Newly discovered fungus spores spurred by heat and drought are killing Seattle street trees

It looks burned, as if blasted with a blowtorch: blackened — and dead.

This maple, in a row of trees along the parking strip at the driving range at the Jefferson Park Golf Course, is a victim of a disease killing Seattle’s street trees.

First detected here in 2020, the disease is caused by a fungus that also can pose risks to human health.

So-called sooty bark disease is named for the black, powdery patches that are the telltale marks on tree bark of the fungus Cryptotostroma corticale. At least 46 street trees have been observed to be suffering or killed by the disease so far in the city, but many more trees may be infected, said Nicholas Johnson, an arborist for Seattle Parks and Recreation.

The disease has emerged as a growing concern because it is expanding in the variety of trees it infects, including native Pacific dogwood and big leaf maple.

Trees are critical for cooling in urban areas made warmer by climate change, and in Seattle, are grieved individually when lost to development. Greenbelts are cherished play places for kids and habitat for wildlife. Even with their sidewalk-busting ways, street trees are fiercely defended.

The new disease further threatens the city’s canopy.

The fungus’ spores also are allergenic and can cause a debilitating inflammation of the lungs in humans under prolonged contact with infected wood. Diseased trees in Europe are considered an occupational hazard, suffered by people with intensive job-related contact with wood, such as mill workers.

Healthy walkers or mushroom pickers or other forest and park users are not considered to be at risk in simply passing by an infected tree, though people who are immunocompromised or who have underlying lung disorders are more at risk.

Seattle Parks has recovered samples of the fungus from unhealthy living trees throughout the metro area and confirmed that sycamore maple, big leaf maple, Japanese maple, Norway maple, Pacific dogwood and horse chestnut are all at risk.

It is unclear whether the disease recently arrived, or it if is just now emerging because of other factors, including longer and hotter droughts experienced in the region in recent years and especially this summer, with its record heat.

Before the Seattle outbreak, the disease appeared only in Europe. It apparently was introduced from the Great Lakes region of North America, where it does not cause disease. Sooty bark disease spread to the U.K. on exported lumber shortly before 1945, and it has since spread through much of Europe.

As it grows, the fungus destroys the vascular tissues of trees, depriving them of the food and water they need to survive. Eventually the fungus fruits, sometimes pushing the bark right off the tree. The fungus spews spores volcanically, by the trillions.

In a recent visit to the trees along the sidewalk at the driving range, Johnson pointed out maples in various stages of infection. City workers will wear special gear, including respirators and protective suits, when they cut down the dead maple, sometime this fall.

There is no known treatment for trees stricken with the disease.

Street trees tend to live in stressed environments, with roots confined and damaged by parking lots, sidewalks, roads and foot traffic. That makes them more vulnerable to damage, particularly in hot, dry conditions, which stress all trees. It is likely that many more trees in Seattle and perhaps beyond are harboring the fungus, but are so far asymptomatic, Johnson said.

Trees can decline and die for any number of reasons; sooty bark disease is just one potential culprit.

Dan Omdal, forest pathologist for the state Department of Natural Resources, said the agency is monitoring the situation around Seattle, with an eye on whether the disease will spread to natural forests.

Gary Chastagner, plant pathologist and extension specialist for Washington State University at the Puyallup Research and Extension Center, has partnered with the U.S. Forest Service on a grant to further detect where the disease is occurring in Seattle and the Puget Sound region. So far the disease has only been documented in the Seattle metro area.

Also on the research agenda: determining a protocol for handling diseased material. So far, Parks has been putting cut diseased trees in a fenced-off pit and covering them with wood chips.

Trees stressed from heat

Tom Hinckley, former director of the Center for Urban Horticulture, said the recent record heat wave was damaging to many trees and plants; some even show scorch marks.

The heat-island effect of urban areas boosts damage to trees — and root damage from construction can make them particularly vulnerable.

Trees are normally resilient and have many defenses against heat and drought. They size their leaves according to sun or shade, with the smallest leaves exposed to extremes of light and heat at the top and outer edges of the tree. More luxuriant shade leaves grow within the canopy and below the crown. Trees also create a waxy cuticle on the surface of leaves or needles to retain moisture.

When moisture is scarce, a tree can shut the stomata — openings in its leaves and needles through which it breathes — to conserve moisture. It can drop leaves, fruit, buds and branches to reduce the area the tree’s vascular system must service. It will forgo growth and reproduction to a more propitious time.

But all those defenses can be defeated in extreme conditions, particularly if they are repeated, and if drought is combined with heat, Hinkley said. That is when trees are particularly susceptible to disease, pests and death.

As the region’s climate changes, the best defense is to plant heat- and drought-tolerant varieties of trees, said Johnson, of the Seattle parks department. Existing plantings can be babied with mulch and water.

But diseases and pests are increasingly a reality for trees in Seattle, Johnson said, adding that since 2014, hotter, drier weather is teaching cruel lessons, and now posing new risks.

The allee of maples gracing the Jefferson Park golf course has a gap tooth where the sooty bark diseased maple stands. It’s a hazard tree now, after gracing the street for decades. How many more are infected in this long row, and elsewhere in the city, is unknown.

Time, and weather, will tell.