SNC on Douglas-fir
A Douglas-fir with low foliar retention, a symptom of Swiss needle cast. Photo by Alan Kanaskie, Oregon Dept. of Forestry.

The fungus that causes Swiss needle cast (SNC), Phaeocryptopus gaeumannii (T.Rohde) Petrak, is found throughout the range of its only host, Douglas-fir. The disease is most damaging near the Pacific coast due to the fungi-favorable climatic (mild winters and wet springs and summers) and topographic conditions. SNC can reduce growth of host trees, alter wood properties, and affect stand structure and development.

In late April and May of 2015, an aerial survey covering 2.6 million acres was flown to detect and map the distribution of SNC symptoms in coastal Washington.

The observation plane flew at 1,500 to 2,000 feet above the terrain, following north-south lines separated by 3 miles. Observers looked for areas of Douglas-fir forest with obvious yellow-brown foliage, a symptom of SNC. Patches of forest with these symptoms were sketched onto computer touch-screens displaying topographic maps or orthophotos and the position of the aircraft. Each patch was classified for degree of discoloration as either “S” (severe) or “M” (moderate). Patches classified as “S” had very sparse crowns and brownish foliage, while those classified as “M” were predominantly yellow-brown foliage with slightly denser crowns than those classified as “S”.

Nearly 350,000 acres of symptomatic Douglas-fir were mapped, which is an increase from the 230,000 acres mapped in the 2012 aerial survey. The survey boundaries were similar to those in the 2012 survey.

Severely symptomatic stands were generally located near the coast and the Grays Harbor area. The cause of the dramatic increase in acreage mapped from 2012 to 2015 remains uncertain, in part due to our ground plot network not extending as far east as the mapped aerial survey and the potentially confounding impacts of an unusually dry and warm winter and spring in 2015.

Forty-seven ground sites across the range of the aerial survey were surveyed for Douglas-fir foliar retention and SNC severity. An average of 2.3 years of foliage were on the trees across all sites. Healthy Douglas-fir carry three or more years of foliage. Previous growth impact studies conducted by Dr. Doug Maguire, and other researchers at Oregon State University, have estimated that growth losses may be as high as 20-30% when foliar retention ranges from 2.1 to 2.5 years.

Douglas-fir is the only host of this disease, therefore forest managers can grow non-host species such as red alder, western redcedar, western hemlock and Sitka spruce in efforts to reduce damage from SNC. However, it should be noted that if Douglas-fir have more than three years of foliage on the branches, then loss impacts are likely minimal to none.

Acknowledgements: Funding for the SNC survey was provided by the Quinault Indian Nation and US Forest Service, which is an equal opportunity provider. The survey was conducted by the Washington DNR and the Washington Department of Fish and Wildlife.

For information on the Swiss Needle Cast Cooperative, visit: http://sncc.forestry.oregonstate.edu/