Cottonless Cottonwood Leaf Rust: One Tree Affected, One Healthy

Two 5-year-old cottonless cottonwoods in Denver, 15-20 ft apart. One yellowing/losing leaves July 15; other dark green. Owner asks if dying.

Diagnosis: Cottonwood Leaf Rust

Fungal disease requiring two hosts: cottonwood and a conifer (likely nearby). Yellow/orange pustules on leaf undersides in midsummer.

Why Only One Tree?

Genetic resistance varies. Some families immune. Healthy tree may be resistant cultivar.

Management

• Rake and destroy fallen leaves (reduces spores).
• Cool, damp weather favors rust.
• Confirm with CO State Forester or Denver City Forester (free).

Rust rarely kills established trees but causes premature defoliation.