Morris, R.C.; Filer, T.H.; Solomon, J.D.; McCracken, Francis I.; Overgaard, N.A.; Weiss, M.J. Insects and Diseases of Cottonwood. New Orleans, LA. U.S. Department of Agriculture, Forest Service; Southern Forest Experiment Station; State and Private Forestry Southeastern Area. Gen. Tech. Rep. SO-8. 1975. 41 p.
Leaf rust caused Melampsora medusae Thum causes economic losses in nursery stock throughout the geographic range of cottonwood. Above latitude 40 degrees, rust may cause premature defoliation in plantations. This defoliation not only causes growth losses; it weakens the trees and predisposes them to other pathogens, which cause cankers and mortality.
Cottonwood leaves are infected in early spring by wind-borne spores produced on conifers or by spores that overwintered on cottonwoods in frostfree areas. The first evidence of the disease is small yellow spots (masses of spores) on the lower leaf surfaces. These rust spores can be blown to new leaves and cause additional infections. If humidity is high, infection may spread until the entire leaf is covered by yellow spores. Defoliation usually occurs when rust covers over 50 percent of the leaf. Late in the season, the disease is easily detectable by the vast amount of rust spores, which give the leaves a dusty yellow color.
Rust-resistant clones developed by the Southern Forest Experiment Station in Stoneville, Mississippi, are available through several State forest nurseries. Stoneville clones 75 and 92 are not defoliated by rust and should be favored in areas when summer defoliation occurs.
