Time to manage needle cast diseases of Douglas fir and spruce

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Now is the time to begin managing the two Douglas fir needle-casting diseases called Rhabdocline needle cast and Swiss needle cast and the needle-casting disease of spruce called Rhizosphaera. The fungi causing the needle-casting diseases of Douglas fir and spruce require free moisture and warm temperatures very typical of May and early June in Michigan. On trips this week to the western part of Michigan, we saw the beginning of spore release. Because of the requirement of free moisture, needle-casting diseases are generally more severe on the lower half of trees. This is due to the denser foliage, complex and therefore older branching patterns, and because this portion of the tree is located in the weed zone. Weeds can interfere with proper air flow in plantations leading to increased free moisture on the needles.

While monitoring spores of Rhabdocline in 2006, we saw the spores of Rhabdocline come in two distinct waves, early in May and later in early June. Therefore, it would be best to design your spray schedules to accommodate these times. At least two sprays should be applied to manage these two spray release waves. If you have planted the Douglas fir seed-source called Shuswap which is resistant to Rhabdocline needle cast, you should still determine the amount of Swiss needle cast infection the trees are carrying. Generally, sprays for Swiss on Shuswap, can be made two years prior to the potential harvest date.

To determine if your trees warrant chemical management, look for signs of the fungal fruiting bodies on the needles. These fruit bodies are producing the spores that will infect the new needles as they break bud. With a hand lens, scan the discolored needles. For Rhabdocline on Douglas fir, look for needles with elongated brown splotches and on the bottom side of the needle, look for areas where the outer portion of the needle’s epidermis (skin) appears raised. If you see these, then examine two-year-needles on 50 or more trees scattered throughout your plantation. If you find fruiting bodies on 20 percent or more of the trees, consider treating the plantation.

In the case of Swiss needle cast of Douglas fir and Rhizosphaeria of spruce, randomly select 20 or more trees scattered around the plantation, and remove three sample branches from each tree. Examine the white rows of stomata on 2-year-old needles and if half of the branches have fruiting bodies on more than 10 percent of the needles, considering treating the entire plantation. Two sprays are more effective than one.

If growing Douglas fir, consider planting the Rhabdocline resistant seed source Shuswap to reduce Rhabdocline needle cast problems.

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