When Fungus Attacks

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Swiss needle cast symptoms may show up in your woods if you have Doug fir planted on your land.

Credit: Robert L. James

Swiss needle cast only affects Douglas-firs. It doesn't normally kill trees, although it slows their growth dramatically.

What Does It Do to my Trees?

Swiss needle cast is caused by the native fungus Phaeocryptopus gaeumannii. The fungus is spread from tree to tree by wind or water. Most of the time, it doesn't cause a problem. In fact, it lives in almost every Douglas-fir tree needle.

The disease occurs when the fungus clogs the stomates, the tiny holes in the needles that trees use to breathe.

Usually the fungus only affects needles that are several years old. But, in some cases, particularly in damp years or wet climates, it damages new needles. That causes Swiss needle cast.

Swiss needle cast leaves some tell-tale signs on Douglas-firs:

• Yellowing foliage. Needles turn yellow or mottled. This symptom is visible from the sky--it's used to map the spread of the disease from airplanes.
• Sparse foliage. As needles drop off, the trees look thin. This can make them less attractive as Christmas trees.
• Fruiting bodies. On the undersides of the needles, miniscule black specks appear in two lengthwise bands. These are the part of the fungus that will send spores to other plants.

Learn what to do if you think your trees have Swiss needle cast.