Reports on Plant Diseases

RPD No. 636 - Canker and Dieback Diseases
of Woody Plants

April 1997

Canker and dieback diseases are most common and conspicuous on trees and shrubs under stress from any of the following: transplant shock, an excess or deficiency of water, prolonged exposure to extremely high or low temperatures, sudden hard freezes in mid- to late fall or spring, summer or winter sunscald, frost cracks, nutritional imbalances, extensive defoliation, soil compaction, changes in the soil grade, mechanical injuries (such as hail, wind, a heavy ice or snow load, fire, lawn mowers, construction equipment or vehicles, or rodents), pruning wounds, root rot, insect borers, nematodes, dogs, or improper digging, storage, and shipping. Cankers are also common in nurseries and plantations where the plants are grown closely together.

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Figure 1. Phomopsis canker
damage on Russian Olive.
Note dieback of twigs.

A canker is a localized diseased area or lesion in the bark of a woody plant and often results in an open wound. Starting as a definitely marked dead area, usually round to oval or elongate in shape, a canker may enlarge and girdle the twig, branch (limb), or trunk. Nearly all canker pathogens are wound parasites. The death of the bark and underlying cambium tissue is usually associated with a dead bud, branch stub, or twig, or with some type of mechanical injury and extends radially from the wound. When the water conducting tissue (xylem) is killed or blocked, the most prominent symptom is a gradual or rapid wilting, withering, and dieback starting at the shoot tip and progressing down the branch — all of this being a secondary effect of the girdling canker some distance below. The leaves on affected parts are often smaller than normal, first pale green then yellow or brown, curled, and sparse. Large or multiple trunk cankers may result in severe girdling and subsequent death of the affected tree or shrub. When a canker forms on the trunk near the soil line, it is called crown canker or collar rot (Figure 5).

Cankers may develop as conspicuous, slightly sunken or flattened areas of bark (Figures 2 and 3), or as inconspicuous diseased areas that cannot be detected by examining the bark surface. The diseased bark of conspicuous sunken cankers often cracks within or at the margins, exposing the wood beneath (Figures 3 and 4). In other cankers, such as those caused by species of Nectria, Urnula (Strumella) (on oaks), and Eutypella (on maples), swollen ridges of callus form a targetlike area (Figure 6). On woody plants with light-colored bark, diseased bark that does not become depressed may turn some shade of brown or black (Figures 7 and 8). Such cankers are conspicuous because of their abnormal color. Inconspicuous cankers usually appear as brown to black diseased areas just under the bark when the thin stromata or outer bark is cut away (Figure 5). Many canker diseases caused by fungi are indicated by raised, speck-sized or larger, dark fruiting bodies (pycnidia, perithecia, stromata acervuli) in the diseased bark (Figure 1). Many fruiting bodies break through the bark and are seen as dark spots or bumps. Oozing of gum, resin, or sap is a common sign of certain cankers (Figure 8).

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Figure 3. Cytospora
canker of poplar kills
thousands of poplar
each year
(IL Nat. Hist. Survey).

Most cankers are caused by fungi although a few affecting fruit and nut trees are caused by bacteria, especially by species of Erwinia, Pseudomonas, and Xanthomonas, viruses, and other types of pathogens. The host pathogen relationships and the pathogenicity of numerous species of these fungi have not been thoroughly investigated. Many of the fungi are known to be only weak or secondary pathogens. Some have both perfect (sexual or telamorph) and imperfect (asexual or anamorph) states, which may or may not be present at the same time on the same plant. Thus, there is undoubtedly some duplication in the list of host plants for many of the genera. Other minor host plants, especially shrubs, could be added to many of the genera.

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Figure 6. Nectria canker on
sugar maple stem.

Perennial Cankers

A canker producing fungus enters through a mechanical wound in the bark (such as a branch stub or mowing bruise) and invades and kills the bark, commonly during a dormant period. At the same time, the host plant attempts to limit invasion by the fungus by producing a layer of callus cells around and over the edge of the invaded tissue, but the fungus invades the callus tissue during the next dormant period. The host then forms new callus tissue. This cycle of invasion by the fungus and formation of callus tissue by the plant may be repeated for several years. The extent and shape of the resulting canker is determined by the growth of the pathogen and the amount of callus formation. The extent and shape of the resulting canker is determined by the growth of the pathogen and the amount of callus formation.

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Figure 8.
Botryosphaeria
canker on
sweetgum trunk.
(IL Nat. Hist. Survey).

Annual Cankers

Cankers contain little or no callus and increase rapidly during a single season. Branches or even entire trees, especially young ones, may be girdled in one growing season. Most are caused by fungi that attack stressed or weakened plants. Most canker fungi are restricted to invasion of bark tissues. Some, however, colonize both the bark and the underlying xylem tissue. Stem diseases that result from a simultaneous canker and wood decay are called canker rots.

Canker fungi usually produce their fruiting bodies in recently killed bark. The spores produced by these fruiting bodies serve as inoculum for new infections, which occur mostly during wet or damp weather. The spores (mainly conidia and ascospores) are spread primarily by air currents, splashing rain, and contaminated pruning and shearing tools; a few are spread by contaminated insects and birds. The spores may ooze out of fungus fruiting bodies in long tendrils or cirrhi (Figure 9) during wet weather.

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Figure 9. Chestnut
blight canker.