SWISS NEEDLECAST OF DOUGLAS-FIR

Douglas-fir trees are susceptible to two needlecast diseases, Swiss Needlecast and Rhabdocline. The more common of these two diseases we see in southeast Michigan is Swiss Needlecast.

Douglas-fir needles infected with Swiss Needlecast will show a yellowing and browning of the tips in the spring and summer but never any mottling. Needles are cast one or two years after infection during the late fall and winter. In severe infections of Swiss Needlecast only the current year’s needles may remain on the tree.

To confirm that Swiss needlecast has infected your tree examine the undersides of the needles in the spring. Look for two rows of small black fruiting bodies, one on each side of the mid-rib of the needle. The black fruiting bodies protrude from the air exchange holes (stomata), which normally appear as small white dots. Spores from Swiss Needlecast are mostly moved by rain splash or irrigation systems. Thus, Swiss Needlecast disease usually spreads only to nearby trees. New needles are susceptible to Swiss Needlecast infection when all buds have broken and needles are elongating.

Cultural Control of Swiss Needlecast

Weed control under Douglas-fir trees and adequate plant spacing allow better air circulation and thus the drying of rain and dew moistened needles. Such practices can have a major impact on the severity of this disease because Swiss Needlecast primarily affects the lower branches and infection depends on prolonged periods of needle wetness.

Chemical Control of Swiss Needlecast

Several fungicides are registered for the control of Swiss Needlecast. Infection usually begins in late spring and continues into the early summer. Fungicide applications should be applied when new shoots are 1/2 to 2 inches long.

Depending on general weather conditions in the spring two or three fungicide applications should be sufficient to control Swiss Needlecast.