By Rachel Brooks, forest pathologist, Washington State Department of Natural Resources, rachel.brooks@dnr.wa.gov

The fungus Cryptostroma corticale, native to the Great Lakes region of the U.S. where it only survives on dead material and does not affect living plants, has recently been spotted acting more like a pathogen on maple trees in Seattle.

This is not the first time this fungus has caused disease outside of its native range: sometime before 1945 it was accidently introduced to Europe where it now impacts maple trees, especially sycamore maples (Acer pseudoplatanus).

Wilted leaves, branch dieback, cankers, stained wood, and tree death can be observed on infected trees. Fittingly named “sooty bark disease of maple” this disease is often identified by areas of tree bark which have split open to reveal soot-like stromatal tissues (gray-black fungal mats). These tissues release large amounts of airborne spores that are allergenic, and those handling spore-producing material should wear personal protective equipment to minimize spore inhalation.

In Washington, C. corticale was first identified on a dying sycamore maple back in 1969 in Whitman County, though reports since then have been nonexistent, presumably because the limited number of non-native sycamore maples minimized the fungus’ ability to establish.

However, in 2020, Seattle Parks and Recreation, in collaboration with the Seattle Committee for Invasive Pests, confirmed the presence of this pathogen on numerous dying or dead trees scattered throughout the city. Molecular testing positively confirmed occurrences in sycamore maple, bigleaf maple (Acer macrophyllum), red maple (Acer rubrum), Norway maple (Acer platanoides), and horse chestnut (Aesculus hippocastanum). All of the sampled trees displayed typical symptoms and signs associated with sooty bark disease.

Without further research, we can’t be certain that C. corticale caused the dieback or death of these trees – all that we know for sure is it was present on them.

In Europe, research has shown that this disease increased after hot and dry summers, which have become increasingly frequent in the Northwest. Since these hot droughts both limit the water available in the soil and increase the water pulled out of the tree by the air, drought-stressed trees have become a common sight in parts of Washington, especially in developed areas. So perhaps the presence of C. corticale on these trees is more indicative of our tree’s increased levels of stress due to our changing climate and less of the pathogen’s presence or ability to cause disease. However, stressed and dying trees have never been an uncommon sight in cities, Seattle included, yet 2020 was the first year reports, and many reports, of this disease were received, which is more suggestive of a recent invasion.

A few other things to note: It is surprising to see red maple on the list of trees, as red maple is native to the Great Lakes region. It would be easy to hypothesize that this tree species, which co-evolved with C. corticale, would be resistant to disease, while tree species not native to the region would be more susceptible. Because there does not appear to be any reports of C. corticale causing disease on red maple trees in its native range, it is hard to know exactly what is going on. Furthermore, the fact that bigleaf maple, a tree that has not co-evolved with C. corticale, is included in this list is concerning. Our region has already seen declining bigleaf maples, which so far has not been attributed to a single pest but just broader environmental conditions. Could this fungus find its way to our natural areas as well?

What specific role this fungus will play – whether it is just a side effect of climate change or a new invasive – is not yet known. Further investigations on the distribution and host range of this pathogen are warranted to better understand how well it is established here in the Pacific Northwest and its potential impact on hardwoods in the urban and forestry setting.

If you think you have a sooty bark disease infected tree, a laboratory analysis is required to confirm C. corticale identification, a process your local arborist should be able to help you with.