Swiss needle cast is a fungal disease that causes needle discoloration on Douglas fir trees. Symptoms are usually most visible on the inner portions of lower branches, where infected needles turn yellow-green and brown and then fall off. Tiny black fruiting structures of the fungus can be seen with a hand lens, appearing as black dots in two rows on the undersides of discolored and some green needles. Affected branches typically look sparse.
The causal fungus, Phaeocryptopus gaeumannii, attacks only Douglas fir. Infection occurs in the spring during budbreak and shoot elongation, although needles may not die until two or three years after infection.
Swiss needle cast is most prevalent in moist environments, so spacing and weed control that enhance airflow can help to prevent this disease. New Douglas fir trees should be inspected carefully for the fruiting structures of the fungus before purchase, and diseased trees avoided. If symptoms appear, the disease may be managed using fungicidal sprays (Bordeaux mixture or chlorothalonil), but timing of the sprays is important. Fungicides should be applied in the last two weeks of May, and again four to six weeks later. Fungicide sprays should be repeated yearly until symptoms have disappeared.
- Black fruiting bodies of the Swiss needle cast fungus on Douglas fir needles.
- Swiss needle cast damage on Douglas fir.
Images are from the USDA Forest Service - North Central Research Station Archives.