Tree cankers: ID and tree-level disease risk

A canker is a sunken, dead area in bark or cambium caused by a fungal or bacterial pathogen. Cankers vary enormously in severity -- from cosmetic blemishes on otherwise healthy trees to girdling infections that kill large branches or entire trunks within a season. The key variable is not the.

—- title: "Tree cankers: ID and tree-level disease risk" slug: how-to-identify-tree-cankers hub: problems category: "Identification guide" description: "Identify tree cankers by sunken, discolored bark with clear margins. Learn which canker diseases are fatal and which trees can compartmentalize and survive." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

A canker is a sunken, dead area in bark or cambium caused by a fungal or bacterial pathogen. Cankers vary enormously in severity — from cosmetic blemishes on otherwise healthy trees to girdling infections that kill large branches or entire trunks within a season. The key variable is not the disease itself but the tree's ability to compartmentalize the infection and wall it off from healthy tissue.

What a canker looks like

Per Penn State Extension, cankers share these features:

• Sunken or flattened area of bark: The cankered bark is depressed relative to surrounding healthy bark
• Discolored bark: Brown, dark, or orange-brown rather than the normal gray or brown of healthy bark
• Distinct margins: A clear boundary between living and dead tissue (though this boundary may be obscured in severe infections or weak-willed compartmentalizers)
• Cracked or loosening bark: Over time, dead bark cracks and peels; underneath, the wood may be stained brown or black

Per NC State Extension, the most reliable canker confirmation: cut away a small portion of bark at the boundary and look at the cambium. Living cambium is green; dead cambium is brown. If brown extends into the wood, the cambium is dead at that point.

Cytospora canker (Cytospora spp.)

One of the most common canker diseases of landscape trees in the northeastern and midwestern United States. Per Penn State Extension, Cytospora canker attacks stressed woody plants — particularly Norway spruce, blue spruce, Colorado spruce, and deciduous trees including fruit trees, aspen, and willow.

Signs:

• Sunken, dried-out sections of bark, usually beginning on lower branches and progressing upward
• Orange, red, or pink spore tendrils (called cirri) extruding from bark during wet weather — per Penn State Extension, these colored spore masses are diagnostic for Cytospora
• Resin soaking from the canker area in conifers
• Blue spruce: dead lower branches progressing upward over multiple years

Prognosis: Per Penn State Extension, there is no fungicide cure for established Cytospora infections. Management is limited to removing infected branches (pruned 6 inches below the canker into healthy wood), reducing stress, and replacing heavily infected conifers.

Botryosphaeria canker (Botryosphaeria and related genera)

Per NC State Extension, Botryosphaeria canker affects a wide range of woody ornamentals including oaks, pecans, apples, dogwoods, serviceberry, and redbud. It is an opportunistic pathogen — attacks stressed or wounded trees most severely.

Signs:

• Elongated, sunken cankers on branches and stems
• Black pycnidia (small, hard fruiting bodies) visible as tiny black dots on the dead bark surface
• Dieback of individual branches while the rest of the tree remains healthy

Management: Prune infected branches below the canker in dry weather. Per NC State Extension, avoid pruning during wet conditions that favor spore dispersal.

Nectria canker (Nectria galligena and N. cinnabarina)

Per Cornell Cooperative Extension, Nectria canker is one of the most common cankers on apple, pear, maple, beech, and other hardwoods. Two species:

• Nectria galligena (target canker): Creates a distinctive "target" or concentric ring pattern on bark as the tree lays down callus tissue each year and the pathogen repeatedly breaches it. The annual rings of callus and bark death create a visual bulls-eye. Per Cornell Cooperative Extension, this target pattern is diagnostic for Nectria galligena.
• Nectria cinnabarina (coral spot canker): Produces bright orange to salmon-pink cushion-like fruiting bodies (sporodochia) on infected bark. Per Penn State Extension, the coral-colored spots are diagnostic.

Butternut canker (Ophiognomonia clavigignenti-juglandacearum)

Per USDA Forest Service, butternut canker has killed an estimated 75–91% of butternuts across much of the species' range since its discovery in the late 1960s. It causes elongated, sunken, black cankers on branches and trunk. No effective treatment exists. The disease has been noted in the invasive section because of its severity.

Bacterial canker (Pseudomonas and Xanthomonas)

Bacterial cankers are less common on woody plants but significant on stone fruits (cherry, plum, peach, apricot). Per Penn State Extension, bacterial canker on cherry and peach causes:

• Sunken, water-soaked cankers on branches in spring
• Amber, watery gum oozing from cankers (bacterial gummosis)
• Branch death above the canker

Caution: Gummosis (sap oozing) on stone fruit can also be caused by Cytospora (a fungal pathogen), environmental stress, or borers. Per Penn State Extension, laboratory diagnosis distinguishes bacterial from fungal canker when the cause is not clear.

Canker comparison table

Management principles

Per Penn State Extension:

1. Prune infected branches at least 6–8 inches below the visible canker margin, into healthy green wood. Disinfect tools between cuts with 10% bleach or 70% alcohol.
2. Prune in dry weather to avoid splash-spread of spores.
3. Reduce stress: Most canker pathogens are opportunistic — they infect stressed trees most readily. Adequate irrigation during drought, proper mulching, and avoiding injury reduces risk.
4. Remove severely infected trees when cankers girdle the main trunk — recovery is not possible.
5. Fungicides: Not effective for established canker diseases in most cases. Preventive copper sprays are used for bacterial canker on stone fruits.

Recommended gear: Best dogwood cultivars (Cornus) — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.

Frequently asked questions

There is oozing sap at the base of my cherry tree. Is this canker? Possibly. Per Penn State Extension, amber gum at the base or on branches of cherry or peach can indicate bacterial canker, Cytospora canker, or peach tree borer damage. Examine the area around the gum for sunken, dark, discolored bark (canker) vs. D-shaped exit holes (borer). If bark is sunken and discolored, canker is likely.

How do I tell a canker from a gall? Per Penn State Extension, galls are abnormal growths of plant tissue — they are raised, not sunken. Cankers are dead, sunken areas. A gall feels solid because it is living plant tissue; a canker has dead, dried bark over dead wood.

Can a tree with a trunk canker survive? Per Penn State Extension, trees can survive trunk cankers that do not encircle (girdle) the trunk — if the canker occupies less than about 25–30% of the trunk circumference, the tree can often compartmentalize it. If the dead zone encircles the trunk, the tree cannot transport water and nutrients above the canker and will die from that point upward.

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Sources:

1. Penn State Extension — Canker diseases of trees
2. Penn State Extension — Cytospora canker
3. Cornell Cooperative Extension — Nectria canker
4. NC State Extension — Botryosphaeria canker
5. USDA Forest Service — Butternut canker
6. Penn State Extension — Bacterial canker