Bur Oak

(Quercus macrocarpa)

Bur Oak is one of the toughest and most adaptable oaks for Colorado. It tolerates drought, alkaline soils, pollution, and harsh winters better than most other oak species. While generally resilient, it can experience issues with Kermes scale, gall wasps, two-lined chestnut borer, oak wilt (rare), and chlorosis in extreme soil conditions.

Seasonal Pest & Disease Calendar

Problem Type Active Season
Kermes scale Insect/scale Spring crawlers; visible year-round
Gall wasps (various) Insect Spring–Summer
Two-lined chestnut borer Insect/borer Summer
Oak lace bug (less common) Insect Summer
Anthracnose / leaf spot Fungus Spring–Summer
Oak wilt (rare in Colorado) Fungus Spring–Summer
Chlorosis (nutrient stress) Abiotic Spring–Summer
Environmental drought stress Abiotic Summer–Fall

Pest & Disease Guide (Homeowner-Friendly)

Kermes Scale

Common symptoms
  • Sticky honeydew & sooty mold
  • Yellowing foliage
  • Round brown scale bumps on twigs
  • Twig dieback
Treatment & management
  • Treat crawlers in late spring (pros)
  • Horticultural oil in late winter
  • Prune out heavily infested twigs

Gall Wasps

Common symptoms
  • Round, spiny, or irregular galls on leaves or stems
  • Leaves may distort or drop early
Treatment & management
  • Usually cosmetic
  • Do not treat unless extremely severe
  • Rake and remove fallen leaves
  • Promote tree vigor through watering & mulch

Two-Lined Chestnut Borer

Common symptoms
  • Thinning canopy
  • D-shaped exit holes
  • Branches die back from top downward
Treatment & management
  • Water deeply during drought
  • Reduce stress (borers attack weakened trees)
  • Preventive trunk sprays during outbreak years

Oak Lace Bug

Common symptoms
  • Speckled, stippled leaves
  • Tiny insects on undersides of leaves
  • Premature browning in late summer
Treatment & management
  • Hose-rinse leaves
  • Encourage natural predators
  • Professional treatment for severe cases

Anthracnose / Leaf Spot

Common symptoms
  • Brown patches along veins
  • Distorted or curled leaves
  • Early defoliation during wet springs
Treatment & management
  • Usually cosmetic
  • Clean up fallen leaves
  • Improve airflow; avoid overhead irrigation

Oak Wilt (Rare in CO)

Common symptoms
  • Sudden leaf wilt
  • Brown margins moving inward
  • Dieback progresses over weeks
Treatment & management
  • Immediate professional evaluation
  • Preventive fungicide injections may help
  • Avoid pruning April–July

Chlorosis (Iron Deficiency)

Common symptoms
  • Yellow leaves with green veins
  • Poor growth
  • Sparse canopy
Treatment & management
  • Soil iron treatments or trunk injections
  • Reduce overwatering & improve aeration
  • Mulch to protect feeder roots

Environmental Drought Stress

Common symptoms
  • Leaf scorch
  • Early fall color
  • Tip dieback
Treatment & management
  • Deep watering every 3–4 weeks in summer
  • Maintain mulch ring
  • Avoid compacting soil above roots

When to Worry vs. When It’s Cosmetic

When to Worry (action recommended)

  • D-shaped exit holes or top-down dieback (possible borer)
  • Heavy scale infestation causing twig death
  • Rapid leaf wilt in mid-summer (possible oak wilt; rare but serious)
  • Severe chlorosis leading to branch decline
  • Repeated early leaf drop year after year

These issues call for professional evaluation and timely intervention.

Mostly Cosmetic (monitor only)

  • Light gall formation (very common and usually harmless)
  • Mild anthracnose in wet springs
  • Minor stippling from lace bugs
  • Occasional scorch during drought
  • Slight yellowing in early summer that resolves with watering

These typically resolve with seasonal changes and proper watering.