Cottonwood

(Populus deltoides – Plains Cottonwood; Populus fremontii – Fremont Cottonwood; Populus angustifolia – Narrowleaf Cottonwood)

Cottonwoods are some of the fastest-growing, most drought-tolerant shade trees in Colorado. They are extremely hardy but prone to branch breakage, borers, cankers, leaf spots, and root-related issues due to their aggressive root systems and sensitivity to soil compaction.

Seasonal Pest & Disease Calendar

Problem Type Active Season
Cottonwood borer Insect/borer Summer
Poplar twig borer Insect/borer Spring–Summer
Oystershell scale Insect/scale Spring crawlers; visible year-round
Aphids (several species) Insect Spring–Fall
Leaf spot / Marssonina Fungus Spring infection; symptoms Spring–Summer
Cytospora canker Fungus Spring–Fall
Slime flux / wetwood Bacterial Spring–Fall
Drought stress / root issues Abiotic Summer–Fall

Pest & Disease Guide (Homeowner-Friendly)

Cottonwood Borer

Common symptoms
  • Large round exit holes
  • Sawdust-like frass at trunk base
  • Branch or trunk weakness
Treatment & management
  • Keep tree healthy—borers attack stressed trees
  • Remove severely infested branches
  • Professional preventive treatment in high-pressure areas

Poplar Twig Borer

Common symptoms
  • Dieback at branch tips
  • Small holes at twig bases
  • Sparse canopy over time
Treatment & management
  • Prune out infested twigs
  • Encourage natural predators
  • Professional sprays if widespread

Oystershell Scale

Common symptoms
  • Hard, shell-like scales on bark
  • Branch dieback in heavy infestations
  • Yellowing or thinning canopy
Treatment & management
  • Treat crawler stage in late spring
  • Horticultural oil in late winter
  • Prune heavily infested branches

Aphids

Common symptoms
  • Sticky honeydew
  • Sooty mold
  • Leaf curling or distortion
Treatment & management
  • Hose-rinse foliage
  • Encourage beneficial insects
  • Systemic treatments if populations are heavy

Leaf Spot (Marssonina)

Common symptoms
  • Brown to black leaf spots
  • Early leaf drop
  • More common in wet spring weather
Treatment & management
  • Usually cosmetic
  • Rake and remove leaves
  • Improve airflow; avoid overhead irrigation

Cytospora Canker

Common symptoms
  • Sunken, discolored bark patches
  • Resin oozing
  • Branch dieback from tips inward
Treatment & management
  • Prune out infected branches back to collar
  • Sanitize tools
  • Improve tree vigor—no chemical cure

Slime Flux / Wetwood

Common symptoms
  • Oozing liquid from trunk
  • Sour or alcoholic smell
  • Discolored streaks on bark
Treatment & management
  • Mostly cosmetic
  • Do NOT seal the wound
  • Reduce stress (water deeply, mulch, avoid injury)

Drought Stress / Root Decline

Common symptoms
  • Leaf scorch
  • Early yellowing
  • Premature leaf drop
Treatment & management
  • Deep water every 2–3 weeks in summer
  • Maintain mulch ring
  • Avoid soil compaction over root zone

When to Worry vs. When It’s Cosmetic

When to Worry (action recommended)

  • Large structural limbs cracking or dropping
  • Borer exit holes or piles of frass at base of tree
  • Progressive dieback in upper canopy
  • Heavy scale covering bark
  • Cankers expanding or multiple branches dying back
  • Significant wetwood/slime flux with canopy thinning

These issues require professional arborist evaluation, especially due to cottonwoods’ massive size and potential hazard.

Mostly Cosmetic (monitor only)

  • Minor leaf spotting in early summer
  • Occasional honeydew from aphids
  • Some tip dieback from poplar borers
  • Light wetwood seepage
  • Early fall color during hot, dry years

These issues typically resolve with proper watering and seasonal recovery.