Maple Bark Disease

What is Maple Bark Disease?

Maple Bark Disease is a form of “hypersensitivity pneumonitis,” an inflammation of the airways or lungs, caused by a fungus called Cryptostroma corticale. Susceptible people can get Maple Bark Disease by breathing in Cryptostroma corticale spores. The fungus grows on maple and other broadleaf trees. The fungus does not cause an infectious disease in people - Maple Bark Disease is the reaction some people experience after exposure to the fungus.

Occasional and limited exposure to Cryptostroma corticale spores is not a serious disease risk for most people. The benefits of outdoor recreation and activities far outweigh the risks of acquiring Maple Bark Disease. It is a very rare disease, with the greatest risk for those with occupational exposures.

What is Cryptostroma corticale?

Cryptostroma corticale is a fungus associated with some broadleaf trees. It does not infect conifers like Douglas fir or cedars. When trees are infected by Cryptostroma corticale, it is called Sooty Bark Disease. The fungus primarily infects maple trees. Sooty Bark Disease describes the mats of black fungal spores the fungus produces. Infected trees can give off large amounts of airborne spores. Symptoms on trees also include leaf wilting and branch dieback. Wind and rain spread the spores.

Where is Cryptostroma corticale in Washington?

The fungus was detected in Whitman County in 1968 and Lewis County in 2007. In 2020, scientists confirmed it on trees from many areas around King and several other counties that border Puget Sound.

Since 2020, Cryptostroma corticale has been confirmed on these tree species in Washington:

  • Sycamore maple (Acer pseudoplatanus)
  • Bigleaf maple (Acer macrophyllum)
  • Norway maple (Acer platanoides)
  • Japanese maple (Acer palmatum)
  • Field maple (Acer campestre)
  • Vine maple (Acer circinatum)
  • Full moon maple (Acer japonicum)
  • Red maple (Acer rubrum)
  • Horse chestnut (Aesculus hippocastanum)
  • Pacific Dogwood (Cornus nuttallii)
  • Cherry plum (Prunus cerasifera)

In Europe, Sooty Bark Disease outbreaks follow hot summers. Trees in cities with high pollution levels and higher temperatures are at risk of greater infection rates. Based on information from Europe, Sooty Bark Disease may become more common in the Northwest as changes in climate cause warmer, drier summers.

What Increases the Risk of Maple Bark Disease?

There is a low risk of breathing in wind-blown spores. Exposure risk increases when handling or working with infected wood:

  • Logging
  • Cutting or chipping wood
  • Spreading infected wood chips

Repeated and close exposure and work that aerosolizes large amounts of Cryptostroma corticale spores have the greatest risk of causing Maple Bark Disease. Maple Bark Disease does not spread from person to person or from animals to people.

Occasional and limited exposure to the spores is not a serious disease risk for most people. Outdoor recreation and activity benefits far outweigh the risks of Maple Bark Disease.

What are the symptoms of Maple Bark Disease?

Most people experience mild allergic reactions similar to seasonal allergies. These reactions go away when exposure stops. Prolonged exposure to spores can cause symptoms of acute respiratory infection or reaction.

Symptoms of Maple Bark Disease include:

  • Nausea and vomiting
  • Wheezing or bronchial hyper-responsiveness
  • Unexplained shortness of breath
  • Cough
  • Fatigue
  • Fever
  • Diarrhea
  • Unintended weight loss

Please consult a health care provider if you experience symptoms after exposure to Sooty Bark diseased wood.

Can Maple Bark Disease be serious?

Years of exposure to large amounts of spores can cause lung fibrosis and long-term respiratory problems. Workers who handle wood regularly are at greatest risk. Chronic hypersensitivity pneumonitis like Maple Bark Disease could lead to a total loss of lung function and death.

How soon do symptoms appear?

Acute symptoms, from occasional high-level exposure, usually occur within a few hours of exposure. Chronic symptoms from long-term, low-level exposure may occur after weeks, months, or even years.

What is the treatment?

Leaving the immediate area or removing the source of the fungus will, in most cases, resolve the symptoms. Serious cases may need immunosuppressive drugs like corticosteroids, antiproliferative drugs, or antifibrotic drugs.

Who is at risk?

Those most at risk for Maple Bark Disease are people whose work brings them into regular and close contact with trees, such as loggers, foresters, and paper mill workers. People with compromised immune systems, lung disease such as asthma or underlying allergic disease may also be at higher risk.

What can I do to prevent Maple Bark Disease?

Arboreal professionals should confirm Sooty Bark Disease. Cordon off infected trees to prevent people from touching them. Have a professional remove the trees. People who work with infected trees should wear standard personal protective equipment:

  • Goggles
  • Overalls
  • Gloves
  • Boots
  • Respirator (N95 mask)

Removing infected trees in damp weather may help reduce the risk of aerosolizing large number of spores.

Ways to reduce sporulation:

  • Bury infected material locally.
  • Transport infected material in covered containers to burn in a waste incinerator.

We currently don't know if composting infected materials reduces sporulation. Cryptostroma corticale can produce spores on dead wood. Handling infected firewood may put you at risk.

More Resources

For Public Health Partners

Sooty bark disease of maples, Journal of Occupational Medicine and Toxicology, 2021

Hypersensitivity Pneumonitis, NIOSH

Hypersensitivity pneumonitis, Medscape

Hypersensitivity pneumonitis abstract, Nature Reviews Disease Primers, 2020

For the Public

Hypersensitivity Pneumonitis, American Lung Association

Sooty Bark Disease, WSU Puyallup

Forest Health Watch – The Forest Health Watch Program facilitates research and education about the health of urban and rural forests in the Pacific Northwest.