Anthracnose occurs in the spring and affects not only the leaves, but also the buds and “flowers”. The buds may fail to open or they may produce stunted flowers. These have many circular to elongated spots with light tan centers. Margins of these spots are purple to brown. Often the flowers abort before development. Foliage spots (1-2 mm. in diameter) are raised at the margins. They are purple at the edges and yellow-gray in the center. Later centers of spots may fall out causing a “shothole” effect. Dozens of spots may be present on a single leaf and may be scattered or concentrated at tip, margin, or midrib. Twisting and malformation of the leaves are common. In addition to floral and foliage spots, infected areas may also occur on petioles, stems, and fruit clusters. All three areas have spots similar to those on the foliage.
Other diseases which may be confused with anthracnose include Septoria and Ascochyta leaf spots. Septoria usually begins around July and unlike anthracnose has more angular lesions that are between the veins. Ascochyta spots may be larger (6 mm. in diameter) than anthracnose, and tissue discoloration may extend outside of their borders. Occasionally the leaves may totally blacken. This disease may occur as early as June.
Wet, humid weather at certain stages of plant development is required for infection. Homeowners may obtain effective control by removing and burning infected plant parts. Various fungicide sprays are recommended by authors of ornamental handbooks.
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.