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CORVALLIS, Ore. -- Over the last decade, a fungal disease known as Swiss needle cast has intensified within the Douglas-fir forests of the Oregon Coast Range, according to the most recent scientific surveys.

Results from aerial analyses in 2015 indicate a slight expansion, 0.6 percent, in the affected area over 2014, but it remains the most significant threat to Douglas-fir plantations in western Oregon, says David Shaw, Oregon State University forest health specialist in the College of Forestry. Shaw is director of the Swiss Needle Cast Cooperative at Oregon State, which leads efforts to understand the disease and determine how best to manage it.

Symptoms of the disease, which is native to the Northwest, have spread as much as 30 percent in one year. Caused by a fungus that reduces the growth of Douglas-fir trees, it now affects over 590,000 acres of trees in Oregon, an area more than four times larger than what was found when surveying began in 1996. The annual economic loss has been estimated at $128 million.

The fungus, Phaeocryptopus gaeumannii, doesn't kill trees outright, but the annual growth of the most infected stands can be reduced up to 50 percent. Application of fungicides is generally not recommended. Where the disease is severe, researchers suggest planting species other than Douglas fir, such as western hemlock, western redcedar and Sitka spruce.

Scientists have reported the results in Forests, a professional journal, of two decades of aerial surveys conducted by the Oregon Department of Forestry. Researchers in the U.S. Forest Service and the Weyerhaeuser Corporation co-authored the report.

"Sustained growth losses over the previous 20 years have resulted in millions of dollars in lost timber and tax revenues," said Gabriela Ritokova, lead author and assistant director of the Swiss Needle Cooperative. "In many cases, mid-rotation stands in the hardest hit areas have remained in an unproductive state, with managers hoping for a reprieve in disease levels."

The pathogen disperses as tiny spores that land on Douglas-fir needles and plug needle openings that normally carry air and water into and out of the tree. Like a clogged drain, the needle loses the ability to function. Infected trees can be visually identified from the air because, in the spring, they turn slightly yellow in contrast to the deep green of healthy trees. While unaffected Douglas-fir trees often retain needles for three years or more, needles on infected trees may fall off in two years or less.

Standard forest management practices neither increase nor decrease the severity of the disease, researchers said.

"The start of the epidemic is thought to be partially the result of planting Douglas-fir that originated from seed sources outside the area being planted," said Ritokova, who is also a faculty research assistant at Oregon State.

"The correlation between disease severity and climate factors, such as spring moisture and warm winter temperatures, raises the question of a link between disease expansion and climate change. Those factors, in combination with lots of Douglas fir and with large springtime fungal spore production, have us where we are now."

In addition to conducting aerial flights over the Coast Range, Ritokova and her colleagues have been monitoring fungus infection in 10- to 23-year-old Douglas-fir trees in the western Cascades. Samples from 590 trees analyzed in 2001, 2006 and 2011 show that Swiss needle cast is present but, with few exceptions, is of limited concern.

Swiss needle cast was first discovered among Douglas-fir trees planted in Switzerland in the early 20^th century.

The research was carried out by the Swiss Needle Cast Cooperative, a consortium of OSU, U.S. Forest Service, Bureau of Land Management, Oregon Department of Forestry, Stimson Lumber, Starker Forests, Cascade Timber Consultants and Weyerhaeuser Corporation. The cooperative recently completed installation of a research and monitoring plot network across western Oregon and southwest Washington to enable scientists to study disease abundance, growth impacts and climate.