Swiss needle cast, caused by the fungus Phaeocryptopus gaeumannii, is a disease that we see on Douglas fir in Iowa. The symptoms are very similar to Rhizosphaera needle cast on blue spruce. Infected needles turn a yellow-brown color and fall from the tree. The undersides of the infected needles will show rows of small black fungal fruiting structures along the length of the needle. Symptoms typically begin on the lower branches.
If symptoms of this disease are present, the foliage should be sprayed with a fungicide such as Bordeaux mixture or chlorothalonil (trade name Daconil 2787) in the last two weeks of May and again 4 to 6 weeks later. Also promote good air circulation by adequate tree spacing and weed control, improve tree vigor through mulching and watering when needed, inspect trees for disease before planting, and do not plant susceptible species next to infected trees.
This article originally appeared in the April 14, 1995 issue, p. 46.