When it comes to diagnosing tree diseases, many of us are focused on the leaves. They’re the easiest to observe, and are the most noticeable by the client.

The danger of focusing too much on the leaves is that it takes attention away from the other two major sections of woody plants: roots and stems. In fact, in terms of maladies that can cause significant damage to most trees, stem and root damage is usually more likely to permanently injure a tree than leaf issues. Borers, bark/sapwood cracks, heartwood decay, root girdling, root hair desiccation and, yes, cankers, are serious problems indeed.

Cankers cause damage to branches, main stems and trunks by killing the cambium and sapwood in localized areas. 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.

Photos Courtesy of UNL Plant Pathology.
Cherry stem canker. Fireblight shoot dieback.

Common canker diseases

Cytospora cankers of poplars, cottonwoods and willows
The cytospora fungus causes branch dieback and localized injuries on old and young trees alike. Cankers on stems are usually elongated and oval in shape, slightly sunken, discolored areas of the bark. As trees try to compartmentalize the injury, callus tissue usually forms at the margins of the canker. As a result, the bark often splits, further expanding the injured area. Twigs can be killed without the formation of a localized canker.

The color change of the bark is the most visible identifying characteristic of cytospora canker. Infected bark may be yellow, beige, brown, reddish-brown, grey or black. It is the contrast with the color of the unharmed portions of the stem that makes it noticeable. Diseased sapwood and cambium tissues often become reddish-brown or black and slightly soft as they begin to deteriorate.

Control of cytospora canker is difficult. Healthy trees are less susceptible to infection and subsequent damage. Strive to maintain a proper balance of nutrients, water, pest control agents, mulch application and other management tools to maintain optimal vigor of trees. Damaged limbs should be removed immediately upon notice.

Fireblight stem canker. Thyronectria canker.

Thyronectria cankers of honeylocust
Also a disease of elm, oak, hickory, ash and willow, thyronectria canker causes dieback of branches, reduced foliage throughout the tree, premature fall coloration and early leaf drop. Some variability in size and appearance exists, with some being expressed as slightly flattened, while others are distinctly sunken with large callus ridges on the margins of each localized infection. On thin bark portions of the tree, stems usually discolor to a reddish-yellow color. Where bark is thick at the bole of the trunk, no discoloration is evident.

The fungus overwinters as mycelium and fruiting structures on infected trees. Spread of the disease throughout the tree is enhanced as a result of mechanical wounds, storm injury and normal pruning cuts. Fresh bark wounds are readily infected by the fungus. The fungus itself grows in the cambium and outer xylem, where it eventually kills the cambium and surrounding tissues. Cambial death is the cause of the vascular dysfunction of the tree.

Cankers at the tree base and on lower scaffold limbs are serious, as they usually progress to cause death. Main stem or branch crotch cankers can cause complete girdling. In some cases, healthy trees are able to compartmentalize and ward off the infection. However, if a tree is under moderate or severe stress, the fungus usually incites a downward spiral of symptoms, resulting in a loss of the tree.

As with most cankers, thyronectria canker should be prevented, rather than controlled. Physical wounds and damage should be avoided if possible. Stressors such as compaction, drought, traffic, construction damage and over/underwatering should be avoided as well.

Keep trees healthy by meeting their needs.

Fireblight cankers of pear, crabapple, cotoneaster and others
Most species of pome fruits and the rose family are susceptible to fireblight. This includes many species, including mountain ash, pyracantha, crabapple, pear, quince and hawthorn. Unlike the others mentioned thus far, fireblight is caused by a bacterium, not a fungus. It infects the tree through blossoms and leaves near the terminal buds.  

Fireblight is relatively easy to spot. When infected, leaves quickly wilt and turn black, but remain attached to the twigs. In most cases, the bacteria move towards the trunk of a tree or crown of a shrub, forming dark, sunken stem cankers. If the infection is especially virulent, it may move into the root system as well, causing similar damage.

The bacteria overwinter in tissues adjacent to the canker margins. In spring, under warm and moist conditions, the bacteria multiply and invade healthy tissue. High humidity accelerates the development of the disease.

Pruning to remove infected twigs and branches with cankers can limit the spread of the disease. Branches should be cut at least 1 foot beyond the affected areas on each side of the canker. Although not completely effective, sterilizing pruning tools between cuts with mild alcohol or bleach solutions will reduce the chance of spreading the disease to other woody plants. Some studies have indicated that copper compounds applied early in the disease cycle can have a suppressive effect. The antibiotic compound streptomycin can also be useful to reduce damage. To be effective, three to four applications, beginning at the pink stage of bloom and every five to seven days thereafter, must be made.

Black knot cankers of cherry and plum.
Another easy disease to identify, black knot of stone fruits is an aptly named disease. If you can imagine a rope knot, elongated and sprayed with black paint, you’ve created the correct image of this malady. Some pathologists refer to them as elongated swellings or corky outgrowths. Fungal infections tend to predominate on the younger stems. They begin greenish and soft, but become black and hard with time.  

Soon after trees push out new growth in spring, spores are re-leased from fungal fruiting bodies along the surface of previous seasons’ knots. Cool, rainy periods favor the development of the disease. Black knot damages the tree by reducing the flow of water and nutrients to the ends of infected branches.

As with other canker diseases, pruning is the main control method. Like fireblight, preventative applications of a fungicide such as lime sulfur made during blooming and repeated several times thereafter at weekly intervals may help to reduce the spread of the disease.

Ways to limit damage

As indicated above, fungicide applications to limit the damage from cankers are generally not an effective technology. For the most part, control measures revolve around three methods:

1. Prune out damaged areas. Pruning is most effective when the disease is identified early on in the life cycle. A full-blown, raging canker infection is difficult to limit through pruning.

2. Choose disease-resistant cultivars. Ask your suppliers for available selections—new cultivars of disease-resistant plants are developed each year, as well as the ones currently available. For example, it is well-documented that some honeylocust cultivars are more resistant to thyronectia cankers than others. Variability amongst crabapple cultivars also exists, sometimes on a state-by-state or region-by-region basis.

3. Keep trees healthy by meeting their needs. This involves preventing drought and providing minimal amounts of nutrition. 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.

John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb.