Insects and Diseases of Trees in the South

by Anonymous

COTTONWOOD RUST

Rust infected cottonwood leaf.

Cottonwood rust, caused by Melampsora medusae, is probably the most important leaf disease of cottonwoods wherever they are grown. In the Lower Mississippi Valley, all sizes of eastern cottonwood trees may be infected with this rust. However, the disease is probably of most importance in cottonwood nurseries.

In mid-summer, yellow to orange pustules containing spores of the fungus form on the under surface of the cottonwood leaves. In late summer and early fall, dark brown fungal growths replace the orange structures. Cottonwood may be prematurely defoliated or even killed by successive attacks. The rust may weaken trees and subject them to attack by other disease-causing organisms. Also, there is often a reduction in growth in these normally fast growing species. This is very important since there is presently a wide interest in the use of hybrid poplars for pulp and timber production.

There is generally no accepted control for cottonwood rust. Rust-resistant varieties of hybrid and exotic cottonwoods are being developed and may provide the best control of this disease.