Landscape and Nursery - Canker Diseases on Woody Plants

Canker diseases caused by fungi and bacteria may develop in most landscape trees and shrubs. The following is information on canker diseases and their management.

Cankers are difficult to manage and they may impact aesthetic values and provide openings for other diseases. 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 and cause the entire branch to die.

Canker diseases are often more severe when plants are growing under stressful conditions. Common environmental stresses include drought, flooding, unseasonable freezing temperatures, extreme temperature fluctuations, mineral element deficiencies, defoliation, chemical injury, transplant shock, and mechanical injuries. Fungal cankers usually start at a wound or a branch stub, but some fungi may penetrate healthy tissue or begin as latent infections already existing inside the plant and only causing disease when the tree is under stress. However, some fungi and bacteria aggressively attack trees and cause cankers. With important exceptions such as chestnut blight, dogwood anthracnose, pine tip blight, and a few other diseases, cankers rarely kill their host plants entirely. Cankers not involving pathogenic microbes can also be caused by mechanical injuries such as hail, heat, or cold.

Finding cankers. Woody plants in the nursery and landscape should be inspected for cankers. Look for:

• localized areas of roughened or cracked bark, especially around wounds and branch stubs,
• ridges of callus formation, and
• small red, dark brown, or black pimple-like fungal fruiting bodies in the center of or around the edges of the cankers.

Canker disease management. In the nursery and landscape:

• Prune out cankered twigs and branches being careful to avoid damage to the branch collar.
• Prune trees and shrubs only during dry weather and not in late summer or fall when canker fungi may be active.
• Prevent drought or flooding.
• Control weeds and other competitors, but avoid herbicide injury.
• Prevent mechanical injury.
• Protect trees from defoliating insects and diseases.
• Remove trees weakened by cankers.
• Plant well-adapted species and cultivars, matching the plant with the site.
• Use proper transplanting techniques.
• Alleviate drought stress with mulching and timely watering.
• Avoid excessive nitrogen fertilization.

Reprinted in part from "CANKER DISEASES OF WOODY PLANTS" By John Hartman in the July 16, 2007 edition of the Kentucky Pest News from the University of Kentucky, College of Agriculture.