By Richard Rees, Ph.D.
What does it look like?
Swiss needle cast is caused by a fungus, Phaeocrytopus aguemannii, which results in the defoliation and reduction of growth in Douglas-fir. This foliage disease is most easily identifiable by decreased needle retention, resulting in sparse crowns and reductions in growth. Black fruiting bodies may be found on the underside of infected needles near the base of the tree. Additionally, remaining foliage will appear yellow or brown in color. The symptoms are most severe on older needles located on lower branches of the tree.
Host material and range
The disease is specific to Douglas-fir, affecting no other tree species. It is found on trees located on the West Coast of Oregon from Coos Bay to Astoria and is native to the Pacific Northwest, but has also been known to affect Christmas tree plantations in the Midwest United States. The disease usually occurs in the spring, from May until June, or shortly after the first needle flush — when new needles begin to emerge — and typically affects current-year foliage. However, symptoms of the disease usually do not appear until the next spring following the infection. Swiss needle cast is most easily spread through shipping infected nursery stock.
Although Swiss needle cast is benign in most conditions, it is a serious threat to the Oregon coast range, having sparked an epidemic. Oregon has experienced growth losses in the range of 20 to 55 percent, with an estimated $200 million impact on annual growth per year. Oregon’s moist, coastal climate provides optimal conditions for the spread of Swiss needle cast. Furthermore, the selection at Christmas tree plantations is also compromised because the thinning of the needles results in less attractive, unsellable trees.
The best way to prevent Swiss needle cast is to plant healthy stock and increase the amount of air circulation around the base of the tree. The base should also be free of weeds. The fungus thrives on relatively moist, wet conditions; therefore areas with increased amounts of precipitation would be susceptible to the disease. Conversely, dry spring weather leads to little infection. Avoid planting trees in low-lying areas with poor drainage, and properly space trees for adequate air circulation. Swiss needle cast primarily affects the growth of trees with less than three years of foliage, targeting new needles. If the tree carries three or more years of foliage, it has the ability to withstand the fungus.
Control vegetation around the base of the tree to increase air circulation and reduce moisture. Early detection of the disease will result in minimal losses.
What can you do?
Help isolate pockets of Swiss needle cast by shearing healthy trees first to avoid cross-contamination with workers’ clothing and equipment that may have come in contact with affected trees. Tools should be sterilized with alcohol after shearing any infected plantations to further prevent the spread of the disease. Additionally, research shows that Douglas-fir grown from non-local seed sources may be vulnerable to the fungus. Therefore, seedlings grown from local parent trees with a built-in resistance to the disease should be planted instead.
Richard Rees, Ph.D., is product development manager – fungicides, Bayer Environmental Science, a business group of Bayer CropScience LP.