Christopher Starbuck, Ph.D., is a horticulturist, teaches courses on woody ornamental plants and does applied research on stress management for landscape plants.

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Cantankerous Cankers
by Christopher Starbuck       #Disease   #Pests   #Trees

Fireblight canker on ‘Aristocrat’ pear.

Thyronectria canker on honeylocust

The term “canker” refers to a lesion on a twig, branch or stem, usually caused by a bacterial or fungal pathogen. The appearance of cankers varies, depending on the host and the pathogen. Often, the bark of the affected stem or trunk is sunken and discolored. Fluids may ooze from a canker or fungal fruiting structures may appear on the bark covering or surrounding the lesion. In some cases, lesions remain small and isolated, causing no major problems for the host plant. In other cases, the canker spreads widely, causing death of twigs, branches or even the main trunks of trees. The best known example of the destructive potential of a canker disease is chestnut blight, caused by the fungus Endothia parasitica, which caused the virtual extinction of the American chestnut within 40 years of its accidental introduction to the United States in about 1900.

It should not be surprising that bacterial and fungal pathogens would colonize the bark of a tree or shrub. The sapwood, just under the bark, is a rich source of carbohydrates and minerals. Fortunately, bark provides excellent protection most of the time. However, canker-causing pathogens are opportunistic. Mechanical bark damage from lawn mowers, string trimmers, insects or hail can provide easy access to a pathogen. Damage or stress caused by environmental extremes, such as waterlogging, drought, freezing or high temperature can also reduce a plant’s ability to resist attack by a canker-causing organism. Wrapping the trunk of a newly planted tree with a light-colored material to prevent winter sun scald will greatly reduce the chances of canker development. A wide mulch ring will eliminate damage by mowers and string trimmers. However, mulch should never be more than 1 inch deep right next to the trunk (no volcanoes!).

Black walnuts are showing advanced symptoms of thousand cankers disease in this picture taken Sept. 18, 2009. The tree died the following June.

New Disease Threatens Black Walnut
Thousand cankers disease (TCD) is a recently discovered disorder that has the potential to decimate black walnut trees in the Midwest. Initially recognized in Colorado in 2009, this disease had killed tens of thousands of black walnut trees in Western states. Since then, the disease has been discovered in Tennessee, Virginia and Pennsylvania; all states within the native range of the species. If thousand cankers disease becomes widespread in the Midwest, it will kill millions of trees with an estimated economic impact of more than a billion dollars.

Thousand cankers disease is caused by the fungus Geosmithia morbida, which is spread very effectively by a tiny insect called the walnut twig beetle. Each beetle bores multiple holes through the bark, inoculating the phloem at each location with the fungus and causing a tiny canker. The cankers eventually coalesce, destroying the vascular system of the tree and leading to mortality within three or four years.

The most likely way in which thousand cankers disease will spread within the native walnut range is by movement of firewood or logs. You can help slow the spread by educating your fellow citizens about the dangers of moving these materials around the Midwest. Be on the lookout for walnut trees dying from the top and report them to your local university extension office.

There are thousands of fungal and bacterial organisms capable of causing cankers on woody ornamentals. Fortunately, very few of these cause serious problems, especially if resistant plants are planted and maintained with good cultural practices. Fireblight is a troublesome bacterial canker disease commonly affecting plants in the rose family, especially crabapples and pears. This disease, caused by the bacterium Erwinia amylovora, usually starts at the shoot tips of a susceptible host. If left unmanaged, it can cause cankers on the main trunk, leading to mortality. Thankfully, most modern crabapple cultivars are highly resistant to fireblight. However, most cultivars of ornamental pear, including ‘Bradford’, ‘Cleveland Select’ and ‘Redspire’ are no longer considered highly resistant. Pruning out the “strikes” on branch tips during dry weather will reduce the chances that the bacterium can spread within the tree. Cut well below the obviously infected tissue and dip the shears in alcohol between cuts.

Canker development around a walnut twig beetle gallery in an English walnut.

Certain species of trees are commonly affected by fungal canker diseases, often after being predisposed by environmental stresses or mechanical damage. Cankers (Thyronectria) commonly develop on trunks of honeylocust trees as a result of transplanting stress or winter injury. Cankers usually remain isolated and trees recover as they become established, but severe infections can lead to dieback. Fast-growing trees such as ‘Lombardy’ poplar are generally short lived due to extreme susceptibility to the fungus Cryptodiaporthe populeum. Dieback of other poplars and of willows is commonly caused by either Leucostoma or Valsa canker. Again, these diseases are most likely to develop when the host tree is predisposed by stress.

We should be thankful that bark is such a good defense against the thousands of organisms poised to take advantage of any chink in the armor of our trees and shrubs. To help our woody friends repel invasion by canker-causing organisms, we should prevent sun scald, mulch to prevent mower and string trimmer injuries, irrigate during drought and avoid late summer fertilization that may lead to winter injury.


A version of this article appeared in a March/April 2012 print edition of State-by-State Gardening.
Photography courtesy of Christopher Starbuck and Ned Tisserat, Colorado State University,


Posted: 04/02/18   RSS | Print


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