What is Swiss needle cast?

First described in 1925 in Switzerland, Swiss needle cast is a disease caused by a fungus which attacks Douglas-fir forests. It has been tracked in Oregon since 1981 and has since become increasingly severe, especially during recent years of abnormally heavy rainfall.

Swiss needle cast occurs through out the Douglas-fir region of the Pacific Northwest. It is a particular problem in heavy fog and rainfall regions along the north Oregon coast where Douglas-fir was planted exclusively to replace natural forests of mixed tree species following harvest or fires. It occurs less frequently in inland and some coastal forests where even-aged stands of predominantly Douglas-fir had historically evolved from intense stand-replacement fires. Aerial surveys have mapped nearly 400,000 acres–from south of Coos Bay north into western Washington–that display obvious symptoms.

Trees infected by Swiss needle cast are weakened by a seriously impaired nutrient flow. It's easiest to spot in late winter through early spring, and the needles appear noticeably yellow to brownish yellow. Although the disease rarely kills trees outright, infected trees often lose all but the current year's growth of needles. This allows more light to enter the forest floor, promoting the growth of competing understory vegetation that reduces nutrients available to diseased trees and further inhibits their growth.

One study of an infected stand showed a reduction of more than 20 percent in volume growth. While it hasn't happened yet, scientists fear that trees weakened by the disease will be attacked and killed by opportunistic bark beetles.

Needles of a Douglas-fir infected by Swiss needle cast

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What causes Swiss needle cast?

Scientists suspect that a key cause for its flare up is certain forest management practices made worse in recent years by abnormally wet climate favorable to the fungus.

Historically, much of the coastal forest was a mix of hemlock, cedar, hardwoods, and Douglas-fir. Because Douglas-fir is the most desired commercial species, it has been the choice for replanting harvested areas for the last half century. Some scientists believe this change from tree species diversity to a forest consisting primarily of Douglas-fir has upset the balance between the forest and the fungus, with the wet climate setting the stage for an upsurge in the disease to a point where it has overwhelmed the forests' natural defenses.

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What effect will Swiss needle cast have at the mill?

Research is being conducted on the effect of the disease on wood structure. Badly infested trees may have more "late" (summer) wood and less "early" (spring) wood. It's not yet clear what affect that will have on the quality of lumber and other products made from diseased wood, but an ongoing epidemic of the disease would affect the quantity of timber available for forest products..

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What can we do about it?

Research suggests that Douglas-fir grown from non-local seed sources may be more vulnerable to needle caste. Foresters can help avoid the disease by using seedlings grown from local parent trees that have built up a resistance. There has been reluctance by some forest landowners to replant with less valuable species of the past, but many foresters now are addressing the problem by replanting a mix of species rather than just Douglas-fir in harvested areas and by planting hemlock, which is not susceptible to the disease, in their worst-hit stands.. Selective thinning that favors hemlock and other non-host species is another promising management technique. These measures will move us toward a diverse species composition that will provide some insurance against increasing damage from the disease.

Fungicides and fertilization of affected stands have shown little promise, but research continues on minerals and other soil additives. In severely affected stands, stand replacement may be the best option.

We have much to learn about the disease, and its magnitude will not be known until we have completed more research and monitoring. The Swiss Needle Caste Cooperative, in cooperation with the Oregon Department of Forestry's Insect and Disease Unit, has installed permanent growth monitoring plots on private and public forestlands, and damage from the disease is being tracked through a combination of aerial and ground survey. Data from these studies will help scientists estimate the disease's severity and range as well as changes in stand conditions over time.

We have learned that planting single-species forests in heavy fog and rainfall regions where mixed species of trees historically grew has unintended consequences on forest health. Foresters will continue to learn from experience and research as well as from nature as they seek the best way to manage forests.

References:

• Dr. Greg Filip, OSU College of Forestry
• Dave Odgers, OFRI
• Forest Health Note, Oregon Dept. of Forestry, March 1998.

For more information, visit the Swiss Needle Cast Cooperative Web page.

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