Forests at risk: Swiss needle cast epidemic in Douglas-fir trees unprecedented, still getting worse

Science Daily: The Swiss needle cast epidemic in Douglas-fir forests of the coastal Pacific Northwest is continuing to intensify, appears to be unprecedented over at least the past 100 years, and is probably linked to the extensive planting of Douglas-fir along the coast and a warmer climate, new research concludes.

Scientists in the College of Forestry at Oregon State University have also found that this disease, which is affecting hundreds of thousands of acres in Oregon and Washington and costing tens of millions of dollars a year in lost growth, can affect older trees as well as young stands -- in some cases causing their growth to almost grind to a halt.

….Swiss needle cast is a native fungal disease specific to Douglas-fir that was first described in Europe. It rarely kills trees but causes discoloration, loss of needles and growth reduction, and is common in the Pacific Northwest wherever Douglas-fir grows. However, it caused significant problems only in recent decades along the coast. Starting in 1984, an epidemic began to develop, and it significantly worsened after 1996.

"It's now clear that this epidemic is a new phenomenon, with far more severity and impact than anything we've observed from Swiss needle cast in the past," said Dave Shaw, an assistant professor at OSU and director of a cooperative designed to fight this disease. "We've known of this disease for decades but it was considered a non-issue in terms of forest health. A perfect storm of conditions that favor this fungus has caused a major epidemic that is still growing."

…The new study concluded that warmer conditions, especially from March through August, are associated with significantly reduced growth in diseased trees, which may reflect earlier fruiting of the fungus. Wet, drizzly conditions in May through July are also important. The warm, wet conditions within 20 miles or so of the Pacific Ocean make those areas a hotspot of disease in coastal Oregon and Washington.

"We now know that weather is a driver in the epidemiology and spread of this disease," said Bryan Black, an assistant professor of forestry based at OSU's Hatfield Marine Science Center. "We can't say yet whether climate change is part of what's causing these problems, but warmer conditions, milder winters and earlier springs would be consistent with that."…

Douglas Fir Forest, Bull Run Watershed, Mount Hood National Forest, Oregon, from the US Forest Service