By: Jackie Carroll
Black canker disease can seriously disfigure trees, especially willows. Find out how to keep your tree healthy, and what to do about treating black canker disease in this article.
Black canker is caused by the fungus Glomerella miyabeana. In willow trees, it often accompanies scab. Leaves that develop irregularly shaped spots are the first sign that a tree may be suffering from black canker. The spots appear in late spring or early summer, and the tree looks otherwise normal. Tree owners hardly notice the problem at this point, even though infected leaves may shrivel.
Cankers form at the point where the leaf stem attaches to the twig in late summer, and as the disease progresses, you’ll find cankers where the twigs attach to stems and branches. Cankers can eventually form on the main stem or trunk. In the fall, wounds ooze a sticky, pinkish, velvety-looking substance that contains spores. The spores are transported to different parts of the tree and to surrounding trees by insects.
The size of the canker depends on the natural resistance of the tree. The first year, they may be only an inch (2.5 cm.) in diameter on resistant trees, or more than three inches (7.5 cm.) on particularly susceptible trees. Each year the areas of dead bark around the cankers become larger, but the disease doesn’t kill the tree unless multiple cankers converge to completely encircle the trunk.
Black canker treatment includes pruning and spraying with fungicides. You can’t cure existing cankers with fungicide, but you may be able to prevent reinfections. Treat nearby trees as well to prevent them from becoming infected. Spraying should be carefully timed. Consult your local Cooperative Extension agent for advice on the best time to spray for black canker on trees in your area.
Pruning away infected twigs and branches is an important part of treating black canker disease. Your goal is to remove all of the infected leaves and twigs. Look for dark-colored twigs with shriveled leaves. When the infection completely encircles a twig, it will have a characteristic droop or hook shape at the tip.
There is no cure for trees already damaged by black canker tree disease. Just do your best to keep the disease from spreading to other parts of the tree and to other trees in the landscape. With careful attention to pruning and occasional spraying, your tree can live a long life in spite of the disease.
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Cherry tree enthusiasts look forward to it each spring: a mass explosion of light pink blossoms. What’s not so exciting for lovers of the blossoms and their sweet fruit is the threat of cherry tree diseases. Such pathogens can harm an entire orchard of cherry trees, putting a full halt to any charming blossoms or juicy fruit. When cherry tree growers execute the right preventive measures, they decrease their trees’ susceptibility to infection and increase the chances of a vigorous bloom and delicious harvest.
We’ve collected symptoms, causes, and treatments of seven different cherry tree diseases to help you better protect yours.
|Apple tree with a canker suspected to be black rot. |
Photo used by permission of the grower.
Because the causes of trunk cankers are complex, they often cannot be diagnosed reliably by a simple online search or comparing the symptoms to photos. Tree fruit experts, plant pathologists, and Extension educators are more than happy to assist in diagnosing injuries on fruit trees.
This process often involves examining evidence like photos and spray records, and then sending a sample to a university plant diagnostic clinic for identification. The University of Minnesota offers these services at the Plant Disease Clinic. Many other state universities have their own clinics as well.
The clinic will examine the sample via microscope to determine whether a disease is contributing to the problem. On occasion, they may need to make isolations to get pathogens into pure culture, and DNA sequencing can sometimes also be needed to firmly identify the species involved in the disease.
I encourage fruit growers to reach out to me or your local Extension Educator to assist with this process, because we will help provide follow-up information on "next steps" once the problem is diagnosed (more information below).
|A high quality, close up photo of an undiagnosed apple tree injury. Photo permission from the grower.|
(These descriptions are meant for informational purposes, and are not enough to diagnose the problem)
Black rot (Diplodia seriata): Black rot causes black, blotchy discoloration on the tree trunks and limbs. It is commonly introduced by winter injury. Black rot cannot be treated once it infects the wood, except by pruning out affected limbs.
Fire blight (Erwinia amylovora): A bacterial disease that infects apple blossoms and wounds and moves into the tree. Fire blight causes a "shepherd's crook on small twigs. Antibacterials are used to prevent infection of wounds and flowers. Once the bacterium infects a branch it must be cut out of the tree to prevent infection into the trunk which would require removing the tree. Some cultivars have resistance, such as Honeycrisp, which is not particularly susceptible to fire blight.
Botryosphaeria dothidea: This is one of the many trunk disease fungi that can cause cankers and tree decline. It gradually kills the tree by girdling the wood and preventing water, carbon, and nutrient transport. Diagnosis requires cutting into the interior wood with a plug or a cross section, and conducting culturing and DNA analysis to identify the pathogen.
Winter injury: Trunk injuries are a symptom of winter damage, and they can be an avenue for subsequent disease development. This occurs frequently with quick periods of freezing temperatures in the late fall, before the trees have hardened off for the winter. Once winter injury is present on tree trunks, pathogens can infect by entering the injured tissues. For example, black rot, mentioned above, could get its start through a winter injury canker.
Cankers directly under a branch may be due to winter injury, as that area is highly sensitive to cold stress in the fall, winter, or spring.
The winter injury that promoted canker development may have occurred several years before the grower notices it, as it grows and becomes infected over time. Therefore it is helpful to know the first time the canker was noticed, but also to keep in mind that if the damage could have originated from a past year.
Herbicide injury: Over the last decade, fruit researchers have discovered that application of glyphosate on or near the trunks of apple trees can contribute to apple trunk injury and disease cankers. Glyphosate application to weeds around the trunk can drift onto the trunk, and frequent contact with the trunk can cause cracking and wounds on the bark. Those wounds can let in disease, and is thought to have led to outbreaks of Botryosphaeria infection in the Northeast.
Cornell University fruit tree researchers also observed that glyphosate that drifts onto lower leaves can make those limbs more susceptible to winter injury, even if the leaves show no visible symptoms. Based on informal on-farm observations, researchers are also examining whether paraquat may impact trunk damage, but this has not been confirmed. Source: Cornell Scaffolds newsletter, March 30, 2020.
For citations and more information on this, read Effects of Glyphosate on Apple Tree Health, a publication from Cornell University.
The complex nature of trunk cankers necessitates a combination of:
|Photo: Annie Klodd, University of Minnesota.|
In general, the next steps after diagnosing the cause of a trunk canker depend on the diagnosis. Please either contact me directly ([email protected]), or contact your local Extension educator, to work through the next steps based on the plant disease clinic report.
If the canker contains black rot or another fungal disease, there are no fungicides to stop the canker from progressing. Trunk diseases, once they enter the wood, are not impacted by fungicides.
If we feel that herbicide application may have led to canker development, we can re-visit your weed management program or discuss strategies to prevent drift from neighboring fields.
If the canker is consuming a large part of the trunk and impacting the health of the tree, the tree will either need to be removed or allowed to live out its remaining years as it declines in health. The grower can also attempt to regrow the tree from the base by cutting off the entire trunk well below the canker and re-growing the tree from a water sprout that comes up from the cut stump. The formation of water sprouts is not guaranteed, however, and it is usually more feasible to plant a new tree. This decision depends on the scale of the orchard and goals of the grower.
Author: Annie Klodd, University of Minnesota Extension Educator - Fruit Production.
Reviewed by: Dr. Bob Blanchette and Dr. Brett Arenz, UMN Department of Plant Pathology
The Great Lakes Fruit Workers Group contributed research-based information to inform this article.
Cut back neighboring plants with pruning shears if they create shaded conditions for Italian cypresses, as these plants thrive in full sun exposure.
Incorporate organic material, such as compost, into the top layers of soil if drainage is a problem poor drainage is a significant factor in the development of serious Italian cypress diseases.
Lay a 2- to 3-inch layer of organic mulch, such as pine straw, onto the soil surrounding the Italian cypress without pressing it against the trunk mulch conserves moisture, deters weed growth and assists in maintaining vigorous trees.
Managing the disease depends on maintaining vigorous, healthy trees, preventing the factors that lead to stress, and allowing the tree’s natural resistance mechanisms to inhibit the pathogen. If a tree succumbs to stress, take measures to reverse the condition before the pathogen can invade. Remedial action should maximize the regeneration of the root system and allow the tree to cope with the subsequent strain.
Vertical Mulching. An aggressive way to improve the soil environment and stimulate feeder roots is through vertical mulching. In addition to fertilizing and root zone aeration, vertical mulching increases gaseous exchange in the root system. It also lessens damage from excessive water, provides necessary aeration during wet periods, allows water penetration during drought, and promotes the formation of fine feeder roots. With vertical mulching, a porous matter such as pea gravel, sand, or a mixture of compost with pea gravel, rice hulls, or sand is added to holes drilled throughout the root zone of the tree (Fig. 7). The holes should be 18 to 24 inches deep and a few inches in diameter.
Remedial Pruning. If 15 percent or less of the canopy is affected, use proper pruning practices to remove all dead branches. If specific branches are infected, carefully remove those branches, making sure to remove all of the infected tissue. Sanitation pruning consists of cutting an infected branch 8 to 12 inches below the visible injury or canker. To avoid spreading pathogen during pruning, sanitize the pruning tool before each cut with a 10 percent solution of bleach (one part bleach to nine parts water). To prevent rust, dry and oil tools after use.
Tree removal. If more than 15 percent of the canopy is infected, consider cutting the tree down. B. atropunctatum causes a white rot of the wood and trees killed by it may quickly become a hazard. Since the fungus is already present throughout a stand, destroying the wood to prevent further infections is questionable. Avoid storing diseased wood in the immediate vicinity of remaining trees.
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There are times when you can save a tree that has canker in its trunk - that is, when it just has a small spot of canker. We successfully treated an apricot tree in Ben Nobleman Park Community Orchard by carefully scraping the canker away from the infected spot, using a clean boxcutter as our knife. Our tree recovered and is thriving today.
In the case of this apricot tree in Ben Nobleman Park Community Orchard, we were able to carefully scrape out the damaged tissue from the trunk with a clean boxcutter, right down to the deadwood in the tree. We ensured all of the diseased tissue fell onto a sheet and not onto the soil, so we could remove the pathogen from the site. The tree recovered perfectly and over the years, the wound healed.
This year, canker has reared its ugly head again elsewhere - in our park's two oldest and most beautiful cherry trees. It has hit a number of the tree's thicker limbs.
When it comes to pruning off diseased branches on fruit trees, timing is important. If we were to prune off those thick branches in the autumn, the wounds would not have time to heal before the winter.
Luckily, as trees go dormant in the winter, so do fruit tree pests and diseases. So winter is an excellent time to prune off diseased branches. To learn more about when to prune fruit trees click here.
After pruning off the diseased branches, we may also treat our trees with copper spray to help prevent the spread of the disease. You can learn all about how to prune your fruit trees and protect them from pests and diseases in my premium online fruit tree care courses.
And the moral of the story for organic fruit tree growers? It's essential to know how to recognize fruit tree diseases so you can treat them early on. And the best time to learn more is while your tree is still young and healthy!