Cause The fungus Cryptostroma corticale has been causing dieback primarily in sycamore maples (Acer pseudoplatanus), although box elders (A. negundo) and Norway maples (A. platanoides) can also become infected. The disease has been found in eastern WA for many years and recently in the Seattle area on sycamore maple, red maple (Acer rubrum) and big leaf maple (A. macrophyllum).
The fungus is thought to infect and kill small, inconspicuous branches, then spreads into the heartwood where it continues to proliferate up and down the tree. The rate of growth of the fungus is greater at higher temperatures and is more rapid in an infected tree when the summers are hot. The fungus can grow quickly within the tree if it has been stressed by drought, particularly if multiple years of water stress have occurred. With high temperatures, the fungus grows rapidly from the center of the tree out toward the bark. The fungus kills the bark, causing it to blister. Dark gray masses that blacken with age, stomata, are formed pushing up the bark. These stomata resemble soot, hence the name sooty bark disease. Once spores disperse, the remaining tissue beneath them looks black.
The spores may cause a severe allergic reaction in susceptible people, which has been a problem for those working around diseased maples, such as arborists and loggers.
Symptoms An infected tree may be numerous small dead twigs that line up one side of the trunk, or small cankers on the trunk. Heavily infected trees may have leaves that wilt over part or all of the canopy. Bark sloughs away exposing dark patches of the fungal stroma and powdery spores. Trees may have a greenish-brown-to-greenish-yellow stain of the wood that can be seen when an infected branch is cut into cross-section. The staining is in a single column, not streaky, as occurs with Verticillium wilt. Tree mortality soon follows after signs of the disease appear.
Cultural control Arborists should wear PPE to keep spores out of their eyes and respiratory system when removing trees.
- Scout landscapes, parks, and street trees for incidences of this disease. Early detection will aid in overall control.
- Remove, cover during transport and burn infected trees. Do not save for firewood.
References Goree, H. 1969. Occurrence of Cryptostroma corticale in northwestern United States. The Plant Disease Reporter 53:87
Worrall, J. J. 2020. Sooty-bark Disease of Maple. Forest Pathology. https://forestpathology.org/canker/sooty-bark-maple/