Plum Tree

(Prunus spp. — flowering and fruiting plums)

Plum trees are attractive ornamentals and fruit trees, but they can be prone to black knot, canker, borers, aphids, leaf spot, and environmental stress in Colorado conditions.

Seasonal Pest & Disease Calendar

Problem Type Active Season
Black knot Fungus Visible year-round; spreads Spring
Cytospora canker Fungus Spring–Fall
Peach twig borer / borers Insect Spring–Summer
Aphids Insect Spring–Fall
Shot hole disease Fungus/Bacteria Spring–Summer
Brown rot Fungus Spring–Summer
Environmental stress Abiotic Summer–Fall

Pest & Disease Guide (Homeowner-Friendly)

Black Knot

Common symptoms
  • Black branch swellings
  • Dieback
Treatment & management
  • Prune below infected tissue
  • Destroy cuttings

Cytospora Canker

Common symptoms
  • Gumming
  • Sunken bark
Treatment & management
  • Prune affected limbs
  • Reduce stress

Borers

Common symptoms
  • Flagging shoots
  • Exit holes
Treatment & management
  • Prune damaged shoots
  • Timed treatment if severe

Aphids

Common symptoms
  • Leaf curl
  • Honeydew
Treatment & management
  • Rinse foliage
  • Encourage predators

Shot Hole Disease

Common symptoms
  • Holes in leaves
  • Brown spotting
Treatment & management
  • Clean up leaves
  • Avoid overhead irrigation

Brown Rot

Common symptoms
  • Fruit rot
  • Blossom blight
Treatment & management
  • Remove infected fruit
  • Improve airflow

Environmental Stress

Common symptoms
  • Scorch
  • Poor fruiting
Treatment & management
  • Deep watering
  • Mulch base

When to Worry vs. When It’s Cosmetic

When to Worry (action recommended)

  • Black knot spreading rapidly
  • Heavy canker damage
  • Repeated fruit loss

These can shorten tree life and reduce performance.

Mostly Cosmetic (monitor only)

  • Minor spotting
  • Light aphids
  • Some summer scorch

These often improve with pruning and proper irrigation.