Walnut

(Juglans spp. — Black Walnut, English Walnut, Hybrids)

Walnuts provide shade and edible nuts but face serious challenges in Colorado, especially Thousand Cankers Disease, along with insects, fungal issues, and environmental stress.

Seasonal Pest & Disease Calendar

Problem Type Active Season
Thousand cankers disease (TCD) Fungus + beetle Summer–Fall
Walnut twig beetle Insect/borer Summer
Anthracnose (leaf blight) Fungus Spring–Summer
Aphids Insect Spring–Fall
Walnut caterpillar Insect Summer–Fall
Scale insects Insect Spring crawlers; visible year-round
Environmental stress Abiotic Summer–Fall
Winter injury Weather Winter–Spring

Pest & Disease Guide (Homeowner-Friendly)

Thousand Cankers Disease

Common symptoms
  • Yellowing leaves
  • Thinning canopy
  • Branch dieback
Treatment & management
  • No effective cure
  • Deep watering
  • Remove severely affected trees

Walnut Twig Beetle

Common symptoms
  • Tiny bark holes
  • Frass buildup
Treatment & management
  • Monitor activity
  • Same management as TCD

Anthracnose

Common symptoms
  • Leaf spotting
  • Early leaf drop
Treatment & management
  • Remove fallen leaves
  • Improve airflow

Aphids

Common symptoms
  • Sticky honeydew
  • Sooty mold
Treatment & management
  • Rinse foliage
  • Encourage beneficial insects

Walnut Caterpillar

Common symptoms
  • Defoliation
  • Larvae clusters
Treatment & management
  • Remove larvae
  • Treat severe infestations

Scale Insects

Common symptoms
  • Bumps on stems
  • Honeydew
Treatment & management
  • Dormant oil
  • Treat crawler stage

Environmental Stress

Common symptoms
  • Leaf scorch
  • Yellowing
  • Sparse canopy
Treatment & management
  • Deep watering
  • Mulch base

When to Worry vs. When It’s Cosmetic

When to Worry (action recommended)

  • Progressive thinning (possible TCD)
  • Numerous bark holes with frass
  • Heavy caterpillar damage
  • Large areas of early leaf drop
  • Heavy scale infestation

These may require professional evaluation, especially for Thousand Cankers Disease.

Mostly Cosmetic (monitor only)

  • Light leaf spotting
  • Minor aphid activity
  • Slight scorch in heat
  • Occasional yellowing

Most mild issues resolve with watering and seasonal recovery.