Scientists at the university's College of Forestry said the epidemic in Douglas-fir forests appears to be unprecedented in at least 100 years and is probably linked to a warming climate.
Researchers said they found the disease, which is affecting hundreds of thousands of acres in Oregon and Washington and costing tens of millions a year in lost growth, can affect older trees as well as young stands -- in some cases causing growth to nearly halt.
Swiss needle cast is a native fungal disease specific to Douglas-fir that was first described in Europe. Scientists say it rarely kills trees, but causes discoloration, loss of needles and growth reduction.
The epidemic started in 1984 and 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 Assistant Professor Dave Shaw. "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 study appears in the journal Forest Ecology and Management.