Billions upon billions of ash trees are dying or have been killed by emerald ash borer (EAB) in both urban and forested environments in the U.S. and Canada. (Photo 1) With time, the ash all will be returned back to the soil and air from whence they came. For better or for worse, arborists are often in the middle of this process of removal and renewal.

An initial report from the arboricultural scientific literature revealed ash wood in EAB-affected trees showed evidence of reduced strength. Arborists were warned of the potential hazard of climbing ash even as the trees were being attacked by EAB and were not completely dead. Reduced wood strength from unknown causes was suggested to be creating an unforeseen hazard to arborists. Some tree companies restricted the climbing of ash even in the beginning stages of decline from EAB. (Photo 2) According to a survey by Purdue University, most tree companies restricted climbing or rigging in ash with 25% or more of the crown affected by EAB. (L. Purcell, personal communication)

It has become clearer, based on research at the University of Minnesota, that the deterioration of ash-wood strength is mostly due to the fungal colonization of EAB galleries, cambium and outer sapwood killed by EAB. Importantly, arborists working on dying and dead ash should know that the process of sapwood deterioration by these fungi, and the potential hazard to tree workers and climbers, begins even as trees are initially being attacked by EAB. (Photos 3 and 3 inset)

Most ash trees die over a period of three to six years, giving the fungi ample time to infect and deteriorate sapwood, as ash die slowly from EAB.

Enter the fungi
The early fungal infections of EAB galleries can have several different impacts on ash, and even on EAB itself. One group of fungi are canker pathogens that can enlarge areas of dead bark and cambium killed by EAB. (Photo 4) These fungi invade EAB-attack sites and can progressively attack healthy bark and cambium and enlarge areas already damaged by EAB. Arborists may recognize some of the generic canker fungi names such as Cytopsora, Nectria, Diplodia, Thyronecctria and Botryosphaeria. These fungi likely contribute to the demise of ash, particularly as the health of trees declines under attack by EAB.

A second group of fungi, and likely the most important to arborists, infects EAB-killed tissues and decay sapwood. These fungi have been shown to cause relatively rapid strength loss in sapwood, even in the early stages of decay – before it is readily visible to the naked eye.

The loss of strength in the outer sapwood is important, because even small amounts of decay in the outer wood fibers can result in significant overall loss of branch and stem strength. Contrast this to the strength loss for the same amount of decay in the center of the stem, where it causes a minimal reduction in overall strength. (Figure 1) Stem size or diameter also has a large influence on how quickly the overall loss in strength occurs (Figure 2), as larger-diameter stems take longer to become compromised from sapwood decay due to their size.

The likelihood that the deterioration becomes significant increases the longer the tree remains standing.

Different angles of attack
Researchers also found a number of different fungal species were involved in the colonization of EAB galleries and decay of sapwood from green ash. One of the most common was the well-known turkey tail fungus (Trametes versicolor). (Photo 5)

Most of the decay fungi isolated from EAB galleries caused white rot, where the decay of lignin usually takes place more rapidly than the degradation of cellulose. White rot, as the name implies, leaves affected wood lighter in color than normal sapwood of ash. (Photo 6a) Brown rot fungi also were present in EAB galleries. However, this type of decay, where cellulose is preferentially removed and lignin remains mostly intact, appears to be less common in dying ash trees. (Photo 6b)

It is likely that the different species of decay fungi attacking ash may change locally and on different ash species, but most of the fungi identified from green ash in Minnesota can be found anywhere ash grow. One interesting group of white rot found in EAB galleries were several crust fungi that cause a “jelly rot” in the genus Phlebia. These fungi are typically found on dead wood and are not commonly associated with sapwood decay of living trees. (Photo 7)

A third wave
A third group isolated from EAB galleries was insect-attacking fungi. A number of different insect-killing species were identified, and their role in reducing EAB populations is being investigated by researchers. These fungi are mostly not vectored or brought in by EAB, but are from spore infections of galleries.

The last and largest group of fungi isolated from EAB galleries contains species commonly found where easily available nutrient sources are exposed to the environment.

Entering ash
Most fungi cannot directly penetrate living bark and healthy sapwood; they require wounds or bark openings. Bark splitting that exposes EAB galleries (Photo 3 inset), woodpeckers foraging in infested trees (Photo 4) and eventual killing them or loss of larger areas of bark from prolonged EAB attack provide ample sites for airborne spores to gain access to EAB galleries or dead sapwood.

EAB-killed sapwood and cambium are prime fodder for canker and decay fungi, because the wood lacks any ability to react to their initial colonization. Splitting of bark and death and exposure of sapwood occurring even in the initial attack of ash could allow for fungal infection and deterioration of sapwood early in the death spiral from EAB. In addition, the canker fungi, and even some of the sapwood decay fungi such as T. versicolor, can kill cambium and sapwood ahead of the decay process.

Falling ash
We have learned from watching thousands of ash trees die and eventually end up on the ground that EAB-affected trees fail in many different patterns when subjected to the forces of nature. Most dead and severely declining trees drop smaller branches of varying sizes first. Smaller stems and branches in the upper crown are often the first attacked and first to die from EAB, and are likely the first to be colonized by decay fungi. The impact of wood-decaying fungi is also greatest in smaller-diameter stems simply because there is less wood to decay before failure becomes more likely. (Figures 1 and 2)

After the failure of smaller-diameter branches, many trees “piece themselves down” by dropping progressively larger branches after trees have died. (Photo 8) Some of the variation in where these larger stems fail may be due to where colonization by decay fungi occurs first.

Most main trunk failures occur after trees are dead for a period of time. It is not uncommon to see the main trunk of EAB-affected trees fail at various heights from the base to the point where larger branches are attached. (Photos 9, on page 20, and 10) Without a consistent pattern of failure, it is very difficult to predict where any individual tree is weakest and where it is most likely to fail. Further complicating matters, roots, trunks and branches may have existing columns of decay that can influence where failures occur. (Photo 11)

Many EAB-killed ash also fail at the roots, the root flare or very close to the ground. (Photo 12) Decay of roots can proceed rapidly once trees begin to decline or are nearly dead. Root-decay fungi, such as Armillaria species and others, are likely to be contributing to the root-related failures, although this has not yet been studied. Ample moisture in soil provides ideal conditions for rapid fungal decay of supporting roots at the base of the tree. Similar to above-ground stems, roots also may have existing decay that can contribute to root failures on EAB-killed trees.

Back to where they came from
In the end, arborists are faced with the decision to determine if they should rig in or ascend any individual ash or use other removal methods. What is clear is that the deterioration of ash wood from sapwood decay cannot be assumed to be isolated only to dead branches, stems or trunks. And, again, the process of decay and strength loss occurs as ash is being initially attacked by EAB.
What also is clear is that the longer the tree has been dying from EAB, the greater the chances that these fungi have caused significant stem or root deterioration somewhere in the tree. This places arborists in a difficult position when working on EAB-affected or -killed trees.

Visual inspection or other inspection methods are seldom effective in identifying where the decay and strength loss are the greatest, and pose a direct danger to the work process.

The other side of the EAB story is that dead and dying trees provide good habitat, nesting locations and sources of food for a wide array of birds, mammals and even other insects and fungi. (Photo 13) Eventually, ash, as all living things, will be returned to the environment as carbon dioxide and the base elements they are made of, a process that starts long before we imagined. Don’t be part of that process!

Christopher Luley, Ph.D., is a consultant and tree pathologist in Naples, New York. He published an updated “Wood Decay Fungi Common to Urban Living Trees” in 2022. The field guide can be purchased at the International Society of Arboriculture’s bookstore at isa-arbor.com, or as an online read-only version at treerot.com.