Cytospora and Nectria Cankers on Rochester Trees: Sunken Bark, Oozing Wounds, and Branch Dieback

By Linda Marsh, Pests & Diseases. Last updated: June 26, 2026

If you have noticed a dead, slightly sunken stretch of bark on a spruce, maple, or honeylocust, often with amber resin or dark fluid bleeding from the margin, you are most likely looking at a canker. In the Rochester area, two fungal canker groups dominate: Cytospora (Leucostoma) cankers, common on Colorado blue spruce, and Nectria cankers, common on maple, honeylocust, and other hardwoods. Both are opportunists. They rarely attack a vigorous, well-watered tree, but they move fast through one that is already stressed.

What does a canker actually look like on a Rochester tree?

A canker is a dead, often sunken patch on a branch, trunk, or twig where a fungus has killed the bark and the cambium underneath. The signs to look for are consistent across species:

• Sunken or flattened bark that looks slightly collapsed compared with the healthy bark around it.
• Discoloration, often a darker reddish-brown or purplish tone, sometimes with a distinct margin where dead meets living tissue.
• Oozing, which on spruce shows up as white to amber resin and on hardwoods as dark, wet "bleeding" streaks running down the bark.
• Dieback above the canker, because the lesion girdles the branch and cuts off water and nutrients to everything beyond it.
• Tiny fruiting bodies, pinhead-sized black or orange-red bumps in the dead bark, which is how labs confirm the genus.

On blue spruce, the classic picture is lower branches browning and dying while the top still looks fine, with sticky white resin crusting the affected limbs. On maples and honeylocust, look for a sunken, sometimes target-shaped zone of cracked, callused bark centered on an old wound.

Why do cankers show up on stressed trees and not healthy ones?

Cytospora and Nectria fungi are weak pathogens, and that one detail changes how you treat them. They are almost always present in the environment, living on dead twigs and bark, but a healthy tree walls them off before they get established. The infection succeeds only when the tree's defenses are already down.

In Rochester and the Finger Lakes, the usual triggers are familiar. Summer drought stress is a big one, which is why keeping up with the kind of ongoing plant health care that keeps trees vigorous is a real disease-prevention measure, not just drought relief. Winter injury and spring sunscald crack bark and create entry wounds. Heavy clay and glacial soils that stay waterlogged choke roots and weaken the whole tree. Road salt spray along streets and driveways adds chronic stress, as does soil compaction from construction and mowers. Each of these leaves a tree too depleted to fight the fungus off.

How is canker different from wilt or root rot?

Homeowners often confuse a canker with other dieback diseases, and the distinction matters because the management is different. The key is where the damage is concentrated and what the wood looks like.

If the dieback traces back to a discrete sunken spot on the bark, think canker. If branches wilt with no surface lesion, Verticillium wilt on maples is more likely. If the trouble starts at the root collar in wet ground, a buried trunk or girdling roots and root collar problems deserve a closer look. A certified arborist can tell these apart and, when needed, send tissue to a lab such as Cornell's Plant Disease Diagnostic Clinic.

Can you cure a canker, or only manage it?

There is no spray that reverses an established canker. Fungicides do not penetrate the bark to reach the fungus living in dead tissue, so spraying a cankered spruce or maple is mostly wasted money. What actually works is a two-part approach.

First, prune out the infected wood. Cut well below the visible canker, into clean healthy tissue, during dry weather. On spruce this means removing dead lower limbs back to the trunk. Disinfect tools between cuts so you are not spreading the fungus to fresh wounds. Avoid pruning during wet spring weather, when spores are most active.

Second, and more important over the long run, restore the tree's vigor so it can wall off future infections. That means correcting the underlying stress: consistent watering through dry spells, proper mulch (a flat ring, never a volcano against the trunk), relieving soil compaction, and soil testing to fix nutrient or pH problems. This is exactly the plant-health-care philosophy that treats the tree, not just the symptom.

Which Rochester trees are most at risk?

A few species come up again and again in Monroe County. Colorado blue spruce is the headline case for Cytospora; it is widely planted here and poorly suited to our hot, sometimes droughty summers, which keeps it chronically stressed. Norway and sugar maples, honeylocust, and a range of other hardwoods are the usual Nectria hosts, especially mature specimens that have taken pruning wounds or storm damage. Old, neglected fruit trees pick up cankers too.

If you have a blue spruce losing its bottom from the inside out, or a maple with a sunken bleeding patch and a dead branch above it, it is worth a professional look before the lesion girdles a major limb. When you are unsure whether a tree has crossed from stressed into genuinely declining, that is the moment to call in a certified arborist rather than guess.

FAQ

Is canker contagious to my other trees? Cytospora and Nectria fungi can spread to nearby susceptible trees through spores in wind, rain, and contaminated pruning tools, but they only establish on already-stressed or wounded hosts. Keeping your other trees vigorous and disinfecting tools between cuts is the best protection.

Should I cut down a tree with canker? Not necessarily. A few branch cankers on an otherwise healthy tree can be pruned out and the tree kept. Removal becomes the right call when cankers have girdled the main trunk, killed a large share of the canopy, or compromised structural safety. An arborist can make that judgment.

Why does my blue spruce keep getting Cytospora? Colorado blue spruce is poorly adapted to the warm, sometimes dry summers of Upstate New York, so it lives under chronic stress that invites Cytospora. Consistent watering, good mulch, and removing dead limbs help, but heavily infected specimens often decline over time regardless.

When is the best time to prune out cankers? Prune during dry weather, ideally in late fall or winter dormancy, when fungal spores are least active and the tree is not actively pushing growth. Avoid pruning in wet spring conditions, which spread spores to fresh cuts.

Sources

• Cornell Plant Disease Diagnostic Clinic
• Cornell Cooperative Extension
• University of Minnesota Extension: Cytospora Canker
• New York State DEC: Forest Health
• Monster Tree Service of Rochester (editorial pick)