RHABDOCLINE NEEDLECAST ( Rhabdocline weirii )
Rhabdocline needlecast is a fungus that affects only Douglas-fir, causing infected needles to become discolored and die and eventually be cast from the tree, often making it unsalable as a Christmas tree.
Begin scouting for this disease in early April before budbreak. Scout on slightly overcast days as opposed to sunny ones, as it will make the discoloration of the needles stand out much better. Examine needles near the base of the tree first, as this is where infection is most likely to occur. Infected needles will have one or more purplish-brown bands or spots which are evident on both the upper and lower surface of the needle. Usually just before budbreak, the banded areas will begin to swell and split open lengthwise on the undersurface of the infected needles, in preparation for releasing infectious spores. Once the fruiting bodies rupture, spongy, orange, spore-bearing fungal tissue will protrude from the undersurface when conditions are damp. When this tissue begins to turn black, spore production and dissemination is complete.
Fig. 1. Purplish-brown bands on Douglas-fir needles infected with rhabdocline needlecast.
Three fungicide applications are generally recommended. The first should be made when at least 50% of the buds have broken and the new growth is 1/2 inch long. Make two more applications at two to three week intervals after the first. To help prevent the spread of this disease, plant where there will be good air drainage, and keep the weeds down to encourage airflow between the trees. Rogue heavily infected trees that are close to harvest and that would require excessive time and effort to save. Do so before Douglas-fir budbreak in the spring.
Fig. 2. Severely infected Douglas-fir in a Christmas tree plantation.
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