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Biggest Forest Pest?

What pest problem has the greatest influence on the health of Inland Northwest forests? That is the question I asked myself when considering what subject to address in this article. Bark beetles? Stem decays? Root diseases? Needle casts? Dwarf mistletoes? Rusts? The list to consider goes on and on.

From a historical perspective, many foresters would argue that white pine blister rust has had the biggest impact. Because of this introduced disease, as well as logging practices, the amount of white pine in our damp forests has been reduced from about 50 percent historically, to about 3 percent today. Fortunately, with increased knowledge of our forests, modern day foresters are working to reverse this trend. For example, efforts are underway to plant blister rust resistant white pine seedlings.

When I think of family forests and the most serious pest I encounter on these ownerships, one pest is commonly at the root of the problem – root disease. This fact was brought home to me earlier this year when I walked over a large private ownership and marveled at how little root disease was present. Except for pure pine forests, it seems I can always find root disease on a property of any significant size.

Unfortunately, root disease does decimate many of our forests. Areas of the Inland Northwest have the dubious distinction of containing some of the largest root disease centers in the world. In fact, many knowledgeable people currently consider the largest living organism to be a single root disease fungus that covers 2,200 acres in eastern Oregon!

Since the presence of root disease plays such an important role in our forests, let’s take a brief look at how this malady works and what we can do about it. This disease spreads from the roots of infected trees to the roots of healthy trees via root contact or by tiny root-like structures. Infection can occur in all tree species found in the region, but mainly impacts Douglas-fir and grand fir trees. Pine species and western larch are much less susceptible to root diseases.

Trees may survive for many years following infection with root disease, although tree growth is slowed, and tree crowns often fade from dark green to pale yellow. Other indications of root disease include the presence of excessive pitch at ground level, the occurrence of dark brown stains on the bark, sparse needles, and the presence of numerous Douglas-fir and grand fir snags.

The proper approach to minimize root diseases losses is a function of your ownership objectives and the tree species currently established in your forest. One approach is to harvest the tree species most susceptible to root disease, while leaving trees that are much less susceptible. In a mixed conifer forest, western larch and pine species should be favored over Douglas-fir and grand fir trees. If only Douglas-fir and grand fir trees are present, then small clearings can be created and planted with western larch or pine seedlings. While both approaches will reduce root disease losses, your aesthetic objectives will strongly influence the feasibility of the second method.

– M. Wolcott

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