Cottonwood Leaf Drop: Aphids, Borers, or Rust?

Your 10–12 year old cottonless cottonwood is dropping yellow leaves in clumps with a dark discharge—well before August. Three likely causes:

1. Aphids or Scale (Honeydew + Sooty Mold)

• Sticky “honeydew” on leaves/ground; black sooty mold grows on it.
• Leaves curl, yellow, drop.
• Treatment: Soil-applied systemic imidacloprid (Bayer Advanced Tree & Shrub Insect Control). Roots carry it to leaves; insects die when feeding. Takes 2–3 weeks to reach canopy.

2. Borer Damage (Trunk Wet Spots)

• Dark, wet, oozing areas on trunk; sawdust-like frass.
• Tree “bleeds” to flush out borers.
• Treatment: Trunk spray with permethrin (Onyx) or soil-applied imidacloprid. Target active borer entry points.

3. Cottonwood Leaf Rust (Melampsora spp.)

• Yellow/orange pustules on leaf undersides in midsummer; later dark brown pustules.
• Requires alternate host (larch, other conifers).
• Management: Rake and destroy all fallen leaves to reduce spores. No effective fungicide for large trees. Some cultivars resistant.

How to Diagnose

• Sticky residue + black mold → aphids/scale.
• Trunk ooze + frass → borers.
• Orange pustules on leaf undersides → rust.

Immediate Steps

1. Inspect leaves and trunk closely.
2. Apply soil systemic imidacloprid now (covers aphids, scale, borers).
3. Rake fallen leaves daily.
4. Water deeply weekly if dry.

When to Call a Pro

If borer damage is extensive (many trunk wounds, canopy dieback), or if diagnosis is uncertain, hire an ISA Certified Arborist.

Bottom Line

Early leaf drop in cottonwood is usually insect-related. A soil systemic insecticide addresses the two most common culprits (aphids/scale and borers) simultaneously. Rust is managed by sanitation.