Horse Chestnut

(Aesculus hippocastanum — includes hybrid forms and ornamental selections)

Horse Chestnut is known for its large palmate leaves, showy white flower candles, and distinctive conkers. In Colorado, it is highly prone to leaf blotch, leaf scorch, mildew, aphids, borers, and heat stress. It performs best in cooler, irrigated sites and struggles in hot, dry, or compacted soils.

Seasonal Pest & Disease Calendar

Problem Type Active Season
Leaf blotch (Guignardia) Fungus Spring–Summer
Leaf scorch (abiotic) Abiotic Summer–Fall
Powdery mildew Fungus Summer–Fall
Aphids Insect Spring–Fall
Japanese beetles (Front Range) Insect Summer
Scale insects Insect Spring crawlers; visible year-round
Borers Insect/borer Summer
Canker diseases Fungus Spring–Fall
Environmental heat/drought stress Abiotic Summer–Fall

Pest & Disease Guide (Homeowner-Friendly)

Leaf Blotch (Extremely Common)

Common symptoms
  • Irregular brown patches
  • Yellow halos
  • Early leaf drop (often mid-summer)
  • Lower canopy thins first
Treatment & management
  • Rake/remove leaves
  • Increase airflow
  • Fungicides rarely used

Leaf Scorch (Major Colorado Issue)

Common symptoms
  • Brown leaf edges/tips
  • Crispy leaves
  • Premature fall color
Treatment & management
  • Deep watering every 2–3 weeks
  • Maintain mulch
  • Avoid hot, west-facing exposure

Powdery Mildew

Common symptoms
  • White powdery coating
  • Distorted leaves
  • Slowed growth in shade or crowding
Treatment & management
  • Improve airflow
  • Water at soil level
  • Fungicide if severe

Aphids

Common symptoms
  • Sticky honeydew
  • Sooty mold
  • Leaf curling
Treatment & management
  • Hose-rinse foliage
  • Encourage beneficial insects
  • Systemics for heavy infestations

Japanese Beetles

Common symptoms
  • Skeletonized leaves
  • Clusters of feeding beetles
  • Damage peaks midsummer
Treatment & management
  • Hand-pick early morning
  • Professional control if widespread
  • Avoid planting attractant species nearby

Scale

Common symptoms
  • Hard bumps on twigs
  • Honeydew & mold
  • Reduced vigor
Treatment & management
  • Dormant oil in late winter
  • Treat crawler stage in late spring

Borers

Common symptoms
  • Exit holes
  • Sawdust-like frass
  • Branch dieback
Treatment & management
  • Avoid trunk injury
  • Deep watering
  • Professional sprays if needed

Canker Diseases

Common symptoms
  • Sunken bark patches
  • Dead limbs
  • Often follows stress
Treatment & management
  • Prune in late winter
  • Sanitize tools
  • Improve vigor

When to Worry vs. When It’s Cosmetic

When to Worry (action recommended)

  • Severe defoliation by early summer
  • Persistent leaf scorch despite watering
  • Heavy Japanese beetle damage
  • Multiple cankers or branch dieback
  • Scale infestation with mold
  • Borers with canopy decline

Horse Chestnut is sensitive in Colorado, especially in hot, dry environments.

Mostly Cosmetic (monitor only)

  • Mild blotch or mildew
  • Light aphid activity
  • Early fall color during drought
  • Minor tip burn
  • Occasional beetle feeding

These are common seasonal issues and typically not harmful long-term.