Inland Forests’ Biggest Pests

Fall 2014

What pest problem has the greatest influence on the health of Inland Northwest forests:
Bark beetles? Stem decays? Root diseases? Needle casts? Dwarf mistletoes? Rusts?

White Pine Blister Rust
From an historical perspective, many foresters argue that white pine blister rust has had the biggest impact on forest health. Because of this introduced disease, as well as logging practices, the amount of white pine in our moist forests has been reduced from about 50% historically to about 3% today. Fortunately, with increased knowledge of our forests, today’s foresters are working to reverse this trend. For example, new plantings include blister rust-resistant white pine seedlings.

Root Disease
For family forests the most serious pest is commonly at “the root of the problem”- root disease. Except for pure pine forests, root disease can likely be found 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 2,300 acres in eastern Oregon!

Since the presence of root disease plays such an important role in our forests, let’s examine how it functions and the solutions.

How is it Spread?
This disease spreads from the roots of infected trees to the roots of healthy trees via root contact or by tiny rot-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 disease.

Trees may survive for many years following infection with root disease, although tree growth is slowed. Indications of root disease are:
• Tree crowns fade from dark green to pale yellow
• Presence of excessive pitch at ground level
• Dark brown stains on the bark
• Sparse needles
• Numerous Douglas-fir and grand fir snags

What to Do?
The proper approach to minimize root disease losses depends on ownership objectives and the tree species currently established in the 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. Since larch and pine species need sunlight to survive and thrive, they do not grow well if planted under a tree canopy.