Canker is one of those plant problems that can quietly creep in and leave a heartbreaking mess in your garden. This disease weakens trees, shrubs, and even flowers, often showing up as ugly wounds on their stems or branches. In this article, you’ll learn what canker really is, how it spreads, and what you can do to protect your plants before it’s too late. Let’s get started!
What is Canker?
In plants, a canker refers to a localized area of dead tissue (often sunken, cracked, or discolored) that appears on stems, branches, or trunks. It isn’t one disease but rather a symptom indicating infection by fungi or bacteria, and sometimes it’s triggered by environmental stress such as frost, sunscald, or pruning wounds.
Types of Canker and Pathogens
Before diving into specific examples, it helps to break cankers into clear groups so you can see which pathogens are responsible and how they behave.
Fungal Cankers
These are the most common types and affect a wide variety of trees and shrubs.
Key Pathogens & Examples
| Pathogen/Genus | Common Hosts | Symptoms & Notes |
| Cytospora spp. | Stone fruits, spruce, poplar, willow, ash | Sunken, discolored bark that may ooze resin or gum; classic on peaches and poplar. |
| Botryosphaeria spp. | Redbud, cherry, dogwood, rhododendron | Kills shoots and girdles branches; often linked to plant stress. |
| Phomopsis spp. | Juniper, Douglas-fir, arborvitae, stone fruits | Causes stem and branch dieback. |
| Nectria spp. | Honey locust, oak, maple, crabapple, pear | Distinct target-shaped cankers on many broadleaf trees. |
| Eutypa lata | Grape, stone fruits | Grape canker and dieback issues. |
| Seiridium cardinale | Cypress | Cypress canker—major cause of decline in plantations. |
Other notable fungal cankers include apple canker (Neonectria galligena on apples), pine pitch canker (Fusarium circinatum on pines), and plane anthracnose (Apiognomonia veneta on sycamores).
Bacterial Cankers
These are most noticeable on fruit and ornamental trees.
Key Pathogens & Examples
| Pathogen/Genus | Common Hosts | Symptoms & Notes |
| Pseudomonas syringae | Cherries, plums, peaches, horse chestnut | Dead bark patches and small “shothole” spots on leaves. |
| Xanthomonas citri | Citrus trees | Causes citrus canker with raised, corky lesions on leaves, twigs, and fruit. |
| Clavibacter michiganensis | Tomato, sometimes pepper and eggplant | Leads to wilting, stem cankers, and leaf scorch. |
Additional examples include ash bacterial canker (Pseudomonas savastanoi on ash) and poplar canker (Xanthomonas populi on poplars).
Oomycete-Related Cankers
Not true fungi, but these water molds can be devastating.
Key Pathogens & Examples
| Pathogen/Genus | Common Hosts | Symptoms & Notes |
| Phytophthora ramorum | Oak, tanoak, rhododendron, beech, larch | Causes sudden oak death; bleeding cankers, dieback, and widespread necrosis. |
| Phytophthora kernoviae | Beech, rhododendron | Creates trunk cankers and twig dieback. |
| Phytophthora plurivora, P. cinnamomi, P. acerina | Ash, maple, other hardwoods | Causes stem cankers and root collar necrosis. |
How Canker Spreads?
Canker spreads in several ways that often work together. Pathogens first invade through wounds or weak spots, such as pruning cuts or frost cracks. Rain splash, wind, and dirty tools then help carry spores or bacteria from plant to plant.
Weather conditions like cool, wet periods make infections worse, while old cankers or debris can harbor pathogens until they strike again. Even soil, shoes, or nursery stock can move the disease into new places, keeping the cycle going.
Symptoms & Impact of Canker on Plants and Gardens
Canker symptoms are easy to spot once you know what to look for:
- Sunken, discolored patches or cracks on bark and branches, often darker than the surrounding tissue.
- Oozing sap, resin, or gum from wounds (often called gummosis) combined with callus tissue forming around the area.
- Wilting or yellowing leaves above the infection, sometimes leading to branch dieback.
- On fruit trees, sunken lesions or rotting spots on fruit can appear alongside the bark symptoms.
Typical symptoms of apple canker (A–D) and examples of diseased shoots and buds used for pathogen isolation (E–I).
Cankers don’t just scar plants; they weaken them. Infected branches become fragile and may snap during storms. Fruit trees lose yield or produce damaged fruit, and repeated attacks lead to significant dieback or even death if the trunk is girdled. Young or heavily infected plants often stop growing, turn yellow, and fail to mature.
Beyond the damage, cankers leave trees looking scarred and unhealthy, and the lingering pathogens in dead wood or soil can threaten the rest of the garden.
Prevention and Treatment for Canker
Prevention Strategies
| Method | Description & Figures |
| Plant Health & Site Choice | Trees planted in well-drained soil have up to 40% fewer canker infections. Choosing adapted cultivars and steady care keep plants vigorous. |
| Sanitation | Removing infected wood can cut disease spread by nearly 60%. Always burn or trash debris and disinfect tools with 10% bleach or 70% alcohol. |
| Pruning Practices | Cutting 15–30 cm (6–12 in) below visible cankers during dry weather helps stop reinfection. Large cuts benefit from wound paint on susceptible trees. |
| Avoid Wounding Plants | Preventing bark injuries from tools or insects reduces infection risk by about one‑third, keeping trees stronger and healthier. |
Treatment Strategies
| Method | Description |
| Prompt Removal | Immediately prune out infected branches, cutting into healthy wood and disinfecting tools between cuts. |
| Destruction of Debris | Burn or deeply bury cankered material—never leave it on site. |
| Chemical Controls | Use copper-based fungicides or bactericides after pruning and during risky periods, always following label instructions. |
| Resistant Stock | Replace severely affected or dead plants with resistant cultivars when possible. |
Conclusion
In the end, even a small canker problem can turn into a big one if it’s left unchecked. The Planteyes app makes spotting disease early easy. Just snap a photo, and you can even chat in‑app with an expert for guidance, like talking to a plant doctor in real time. Download Planteyes today and keep your garden one step ahead of canker.
FAQs
What exactly causes canker on plants?
Cankers happen when fungi, bacteria, or even water molds (oomycetes) infect a plant through weak spots like pruning cuts, insect bites, or weather damage.
Can you cure canker once a plant has it?
There isn’t a magic fix, but quick pruning into healthy wood, tool cleaning, and boosting the plant’s overall care can keep it from spreading further.
Which plants are most vulnerable to canker?
Fruit trees such as apples, peaches, cherries, and citrus are common targets, but even oaks, sunflowers, and maples can be hit.
How can a plant identification app help with canker?
Planteyes lets you take a photo of suspicious spots and get an instant ID. You can even chat in‑app with an expert for advice, just like texting a plant doctor.
Is citrus canker dangerous to humans?
No, citrus canker only affects the trees. It won’t harm people, though it does leave fruit blemished and often unsellable.