Ash

(Fraxinus spp.)

Deciduous shade tree common in Colorado landscapes. Ash are highly vulnerable to borers— especially Emerald Ash Borer (EAB)—as well as several native borers, fungal leaf issues, and environmental stress.

Seasonal Pest & Disease Calendar

Problem Type Active Season
Emerald ash borer (EAB) Insect/borer Late Spring–Summer
Lilac–ash borer Insect/borer Spring–Summer (adult flight)
Banded ash borer Insect/borer Early Spring
Redheaded ash borer Insect/borer Spring–Fall
Ash bark beetle Insect/borer Late Spring–Summer
Ash anthracnose Fungus Cool, wet Spring
Ash rust Fungus Spring infection; visible late Spring–Summer
Environmental leaf scorch / decline Abiotic Summer drought

Pest & Disease Guide (Homeowner-Friendly)

Emerald Ash Borer (EAB)

Common symptoms
  • Thinning canopy
  • “D-shaped” exit holes
  • Epicormic sprouts at trunk base
  • Woodpecker activity & bark flecking
Treatment & management
  • Systemic trunk injections (every 2–3 yrs; pros only)
  • Treat BEFORE infestation symptoms for best survival
  • Remove severely infested trees—beetle spreads quickly

Lilac–Ash Borer

Common symptoms
  • Branch dieback
  • Clear-wing moth; larvae tunnel in trunk
  • Sawdust-like frass at trunk base
Treatment & management
  • Preventive trunk sprays in spring (pros)
  • Reduce stress: water deeply, avoid wounding trunk
  • Remove severely infested limbs

Banded Ash Borer

Common symptoms
  • Early spring activity
  • Exit holes similar to other borers
  • Damage often in recently stressed trees
Treatment & management
  • Improve vigor: watering, mulch
  • Preventive sprays when needed (pros)
  • Typically secondary—manage stress first

Redheaded Ash Borer

Common symptoms
  • Tunneling in stressed or newly planted trees
  • Round exit holes
  • Rapid decline under severe infestation
Treatment & management
  • Reduce stress, water deeply
  • Professional preventive trunk spray if high risk
  • Remove dying trees

Ash Bark Beetle

Common symptoms
  • Top-down dieback
  • Small pinholes in bark
  • Fine boring dust
Treatment & management
  • Watering & stress reduction is critical
  • Remove dead branches promptly
  • Preventive sprays may help in outbreak years

Ash Anthracnose

Common symptoms
  • Leaf curling & distortion
  • Brown/black blotches
  • Early leaf drop in wet springs
Treatment & management
  • Usually cosmetic
  • Rake/remove leaves
  • Improve airflow, avoid overhead irrigation

Ash Rust

Common symptoms
  • Orange/yellow swellings on leaves
  • Orange spores on leaf undersides
  • Premature leaf drop
Treatment & management
  • Mostly cosmetic in CO
  • Remove fallen leaves
  • Fungicide rarely needed

Environmental Leaf Scorch / Decline

Common symptoms
  • Browning tips and margins
  • Leaves crisp in midsummer
  • Often drought or root damage related
Treatment & management
  • Deep watering every 2–3 weeks in summer
  • Maintain 2–4" mulch ring
  • Avoid compacting soil or damaging roots

When to Worry vs. When It’s Cosmetic

When to Worry (action recommended)

  • Canopy thinning from the top down
  • D-shaped exit holes or bark splitting
  • Woodpecker damage (flecked bark)
  • Entire branches suddenly die mid-summer
  • Sprouting from the trunk is increasing

These signs suggest borer activity, especially EAB, and require immediate evaluation.

Mostly Cosmetic (monitor only)

  • Leaf spots or curling in wet springs (anthracnose)
  • Light rust spotting without branch dieback
  • Minor scorch during hot, dry periods
  • Small amount of early leaf drop in wet years

These typically resolve with improved watering and simple cleanup.