Cottonwood Leaf Rust: Yellow Spots and Early Leaf Drop

Rust Disease Causes Cottonwood Defoliation

Your cottonwood’s yellow-spotted leaves falling in clusters with blackened twig tips indicate cottonwood leaf rust (Melampsora spp.). Yellow/orange pustules on leaf undersides release spores in midsummer; dark brown pustules follow in fall. The fungus needs an alternate host (often a conifer like larch or pine) to complete its cycle.

Management

• Rake and destroy fallen leaves to reduce spores for next year.
• No effective fungicide for yard trees.
• Some cottonwood families are genetically resistant—explains why one tree suffers and another doesn’t.
• Tree won’t die, but repeated heavy infections can reduce growth.

See this resource for details.