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.
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