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Plum Tree Diseases: Identifying Silver Leaf, Rust and Rot

Most plum problems are one of six things. How to tell them apart by what the leaves, fruit and bark show, and which ones mean the tree is lost.

By Zakaria 4 min read Save to Pinterest — opens Pinterest in a new tab
Ripe blue plums hanging in a cluster on the branch, leaves around them
Photo: Couleur, Pixabay Content License, via Pixabay
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Narrow it by what is affected first — leaves, fruit, or bark. That alone takes six possibilities down to one or two.

Identify by what is affected

Plum problems at a glance
What you seeLikely causeSerious?
Silvery sheen, one branch at a timeSilver leafUsually fatal
Sunken dark bark, amber gum, diebackBacterial cankerOften fatal
Fruit brown, rings of buff pustulesBrown rotCrop, not tree
Yellow leaf spots, orange pustules beneathPlum rustCosmetic to moderate
Fruit hollow, elongated, no stonePocket plumCrop only
Leaves curled, sticky, ants presentAphidsMinor
Small round holes in leavesShot hole — often cankerDepends on bark
Gum from a clean wound, nothing elseGummosis, stressUsually not disease

Silver leaf

  • Silvery sheen on foliage, starting on one branch and spreading over seasons
  • The definitive test: cut a branch over an inch thick. A dark purple or brown stain through the wood confirms it. No stain means false silver leaf, a stress response, which trees recover from
  • Enters through wounds, which is why winter pruning is the main route in
  • Cut affected branches back into clean stain-free wood in summer, well below the stain
  • Burn or bin the prunings. Do not compost or leave them stacked
  • Usually fatal within a few years once it reaches the trunk

Bacterial canker

  • Sunken, dark patches on bark, often oozing amber gum
  • Dieback of the branch above the patch
  • Shot hole — small round holes in leaves where infected tissue drops out
  • Worst on wet, heavy soils and on stressed trees
  • Cut well back into clean wood in summer, sterilising tools between every cut
  • Improve drainage and vigour. A tree growing strongly resists it far better
  • Canker on the main trunk generally means the tree is lost

Brown rot

  • Fruit turns brown and firm, with concentric rings of buff pustules
  • Shrivels into a mummy that stays on the tree through winter
  • Those mummies are next year’s infection, which makes winter clean-up the control
  • Enters through skin wounds from wasps, birds, hail, or fruit touching fruit
  • Thin the fruit so they do not touch, and pick up windfalls
  • Remove every mummy in winter. This matters more than any spray

Prune stone fruit in summer, never in winter

Plums, cherries, apricots and peaches are all pruned between late spring and midsummer, when the tree is in active growth and wounds seal quickly.

Winter pruning is how silver leaf and bacterial canker get in — spores are abundant in damp weather and the cut stays open for months. Observing this one rule prevents more plum disease than everything else on this page combined.

Plum rust

  • Yellow spots on the upper leaf surface, orange-brown pustules underneath
  • Late summer, often after a wet spell
  • Causes early leaf fall, which weakens the tree over several seasons but rarely kills it
  • Rake and remove fallen leaves, which carry it over
  • Improve airflow by thinning the canopy in summer

Pocket plum

  • Fruit elongates into a hollow pod with no stone, greyish and distorted
  • A fungal disease, related to peach leaf curl
  • Remove and destroy affected fruit and prune out affected shoots
  • Does not kill the tree, and a wet spring is usually behind a bad year

Keeping a plum healthy

  • Summer pruning only, and only what is needed
  • Clean, sharp tools, sterilised between trees and between cuts on a diseased tree
  • Do not leave stubs. Cut to the branch collar so the wound can seal
  • Clear fallen fruit and leaves every autumn
  • Mulch, keep it off the trunk, and water in a dry summer
  • Do not plant into wet ground — bacterial canker is far worse on poorly drained soil
  • Burn or bin diseased material. Composting it keeps the inoculum in the garden

Common questions

What are the most common plum tree diseases?

Six account for nearly everything: silver leaf, bacterial canker, brown rot, plum rust, pocket plum and aphid damage mistaken for disease. Silver leaf and bacterial canker are the two that kill trees. Brown rot and rust affect the crop and the foliage but rarely the tree's life. Identify by what is affected first — leaves, fruit, or bark and branches — because that narrows it to one or two possibilities immediately.

How do I know if my plum has silver leaf?

Leaves take on a silvery sheen, usually starting on one branch rather than the whole tree, and the definitive test is cutting a affected branch more than an inch thick: a dark purple or brown stain through the wood confirms it. Silvering alone without the stain can be false silver leaf, which is a stress symptom and recoverable. True silver leaf spreads branch by branch and is usually fatal within a few years.

What causes brown rot on plums?

Monilinia fungus, entering through wounds in the skin — usually caused by wasps, birds, hail or fruit touching fruit. Infected fruit turns brown and firm with concentric rings of buff pustules, then shrivels into a mummy that stays on the tree. Those mummies are the source of next year's infection, which is why removing them in winter matters more than any spray.

When should I prune a plum tree?

In summer, never in winter. Plums, cherries and other stone fruit are pruned between late spring and midsummer when the tree is growing actively and wounds seal fast. Winter pruning is the main way silver leaf and bacterial canker get in, because the spores are abundant in damp weather and the cut stays open for months. This single rule prevents more plum disease than anything else.

Can a plum tree recover from bacterial canker?

Sometimes, if it is caught early and limited to a branch. The signs are sunken, dark patches on bark that ooze amber gum, dieback above the patch, and small round holes in the leaves — shot hole. Cut affected branches well back into clean wood in summer, sterilise tools between cuts, and improve drainage and vigour. Canker on the main trunk usually means the tree is lost.

Is gum oozing from a plum tree always a disease?

No. Gummosis is a general stress response in stone fruit and can follow mechanical damage, drought, waterlogging or pruning, as well as bacterial canker. Clear amber gum from a clean wound is usually a reaction; gum alongside sunken dark bark, dieback and shot-holed leaves points to canker. Look at what else the tree is showing before concluding.

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