Douglas fir needle loss can be attributed to a needle disease caused by the fungus Rabdocline pseudotsugae. The disease begins from infections occurring during cool, moist periods in spring. Succulent, young needles are infected from bud break through shoot elongation. By late summer or fall, infected first-year needles begin to show some yellowing followed by conspicuous brown banding in late fall or early spring, giving the needles a mottled appearance. Needles may begin dropping in winter and continue falling the next season. Eventually, during summer, the tree may be missing most of the previous year's needles.
This disease is often adequately controlled by improving ventilation and air movement around the base of the tree. Weed removal, improved spacing, and removal of diseased lower branches and crowding trees will help. Fungicide sprays can be applied in spring when buds first begin to swell. Continue applications every seven to 10 days for about a month.
Contact your local Extension office for fungicide recommendations.
