Bacterial canker on apricot — symptoms and treatment

Severity: high

What is Bacterial canker

Bacterial canker is caused by Pseudomonas syringae. It attacks bark and can kill entire branches or young trees, which is why its severity is rated high — this is not a cosmetic leaf disease but a structural one. Because the pathogen is a bacterium rather than a fungus, the usual orchard fungicides do nothing against it, and there is no spray that cures an established canker. Management is about protecting the tree from infection and cutting out what is already lost.

Symptoms

The clearest sign is amber-coloured gum on the trunk and limbs — sticky, resinous beads or runs oozing from the bark. Gumming alone is not proof, since stone fruit gums for many reasons, but gum coming from a sunken, darkened patch of bark is characteristic. Peel back the bark at the edge of such a patch and the tissue beneath is discoloured and dead rather than green. In spring, buds fail to open and blossoms wither on affected limbs, and shoots die back from the tip. A canker that girdles a branch kills everything above it.

• Early signs: amber gum beads on bark; a few buds that fail to break in spring
• Advanced signs: sunken bark cankers with gummy exudate, dead buds and blossoms, limb dieback, death of young trees
• Confusable with: brown rot blossom blight, which also kills blossoms — but brown rot produces grey spore masses on fruit and no sunken bark canker with gum

Causes and conditions

The bacterium lives on the surfaces of healthy buds, leaves and bark and turns invasive only when it can get inside. Wounds are the entry route: pruning cuts, frost injury, hail damage, leaf scars and broken twigs. Cool, wet weather is what drives infection, and rain splash and wind-driven rain move the bacteria from tree to tree. This combination is why pruning timing matters so much on stone fruit — a fresh cut made in cool wet conditions is an open wound in exactly the weather the pathogen needs. Trees stressed by waterlogging, poor drainage or nutrient problems succumb far more readily.

Treatment

Prune in dry weather — cultural

Timing: summer. Prune only in dry summer weather. Protect wounds with sealant. Avoid winter pruning on stone fruit. Cut back into healthy wood below any canker and remove the prunings from the site. This is the core of managing the disease — get the timing wrong and pruning becomes the means of infection rather than the cure.

Copper spray (dormant) — chemical

Timing: dormant season. Apply copper at leaf fall and again at bud swell. Avoid pruning in wet weather. Pre-harvest interval: 21 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Prune apricots only in dry summer weather, never during winter or in wet spells
• Seal pruning wounds and avoid any unnecessary bark injury from ties, mowers or stakes
• Plant on well-drained ground — waterlogged, stressed trees are the ones that get cankered
• Remove and destroy dead and cankered wood promptly; do not leave prunings under the tree
• Buy clean nursery stock and choose less susceptible rootstocks where available

Frequently asked questions

Can I save a tree that already has cankers? Sometimes. If the canker is on a limb, cut back into healthy wood well below it in dry weather and the tree can carry on. If it has girdled the trunk of a young tree, the tree is lost — replace it rather than nursing it.

Why is winter pruning such a problem? Winter cuts heal slowly and coincide with the cool, wet conditions that let the bacteria spread. Summer pruning in dry weather leaves wounds that seal quickly, giving the pathogen far less opportunity.

Will fungicide help? No. This is a bacterial disease, so fungicides have no effect on it. Copper is the only spray in the programme, and it works preventively at leaf fall and bud swell rather than curing existing cankers.