Tree disease: a sneaky forest threat

Native tree diseases are a particularly common menace to Oregon’s western forests. Among the most prevalent diseases in Oregon are Swiss needle cast and laminated root rot. Swiss needle cast is a foliage disease that affects Douglas-fir trees. It causes trees to prematurely shed their needles.

Laminated root rot attacks and kills a tree’s root system. This hinders its ability to suck up water and soil nutrients. Trees affected by root disease are also more susceptible to bark beetles and wood borers, and are more likely to fall during a storm or high winds.

But there are ways to combat and prevent the spread of tree disease with active forest management. For instance, planting hardwood tree species such as alders that are not affected by root disease is one way to fight the pathogens. Other management techniques to treat root disease include harvesting a buffer around diseased trees, and breaking the chain of root contact between infected and healthy trees through thinning.

Forest Threats: Tree Disease

Tree diseases such as Swiss needle cast and laminated root rot threaten the health of Oregon’s forests, but active forest management helps prevent or slow the spread of these diseases. 

Disease threats to Oregon forests

Dwarf Mistletoes

Parasitic, flowering plants that can slow tree growth, deform crowns and branches, and eventually kill the trees in which they grow.

Foliage Diseases

Caused by fungi that infect tree leaves or needles and cause them to fall prematurely from the tree. In the case of foliage diseases known as needle blights, the dead or partially dead needles often remain attached to the tree’s branches.

Root Diseases

Caused by fungi and pathogens that attack and kill a tree’s root system. Trees affected by root disease are more susceptible to bark beetles and wood borers, and are more likely to fall during a storm or high winds.

Rust Disease and Stem Cankers

Localized areas of dead bark on a tree branch or trunk are caused by fungi that destroy the inner bark and kill branches and entire trees. White pine blister rust, a stem canker caused by a non-native fungus, has devastated five-needle pine trees growing in Oregon.

Stem Decay

Caused by fungi that enter trees through wounds or small branches.

Sudden Oak Death

A non-native pathogen that has killed hundreds of thousands of tanoak trees on the southern Oregon coast. It has a wide host range and can cause branch dieback and leaf spots on many native plants and trees, including Douglas-fir trees.


9755 SW Barnes Rd., Suite 210        
Portland, OR 97225        
Phone: 971-673-2944        
Fax: 971-673-2946

twitter youtube facebook linkedin