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

It looks burned, like blown up with a blowtorch: blackened – and dead.

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

The disease, which was first detected here in 2020, is caused by a fungus that can also endanger human health.

So-called soot bark disease is named after the black, powdery spots that are the telltale marks on the tree bark of the fungus Crypotostroma corticale. To date, at least 46 street trees in the city have been infected or killed, but many more trees could be infected, said Nicholas Johnson, an arborist for Seattle Parks and Recreation.

The disease has become a growing problem as it spreads through the variety of trees it infects, including the native Pacific dogwood and great maple.

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Trees are critical to cooling in warmer urban areas through climate change, and in Seattle, are individually grieved when lost in development. Green belts are popular playgrounds for children and habitats for wild animals. Even with hers Sidewalks, street trees are fiercely defended.

The new disease continues to threaten the city canopy.

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The spores of the fungus are also allergenic and can cause debilitating pneumonia in humans after prolonged contact with infected wood. In Europe, diseased trees are considered to be an occupational risk for people with intensive work-related contact with wood, such as mill workers.

Healthy hikers or mushroom pickers or other forest and park users are not considered to be at risk if they simply walk past an infected tree, even though people with immunodeficiency or underlying lung diseases are more at risk.

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Seattle Parks has recovered samples of the fungus from unhealthy living trees across the metropolitan area and confirmed that sycamore, large-leaf maple, Japanese maple, norway maple, Pacific dogwood and horse chestnut are all at risk.

It is unclear whether the disease appeared recently or whether it is only now due to other factors, including the prolonged and hotter droughts that have hit the region in recent years, and especially this summer with its record heats.

Before the Seattle outbreak, the disease only occurred in Europe. It was apparently imported from the Great Lakes region of North America, where it does not cause disease. The soot bark disease spread shortly before 1945 on wood exported to Great Britain and has since spread to large parts of Europe.

As it grows, the fungus destroys the vascular tissue of the trees and deprives them of the food and water they need to survive. After all, the fungus bears fruit and sometimes pushes the bark straight off the tree. The fungus spits billions of billions of volcanic spores.

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

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

Street trees tend to live in stressed environments, with the roots being restricted and damaged by parking lots, sidewalks, roads, and pedestrian traffic. This makes them more susceptible to damage, especially in hot, dry conditions that stress all trees. It is likely that many more trees in Seattle and perhaps beyond are home to the fungus, but have so far been asymptomatic, Johnson said.

Trees can decline and die for a variety of reasons; Soot bark disease is just one possible culprit.

Dan Omdal, a forest pathologist with the state Department of Natural Resources, said the agency is monitoring the situation around Seattle to see if the disease can spread to natural forests.

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

Also on the research agenda: Establishing a protocol for dealing with diseased material. So far, Parks has put sick trees in a fenced pit and covered them with wood shavings.

Trees stressed by heat

Tom Hinckley, former director of the Center for Urban Horticulture, said the recent record heatwave damaged many trees and plants; some even show traces of fire.

The heat island effect of urban areas increases damage to trees – and root damage from construction work can make them particularly vulnerable.

Trees are usually resilient and have many defenses against heat and drought. They size their leaves according to sun or shade, with the smallest leaves on the top and outer edges of the tree being exposed to extreme light and heat. Lush shade leaves grow in the canopy and below the crown. Trees also form a waxy cuticle on the surface of leaves or needles to retain moisture.

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When moisture is scarce, a tree can close stomata – openings in its leaves and needles through which it breathes – to conserve moisture. It can drop leaves, fruits, buds, and twigs to reduce the area that the tree’s vasculature must supply. It will forego growth and reproduction at a more convenient time.

But all of these defenses can be defeated in extreme conditions, especially when they are repeated and when drought is combined with heat, Hinkley said. Then 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 tree species, said Johnson of the Seattle Parks Department. Existing plantings can be supplied with mulch and water.

But disease and pests are increasingly a reality for trees in Seattle, Johnson said, adding that hotter and drier weather has taught cruel lessons since 2014 and now poses new risks.

The avenue of maples that adorns the Jefferson Park golf course has a gap in its teeth where the sooty-bark-sick maple stands. It is now a danger tree after gracing the street for decades. How many more in this long line and elsewhere in the city are infected is unknown.

Time and weather will tell.

Lynda V. Cards:
206-464-2515 or lmapes@seattletimes.com; on Twitter: @LyndaVMapes. Lynda specializes in reporting on the environment, natural history and Indian tribes.