When a tree dies, people want to know why. Sometimes the answer isn't

	Dying bishop pines. By Chris Lee, Cal Fire

straightforward; combinations of pests often work together to take advantage of stressed trees.

 

Some of these associations are well-known, such as the one between bark beetles and vascular wilt fungi. Bark beetles carry sticky fungal spores into a tree as they penetrate it and tunnel within the inner bark. The spores germinate inside the beetles' galleries and form fruiting structures that protrude into the galleries so that young beetles pick them up before emerging to fly to a new tree, which they in turn inoculate with the fungus. In such a relationship, both species benefit - the fungi are transported to new host material and the beetles get help from the fungi to overcome a tree's defenses.

 

A less clear-cut relationship is that of bark beetles and root pathogens.

Red turpentine beetle pitch tube. By Chris Lee, Cal Fire

  When bark beetles decimate a conifer stand, it is easy to blame them as the sole mortality agent. Yet, trees that are t argeted by beetles are often already infected and wea kened by root pathogens, making them more attractive to bee tles. Many of the trees currently dying in the Sierra Nevada, Transverse Ranges, Coast Ranges, and Klamath Mountains were   predisposed to beetle attack because of root pathogens such as Heterobasidion root disease.

 

An example of complex tree mortality can be seen in the photo of the

	Leptographium stain. By Chris Lee, Cal Fire

failing bishop pines. The trees had thinning crowns symptomatic of root disease as well as entrance holes and pitch tubes, signs of beetle attack. Closer examination revealed red turpentine beetle larvae in the base of the tree and staining of the xylem tissue that yielded a species of Leptographium, a common blue-stain fungus of conifers in California that blocks the transport of water in the tree. Around several larger neighboring pines that had previously been cut were the fruiting bodies of the root decay fungus commonly called the "velvet-top fungus." The property owner had also recently built a small utility building very close to the trees and surrounded it with heavy gravel fill. This construction likely disturbed the trees' root systems, which intensified pre-existing root infections, further stressed the trees, and encouraged the buildup of beetle populations that introduced the Leptographium.

Newsletter feedback and ideas are welcome.  Please submit comments to caforestpestcouncil@gmail.com. 

When buying firewood for home heating or camping, remember to buy it from local sources, helping to minimize the spread of pests and diseases - Buy It Where You Burn It.  For a list of firewood dealers local to where you live, go to firewoodscout.org.

Sincerely,

Katie Harrell
Communications Director  

California Forest Pest Council