If you’ve been in the tree care industry for more than a season, you know that some maladies are more problematic than others. Aphids, for example, can cause quite a bit of temporary damage to a green ash tree, but compared to the emerald ash borer, they have less of an impact.

Cankers cause damage to branches, main stems and trunks by killing the cambium and sapwood in localized areas. Of course, without cambium and sapwood, water and nutrients cannot move throughout a tree, causing it to die. If the canker disease develops many localized infections and spreads throughout the tree, death or severe damage almost always results.

In terms of potential for long-term injury, cankers are quite serious in that options for treatment are limited. At the present time, no fungicide or bactericide products are effective at controlling canker diseases that infect shade and coniferous trees. As a result, control efforts must emphasize prevention through sound cultural practices.

Rumpled canker appearance. Photo: John C. Fech, UNL

Inspection techniques

Inspection is about the three Rs: repetition, regularity and recognition. Good inspection protocol involves making frequent visits to the job site to closely monitor existing problems and observe the initial stages of new ones.

When inspecting for cankers, keep in mind three recognition factors: shape, size and overall appearance or “the look.” There are many canker species, but they are all oval shaped. Sure, we could debate whether oval is oval, and suggest that certain ones are elongated, ovoid or rounded to oval, but, really, they look like a football without the leather and laces.

The second part of inspecting for cankers is to realize that sizes can vary tremendously, just like they do with truck tires. The size of tire that is appropriate for a Ford Ranger is much different than what’s best for a Dodge Hemi Super-Cab. Size is related to the specific genus and species of canker. For example, consider the ones found when dealing with thousand cankers disease of black walnut versus the ones that develop from thyronectria canker of honeylocust.

Thousand cankers disease cankers are quite small compared to others such as thyronectria
canker of honeylocust. Photo: Lori Stepanek, UNL

“Words mean things … people judge you by the words you use.” This was a line from a radio advertisement a couple of years ago. If I could change this phrase a bit for the purposes of this article, it would be: “Words mean things … people understand you by the words you use.” This is just basic communication strategy, where the sender of the message formulates a set of words that will help transmit his meaning to another person in the most efficient way possible. As arborists, communicating the results of the regular inspections is an important part of caring for trees.

So, what word would I use to describe “the look” of a canker? Rumpled. Hopefully, that word conveys the meaning of an appearance of bark and sapwood that has changed from a smooth or consistent pattern to one that is undulating, shattered or shaggy. If “rumpled” doesn’t do it for you, “crinkled” or “compressed” are other possibilities.

In many cases, small cracks develop at the edges or in the center of the canker. This is due to the desiccation of the outer bark tissues. Certain cankers develop an “oozing” of sorts, with amber colored sap pushed out as an easy recognition feature.

Thousand cankers disease cankers are quite small compared to others such as thyronectria
canker of honeylocust. Photo: Lori Stepanek, UNL

Sunscald cankers

A look-alike condition known as sunscald mimics the appearance of typical cankers. Sunscald cankers are oval, vary in size, and usually appear rumpled or at least cracked in appearance. Unlike the previously mentioned diseases that occur as a result of infection from fungal or bacterial organisms, sunscald occurs when the bark is warmed by the winter sun, resulting in a softening of the tissue followed by a rapid decrease in temperature. When this pattern of fluctuation is repeated several times, many tree species develop canker-like symptoms. One way to distinguish it is that it almost always occurs on the south, west or southwest side of a tree trunk. Obviously, infectious cankers can also occur on these sides, but sunscald develops in these locations on a more regular basis, providing an additional clue for diagnosis.

Sunscald canker appearance on trunk. Photo: John C. Fech, UNL


Inspection is important in terms of controlling cankers, as problematic tree parts can be documented and slated for removal in some cases. These steps often result in the elimination or at least a reduction of the spread of the disease throughout the tree or tree stand. Keep in mind that when pruning damaged areas, pruning is most effective when the disease is identified early on in the life cycle. A full-blown canker infection is difficult to limit through pruning.

While inspection is quite worthy, prevention is essential. Think about it, is it better to remove a problem or avoid it altogether? Think back to your last automobile accident. You may have been in the right, and the police officer ended up giving the other driver a ticket, but you still had to deal with insurance issues, get estimates and have the car repaired. It’s better to avoid a traffic accident. The same is true for canker diseases of trees; it’s much better to prevent or avoid them.

Cracking appearance.
Photo: John C. Fech, UNL Photo: John C. Fech, UNL

Most attempts at prevention fail because they are passive rather than active. For each canker-prone tree on the properties you manage, outline an active prevention strategy and share it with the property owner. In many situations, this becomes the “treatment” instead of something pulled out of a spray tank and applied to the tree. The good news is that prevention treatments can be as profitable as fungicide applications. More importantly, active prevention treatments comprise what trees need most.

Prevention steps

The first prevention strategy is to choose disease-resistant cultivars by asking your suppliers for available selections. There are disease-resistant plants currently available, and new cultivars are developed each year. For example, it is well-documented that some honeylocust cultivars are more resistant to thyronectria cankers than others. Variability amongst crabapple cultivars also exists, sometimes on a state-by-state or region-by-region basis.

Cracks and oozing appearance.
Photo: John C. Fech, UNL

Secondly, keep trees healthy by meeting their needs. This involves preventing drought and providing minimal amounts of nutrition. If the tree in question is growing by itself, one approach is called for; if present in an island of turfgrass, then it’s a whole different ball game. Trees growing in a landscape setting where the roots are intertwined with those of turfgrass plants often receive excessive amounts of nitrogen fertilizer and water, causing them to be more prone to cankers. As a general rule, most woody plants need only a third as much water and fertilizer as grass in order to thrive. Soil testing and conversations with landscape crews can help determine the overall status of the landscape.

Watering and fertilizing to meet the needs of adjacent turfgrass often leads to succulence and carbohydrate and sugar redirection from tree defense, which results in reduced resistance to diseases such as cankers. Avoidance of damage from construction, soil compaction, mower blight, vandalism and trenching will also help to reduce the likelihood of canker infection.