Canker Disease Treatments by SavATree Insects, Mites & Diseases Submitted by Elisa Edelstein Matt Giegerich Reviewed by Expert Erik Lindberg, Assistant Director of Plant Health Care A tree canker is easy to overlook until a branch suddenly dies back or the bark on a favorite shade tree starts to crack and weep. These lesions, while unsightly, are more importantly a warning. Left unchecked, some spread through the bark, girdle branches, and in certain cases kill the tree. The good news is that many cankers are manageable when you catch them early. A canker is a localized area of dead bark and underlying tissue. Fungal or bacterial pathogens often cause many cankers, but environmental injury and other noninfectious factors can also produce them. These pathogens typically enter through wounds from pruning cuts, insect entry points, or mechanical damage, and sometimes through natural openings when a tree is already stressed. Once inside, it kills the phloem and cambium, the living tissues just under the bark that a tree relies on to move nutrients and sustain growth Whether a canker becomes more serious often depends on the tree's health. Healthy, vigorous trees are often better able to compartmentalize infection and limit its spread, while stressed trees may have a harder time containing canker development How to identify cankers on your tree Cankers aren’t always obvious early on, so look for a few things if you think your trees may be suffering from cankers. It’s best to catch cankers early, and reach out to an experienced ISA Certified Arborists if you aren’t sure and want a professional diagnosis. Sunken or discolored patches on bark The classic symptom cankers display is an oval or elongated lesion that looks sunken and appears darker or lighter than the surrounding bark, with a distinct margin between dead and healthy tree tissue. Perennial cankers may form a bull's-eye pattern showing up as successive rings of callus tissue as the tree repeatedly tries to seal off the spread. Cankers on the trunk are generally larger than those on branches and may result in more damage. Oozing, cracking, or dead tissue at wound sites Many cankers weep sap or resin from the affected area. For example, Cytospora canker on spruce trees produces a thick, bluish-white resin at the base of dying branches, while Phytophthora canker causes dark, bleeding sap near the root collar. The bark at and around the canker may shred, crack, peel, or fall away entirely. A foul odor can point to a bacterial infection or secondary decay. Branch dieback above the canker Most people notice dieback as the first visible symptom. If a single branch is dead, with wilted or brown foliage still attached, and it is surrounded by healthy branches, that may be a clue. When a canker girdles a branch and encircles it completely, everything above that point is cut off from water and nutrients and dies. Multiple dead branches scattered randomly through the canopy are a classic sign of Cytospora canker. Common types of canker disease in trees Identifying the specific type of canker matters, because it determines whether treatment is realistic or whether removal is the safer, healthier path. Cytospora canker Cytospora canker is most common on Colorado blue spruce, though it also affects other spruces, pines, poplars, and willows. Look for random dead branches scattered throughout the canopy and a thick, dry, bluish-white resin coating the branch over the canker area. It rarely kills a tree outright, but can badly disfigure a tree over time. No chemical cure exists, so the standard approach is to prune out infected branches during dry weather. Hypoxylon canker Biscogniauxia atropunctata (formerly associated with the name Hypoxylon canker) primarily affects oaks and other hardwoods, and it is often fatal. Not to be confused with Hypoxylon canker of aspen, which is caused by a different pathogen. Its hallmark is bark that peels or sloughs away to reveal a silvery-gray, powdery crust underneath, which shifts through color stages from gray to brown, and black as the disease progresses. It typically attacks trees already weakened by drought. Visible stromata often indicate advanced decline and decay, so removal is usually recommended based on the tree's structural condition and risk rather than solely to prevent spread to nearby trees. There is no cure. Nectria canker Nectria canker is common on honey locust, maple, and oak. Nectria canker is common on honey locust, maple, and oak. The classic perennial form is associated with Neonectria ditissima (formerly classified as Neonectria galligena). Related Nectria and Neonectria species can cause similar cankers and dieback, so exact species identification typically requires lab diagnosis. As a perennial canker, it creates the characteristic bull's-eye or target-like pattern of concentric callus rings. The key identifying feature is a cluster of small, bright red or orange fungal fruiting bodies at the canker margin, usually visible in fall or winter. Infected branches should be pruned well below the visible edge of the canker. Botryosphaeria canker Botryosphaeria canker is an opportunistic disease with an unusually wide host range, affecting stressed out trees and shrubs including oak, elm, apple, ash, juniper, redbud, grapevine, rhododendron, and azalea. Look for dark, sunken patches on the bark and tiny black speckles, the fungal fruiting bodies, erupting from cracks in the affected area. Sudden wilting or branch dieback is common, particularly in plants already weakened by drought or other stress. Like most cankers, it has no reliable chemical cure once established, so management centers on reducing stress through consistent watering and pruning out infected wood well below the visible margin. Phytophthora canker (crown and root collar) Phytophthora canker is caused by a water mold rather than a true fungus, so standard fungicides may not be effective. Phytophthora bleeding canker often affects the lower trunk or root-collar region, particularly where moisture remains excessive. Dark bleeding or oozing sap can be an important symptom, but several other disorders can look similar, so confirmation of the causal agent is important before treatment Improving drainage conditions and avoiding overhead irrigation are essential, and phosphonate-based fungicides may help if you catch the condition early. Life Cycle of Canker Diseases Canker pathogens follow a seasonal rhythm. While the exact timing may vary by region and pathogen, this general cycle helps guide detection and treatment strategies: Spring & Fall – Infection Season Fungal and bacterial pathogens enter through wounds caused by pruning, insects, or weather damage. Moist conditions during these seasons make trees more vulnerable. Summer – Internal Spread Once inside the tree, pathogens thrive beneath the bark. The tree may show no external symptoms yet, but this is the beginning of cankers forming. Fall & Winter – Canker Expansion As the tree slows for dormancy, infected tissues continue to die. This is when visible cankers may form, especially near wounds or where branches meet. Late Winter to Early Spring – Spore Release During wet and warming weather, spores are released and spread to nearby trees by wind, rain, or insects. Year-Round – Tree Decline Once the pathogens that cause cankers invade your tree, they slowly disrupt nutrient flow, which may weaken or kill the tree over time. What causes cankers to develop? Many common canker pathogens are opportunistic and become most damaging when trees are wounded or under stress. The stress factors may include flooding, drought, winter injury, transplant shock, soil compaction, mechanical wounds from lawn equipment, insect borers, and improper pruning. A well-maintained tree can compartmentalize an infection and move on, but a chronically stressed one usually can’t scale the same defense. The single most effective thing you can do is prevent the ongoing stress that makes infection possible. With consistent watering during drought, proper mulching, and care that avoids wounding the trunk and roots, your tree will most likely stay healthy long term. How to treat tree cankers Effective tree canker treatment depends on the canker type, its location, and how far it has progressed. Pruning and removing infected tissue For branch cankers, prune several inches below the visible infection, cutting back to healthy wood at the branch collar. Sterilize your tools between every cut with 70% isopropyl alcohol or a 1 to 10 bleach solution, and never prune in wet weather, when spores spread most easily. One important caution: do not cut into an established trunk canker, because doing so breaks the tree's containment barrier and can renew fungal activity. Dispose of all infected material and never compost it. This is where professional, proper tree pruning to remove infected wood can make a major difference. What about fungicide treatments? Homeowners often hope for a spray that will cure the problem, but the reality is more nuanced. For many canker diseases, including Cytospora, Nectria, and hypoxylon canker, no chemical cure exists once the infection is established. Chemical management is disease-specific. Most established cankers cannot be cured with fungicides, though preventive or supportive treatments may help with certain pathogens (phosphonate treatments against Phytophthora, for instance) when applied appropriately. Improving overall tree health through appropriate watering, fertilization, soil amendments, and other cultural practices may also help reduce stress and improve the tree's ability to cope with canker damage, all of which fall under supportive care. Because chemical and cultural approaches vary by pathogen, treatment should follow an accurate diagnosis rather than a generic product claim. When to call a certified arborist Any tree with multiple cankers, a canker on the main trunk near the root collar, or significant branch dieback should be evaluated by a professional. A certified arborist can determine whether the tree can be saved or whether removal is the safer choice, and can recommend the right tree disease treatment for the situation. Early intervention gives the best odds of preservation. Once a canker girdles the trunk completely, no recovery options remain. The bottom line on cankers A canker is a signal, not just a wound. It tells you a pathogen found an opening and the tree could not fully fight it off. That makes prevention through consistent, attentive care your strongest defense against any tree canker. When you do spot one, identifying the type early and acting quickly, whether that means careful pruning or a professional assessment, gives your tree the best chance of recovery. Contact us today to schedule a consultation with an expert arborist.