[Illustration: 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.