Disease

Cankers

Sunken, dead patches of bark on trunks and branches that can girdle and kill the shoots above them.

What you'll see

• Sunken, darker or cracked patches of bark, sometimes with rings or oozing sap.
• Leaves wilt, yellow or die on one branch beyond the canker, often in summer heat.
• Under the bark, the wood is brown, with a sharp edge against healthy, pale tissue.
• On cherries and plums, gummy ooze and small round holes in the leaves, called shothole.

Why it happens

Fungi or bacteria enter through wounds, pruning cuts or frost cracks and kill the bark. Trees stressed by drought, a poor site or damage are hit hardest.

How to fix it

1. Prune out dead and cankered branches, cutting into healthy wood below the canker, in dry weather.
2. Clean pruners with 70% rubbing alcohol between cuts, and bin the prunings.
3. Prune cherries and plums in dry midsummer, when bacterial canker is least likely to get in.
4. Water deeply in dry spells and mulch the root zone to reduce stress.

Keep it from coming back

• Choose trees suited to your climate and site.
• Keep mowers and string trimmers away from trunks; a mulch ring helps.
• Prune dead or dying branches as soon as you see them.

Sources

• UC IPM: Canker diseases
• University of Maryland Extension: Cankers of trees and shrubs
• RHS: Bacterial canker

General guidance, not a diagnosis. For valuable, edible or outdoor crops, check with a local extension service or plant expert. Use any product exactly as its label says, and keep children and pets away while it dries.