sugar maple, amur maple and ash. |
| Where | Forest Research lists confirmed presence in Canada and the USA from 1898, the United Kingdom from 1945, then Italy, Germany, Austria, Switzerland, the Czech Republic, France, the Netherlands, Bulgaria and Belgium. Forest Health Watch and the California profile record it in Washington State, British Columbia and California. |
| Status in the United Kingdom | Present since 1945 per Forest Research. The page does not label it notifiable. It asks for suspected cases on Acer to be sent through TreeAlert so the service can track whether the distribution or incidence is growing. |
| Report to | Great Britain: TreeAlert. See the United Kingdom guide. |
How the sources say it spreads and progresses
Forest Research says the fungus probably enters through wounds or infects small branches and twigs, then colonises the tree and can live alongside it as an endophyte. It says the colonised wood is stained olive green to brown, which shows in a cross-section, and that when average temperatures stay above 25 degrees C for an extended period the fungus can grow outward, the tree wilts and begins to die, and a black powdery spore layer forms under the bark. The California profile says the airborne spores enter wounds, especially broken branches, that nursery stock is not considered a pathway because this is not a disease of young trees, and that moving cut branches from infected trees is a possible route for long-distance spread. Forest Health Watch ties the disease in the Pacific Northwest to longer and hotter summer droughts, which stress trees and favour faster growth of the fungus.
Symptoms as the sources state them
- Crown. The California profile describes wilting foliage and dieback that starts in small branches and moves to larger ones, with general decline. It says these internal and crown symptoms are very similar to other vascular diseases such as Verticillium wilt.
- Bark. Forest Research says the bark falls away to show a black powdery layer, and that after the spores have gone the surface looks greyish. The California profile describes blistering and shedding bark with long, broad black stripes, and dark, dry, sometimes powdery patches sunken into the bark that run parallel to the stem.
- Season. Forest Research says the disease is most often noticed after periods of hot weather, when the black patches are revealed.
- Time to death. The California profile says tree death can come in as little as one year or take as long as seven years. It says an infected tree can stay without symptoms for years until heat, drought or flooding stress makes the fungus active.
Look-alikes and lab confirmation
The California profile names Verticillium wilt as similar in its internal symptoms. Forest Health Watch offers a laboratory service for samples to confirm whether a tree has sooty bark disease and links its sampling instructions. The sources read give no further look-alike list.
Health notes for people who work around it
Forest Research says the fungus is an allergen and that the billions of spores it produces can cause health problems if inhaled, and points to a review of hypersensitivity pneumonitis. It relays advice from the UK Forest Industry Safety Accord that includes protective equipment against spore inhalation, working when the weather is damp, restricting public access during works, mechanical harvesting in a forestry setting, and not keeping infected logs for firewood. The California profile describes maple bark strippers' disease as uncommon and found among paper-industry workers who debark, cut and chip maple logs, lists breathlessness, fever, night sweats, chills and weight loss as symptoms, and says huge numbers of spores can be released before the tree dies. Forest Health Watch says some people may get hypersensitivity pneumonitis or temporary breathing problems, recommends an N95 mask or respirator for anyone vulnerable who handles possibly infected samples, and links the Washington State Department of Health maple bark disease page. Each of these is advice the named source gives to its own readers. None of the three pages names a treatment for affected trees.
What the sources say about status in other places
The California profile proposes a permanent pest rating of C for the fungus, after detections in El Dorado County in 2019 and Sacramento County in 2022 and 2023. Forest Health Watch records the first Washington specimen in 1968, a Seattle confirmation on 10 trees in 2020, and confirmations in Tacoma, Bellingham, Anacortes and Olympia in 2021.
Credit
The Forest Research material was read under the Open Government Licence v3.0 and is restated here in our own words.