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Washington State University
WSU Puyallup Ornamental Plant Pathology

Sooty Bark Disease


Sooty Bark Disease

Sooty Bark Disease is an emerging concern in the Seattle metropolitan area. The disease is caused by the fungal plant pathogen Cryptostroma corticale.

While this plant pathogen is suspected to have been present in the Seattle area for a while—with a first report of its presence in eastern Washington dating back to 1969—there is consensus the disease is emerging partly because of hotter and longer droughts.

Sooty Bark Disease

Check Before Acting

There are many look-a-likes

There are many organisms and factors that cause similar symptoms. Please consult a plant health specialist, a certified arborist or local urban forest official to confirm the tree is infected before making decisions.

Please note there are many mold fungi with dark spores and similar staining, especially on dead wood. Confirmation of the presence of Cryptostroma corticale requires diagnosis from a laboratory. Collaborators abroad have indicated the fungus can be confirmed by the shape of the spores under a microscope, but we need more information about the other fungi present in the PNW before recommending this method as a reliable form of diagnosis.

Internal fungal wood staining
Fungus on phloem

Potential Health Concern

It is important to confirm if the tree is infected by Cryptostroma corticale when making decisions because of potential health concerns with exposure to the fungal spores.

More information about Maple Bark Disease and the potential health concerns are available at the Washington State Department of Health webpage.

Cryptostroma corticale poses a potential threat to the health of trees and individuals who work with diseased trees. Inhalation of its spores has been recognized as a potential human health concern since the 1930’s. Chronic exposure to C. corticale was linked to a handful of medical cases with respiratory issues in Wisconsin wood and paper mill workers in the 1960s (Article, New England Journal of Medicine). The spores of C. corticale can cause hypersensitivity pneumonitis in humans with similar symptoms to allergic asthma, influenza or pneumonia (Article, Journal of Occupational Medicine and Toxicology). Prior to working with infected trees and wood, individuals are encouraged to consult with their employers and health care professionals about appropriate personal protective equipment.

Cryptostroma corticale spores
Cryptostroma corticale spores. Photo credit: Marianne Elliott

Diagnostic Guide

Diagnostic Guide Cover

More Information is Needed

The emergence this disease and it’s detection on a number of previously unreported hosts raises a number of important research questions. More research is needed to address questions about topics such as:

  • Distribution and spread
  • Plant species susceptibility and host range
  • Diagnostic methods and molecular approaches to improve diagnostic efficiency and capacity
  • Pathogen life history and genetic diversity
  • Factors that affect disease development and vulnerability (site, stress, age of host, etc.)
  • Potential of human mediated dispersal and vectors such as pruning tools
  • Best management practices and worker protection

A small amount of funding from the USFS FHP Emerging Pests program has been obtained to increase capacity for diagnostic services and surveying trees in other cities of the Puget Sound region.


Potential Local Host Species

The presence of Cryptostroma corticale has been confirmed on the following tree species. Tree species were identified by Seattle Parks and Recreation staff and the presence of C. corticale was confirmed by Bartlett Tree Research Laboratories. Additional potential hosts are listed in the Bartlett technical report here.

  • Sycamore Maple (Acer pseudoplatanus)
  • Norway Maple (Acer platanoides)
  • Bigleaf Maple (Acer macrophyllum)
  • Horse Chestnut (Aesculus hippocastanum)
  • Japanese Maple (Acer palmatum)
  • Pacific Dogwood (Cornus nuttallii)


More information
Please check the below links for more information. We will update this list as more resources become available.
Public health

Local reports




Newsletters and other news articles

Recent technical literature (contact us if you need help obtaining a pdf)

  • Braun M, Klingelhöfer D, Groneberg DA (2021) Sooty bark disease of maples: the risk for hypersensitivity pneumonitis by fungal spores not only for woodman. Journal of Occupational Medicine and Toxicology 16:2.
  • Ogris N, Brglez A, Piškur B (2021) Drought Stress Can Induce the Pathogenicity of Cryptostroma corticale, the Causal Agent of Sooty Bark Disease of Sycamore Maple. Forests 12:377.
  • Kelnarová I, Černý K, Zahradník D, Koukol O (2017) Widespread latent infection of Cryptostroma corticale in asymptomatic Acer pseudoplatanus as a risk for urban plantations. Forest Pathology 47:e12344.
  • Cochard B, Crovadore J, Bovigny PY, et al (2015) First reports of Cryptostroma corticale causing sooty bark disease in Acer sp. in Canton Geneva, Switzerland. New Disease Reports 31:8–8.
  • Koukol O, Kelnarová I, Černý K (2015) Recent observations of sooty bark disease of sycamore maple in Prague (Czech Republic) and the phylogenetic placement of Cryptostroma corticale. Forest Pathology 45:21–27.
  • Longa CMO, Vai N, Maresi G (2016) Cryptostroma corticale in the northern Apennines (Italy). Phytopathologia Mediterranea 55:136–138.

Sample diagnostics

Submit a sample

Diagnostic services

Our laboratory received funding from the USDA Forest Service to provide diagnostic services for city staff.

Please contact us to inquire about the process to submit samples for free testing.

Locate symptomatic trees

Locate symptomatic trees

Record GPS coordinates, tree species, and other details such as tree condition.

Please record the number of symptomatic trees you survey and the number of trees that have signs of Sooty Bark Disease.

Look for signs of Sooty Bark Disease

Look for signs of Sooty Bark Disease

Check symptomatic trees for signs of Cryptostroma corticale infection. Note there are many other dark mold fungi, especially on dead woody tissues.

Signs may include:

  • staining that radiates outward within the xylem,
  • black ‘mold’ underneath bark
  • ‘sooty’ mold patches where bark has been shed.
    • patches may grey as the fungus dries out.

Note that signs will likely be different for other tree species.

Collect Samples and Take Photos

Collect Samples

If possible, use a chisel to cut a square from the bark and wood that include fungal tissues. Alternatively, submit small (2-3 inches long and wide) amounts of bark with fungal spores/tissues or cut stem sections with stains. Try to sample into the wood, but include outer parts of bark as well if possible.

Please record plant species and GPS locations of the samples collected.

Take Photos

Please include photos to help identify the plant species you’re collecting from. Send photos when you’re letting us know you’re submitting a sample.

Please include the following photos:

  • Leaves (so we can clearly see leaf shape)
  • Leaf attachment (are leaves attached alternately or opposite of each other on stems)
  • Bark
  • Whole tree
  • Signs of fungus or symptoms of disease (e.g. crown dieback, bark peeling)

Protect yourself and others

Use precautions and consider using protective equipment to avoid direct contact or inhalation of spores.

Please keep samples fairly small. The bigger the sample, the greater the chance we will miss what you actually want tested.

Please send samples to the address below and let us know how many or when they are coming.

Please include the following information:

  • GPS coordinates for the tree
  • The tree species identification
  • Tree condition, e.g. whether it was dead or in poor, fair, or good health.

Submit Samples for Diagnosis

Submit Samples for Diagnosis

City staff and arborists groups can submit samples to our laboratory for diagnosis. Other individuals can contact us or submit samples to the WSU plant clinic (see below).
Samples can be sent to:

ATTN: Taylor McNees / Joey Hulbert
WSU Puyallup Research & Extension Center
2606 West Pioneer
Puyallup, WA, 98371-4998 USA
Click here for our location on Google Maps

Please send us an email ( to let us know to expect some samples from you.

Concerned about your trees?

Learn More

Forest Health Watch

The Forest Health Watch program offers biosurveillance trainings online to raise awareness about the signs and symptoms to look out for.

Have Questions?

Contact Us

Please contact us with any questions. We will do our best to get back to you with answers and resources quickly.

Tree Testing

WSU Plant and Insect Diagnostic Laboratory

The WSU Plant Clinic can provide diagnostic services. Please click here to read their instructions for submitting a sample.


Accelerate Research as a Community Scientist

Interested in contributing to research about Sooty Bark Disease?

Sign up as a Community Scientist through our Forest Health Watch program.

Live near Tacoma?

Sign up as a Tacoma Community Scientist to participate in our Tacoma Urban Heat project during 2022-2023.

Share a Tree of Concern

Upcoming Information Sessions

We hosted three information sessions about Sooty Bark Disease through our Forest Health Watch program between September – November, 2021.

More information sessions are scheduled during the below dates throughout 2022.

  • February 18, 2022
  • April 15, 2022
  • June 10, 2022
  • August 5, 2022
  • September 30, 2022
  • November 25, 2022

All updates are scheduled to occur at 12:00pm PST/PDT. Please visit to register for an upcoming event.

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