New strain of deadly disease infects SW Oregon tree saplings

An aggressive strain of the disease that causes sudden oak death in plants has infected Douglas fir and grand fir saplings in southwest Oregon.

Scientists at Oregon State University, the Oregon Department of Forestry and the U.S. Department of Agriculture’s Research Service report the finding in the journal Plant Disease.

The strain, identified in 2012 and reported first in Europe, spreads more aggressively than its North American counterpart, which has been the subject of intense eradication efforts over the last dozen years in the United States.

Federal, state and local agencies have marshaled their resources to manage the outbreak, said Jared LeBoldus, a forest pathologist at Oregon State University. They’ve quarantined the area where the trees have been infected and cut down and burned sick trees.

In trees, it takes about two years from the time of initial infection to death. So far, there haven’t been reports of dead Douglas fir or grand fir, he said.

“It’s important for people not to get overly worried or concerned,” said LeBoldus, who has a dual appointment in OSU’s College of Agricultural Sciences and College of Forestry. “This pathogen is spreading, but the effort that we have put forward into managing this disease has slowed it down significantly.”

The pathogen travels through the air and produces spores on leaves and branches in the canopies of its host. A water mold, it needs water to spread on its host and the spores have tails that they use to skim along the wet surface of a leaf or branch. The fungus spreads from leaves to branches and once it reaches the stem, death occurs in a matter of months.

The pathogen that causes sudden oak death affects far more than just oaks. About 130 plants that define Oregon landscapes can harbor the pathogen, including rhododendron, madrone and huckleberry.

“It has the potential to change the forest structure in areas that are infested,” LeBoldus said.

Sudden oak death was first reported in North America in 1995 in Mill Valley, California. In 2001, OSU forest pathologist Everett Hansen, now retired, identified what was killing trees in forests in southwest Oregon and led an aggressive program to eradicate the disease. In 2004, USDA formed a rapid response project to limit the spread of sudden oak death in nurseries across the country.

In 2012, a rhododendron suffering from sudden oak death was reported in a Curry County ornamental nursery. Genetic testing in the USDA-ARS Horticultural Crops Lab in Corvallis determined that the pathogen wasn’t the same one as discovered in North America, such as the one that appeared in Oregon in the 2000s.

That strain spread to a tanoak in the forest near the nursery in 2015 and then to Douglas-fir and grand fir saplings over the last two years. Symptoms in fir include wilted tips, brown discoloration of needles and needle loss on young shoots.

“Because this is new to fir trees in Oregon, we don’t really know what is going to happen,” LeBoldus said. “There have been cases in Europe where this strain has killed Douglas fir. We’ve been looking at different host ranges and other species of trees that have been infected. We’re trying to determine if this will be more of a threat to Oregon forests than the North American strain.”