Landscape - Canker Diseases on Woody Plants

Late winter is a good time to inspect trees and shrubs for canker diseases and remove any infected branches. The following is an article on the subject.

Late winter is a good time to prune-out diseased, dying, and dead twigs and branches to improve plant appearance, to reduce disease spread, and reduce pathogen inoculum. Canker diseases on twigs and branches may be easier to see now before emerging foliage obscures them. For some canker diseases, pruning while the plant is still dormant may be the last opportunity to prune without danger of spreading the disease.

What are cankers? Cankers are localized dead areas of twigs, branches, limbs, trunks, and even roots of woody plants. They are often caused by invasion of bark or cambial tissues by fungi or bacteria which then kill phloem, cambium, and the outermost xylem. The bark in an infected area may shrink, crack, and expose the wood beneath. Canker diseases often girdle the infected branch, causing the entire branch to die. Infection occurs through wounds such as mechanical injuries, leaf and fruit scars, branch stubs, and cold-injured or sun-scalded tissues. Many fungi and bacteria that cause cankers normally inhabit the surface of the tree, or possibly exist inside the tree as latent pathogens, and only cause disease when the tree is under stress. However, some fungi and bacteria aggressively attack trees and cause cankers. Cankers, or dead areas on the bark not involving pathogenic microbes, can also be caused by mechanical injuries such as hail, heat, or cold.

Perennial cankers.

Target-spot cankers caused by fungi such as Nectria or Eutypella on hardwoods are roughly circular or elongate with much callus at the canker edges. Wounds and branch stubs are invaded by the fungus during the tree's dormant period. The plant forms callus around the infection site during the growing season, but the fungus invades more tissue the following dormant period. This back-and-forth struggle between the tree and the pathogen creates concentric ridges of callus tissue. Although infection spread is relatively slow and target cankers seldom kill the tree, they do weaken the tree structure and detract from its appearance.

Annual cankers.

Weak parasites normally don't cause disease problems unless the tree is under environmental stress and low in vigor. Infection occurs during the dormant season, but during the growing season host callus tissue walls off the canker and prevents further spread. Although annual cankers do not persist, continued stress makes it likely that more cankers will form and it opens the possibility of invasion by other diseases.

Diffuse cankers.

Fungi such as Cytospora, Botryosphaeria, Hypoxylon, Phytophthora, or Cryphonectria (chestnut blight) and bacteria such as Erwinia (fire blight) produce cankers with little callus at the margins. Because the pathogens invade so rapidly, the tree tissue at the canker margin is killed and branches or whole trees are girdled and killed, sometimes in one season. Some diffuse cankers are favored when the tree is under stress, but most are not. Canker blights are diffuse cankers in which the disease develops rapidly and kills collateral branch and foliage tissue by way of girdling; canker-rots are diffuse cankers that cause significant internal wood decay.

Common fungal cankers include:

Botryosphaeria canker of many kinds of trees and shrubs
Cryphonectria canker of chestnut (chestnut blight)
Cytospora (Leucostoma) canker of fruit trees and spruce trees
Discula canker of dogwood (dogwood anthracnose)
Hypoxylon canker of oaks
Nectria canker of many kinds of trees
Phomopsis canker of a variety of trees and shrubs
Seridium canker of Leyland cypress
Sphaeropsis canker of Austrian and Scots pines (pine tip blight disease)
Thyronectria canker of honey locust and other woody plants

Pine tip blight, dogwood anthracnose, and chestnut blight are observed to kill trees or at least cause significant dieback. The most common bacterial canker occurs mainly on apple, crabapple, pear and flowering pear and is caused by Erwinia, the fire blight bacterium.

Canker disease management.

For canker disease management, integration of several cultural practices may be needed. Inspect woody plants in the nursery and landscape for cankers. Look for: dead twigs and branches, especially the area between diseased and healthy tissue localized areas of roughened or cracked bark, especially around wounds and branch stubs ridges of callus formation small red, dark brown, or black pimple-like fungal fruiting bodies in the center of, or around the edges of, the cankers. Prune-out cankers during dry weather, preferably when trees are dormant, and avoid pruning during the growing season when canker fungi may be active. When pruning, be careful to avoid damage to the branch collar. Plant well-adapted species and cultivars, matching the plant with the site. Use proper transplanting techniques.

Promote tree vigor so that the tree's natural resistance to disease can be expressed and wound healing can begin promptly and develop rapidly. Apply mulch, water plants during dry periods, and aerify compacted soil. Apply fertilizer only where there is a known mineral element deficiency. General fertilization, especially with nitrogen, can make some canker diseases worse. Control weeds and other competitors, but avoid herbicide injury. Prevent mechanical injury. Protect trees from defoliating insects and diseases. Remove trees weakened by cankers.

Adapted from "PRUNE-OUT CANKER DISEASES FROM LANDSCAPE TREES AND SHRUBS" By John Hartman in the March 21, 2005 edition of Kentucky Pest News from the University of Kentucky College of Agriculture.