Septoria Leaf Spot

Morris, R.C.; Filer, T.H.; Solomon, J.D.; McCracken, Francis I.; Overgaard, N.A.; Weiss, M.J. 1975. Insects and Diseases of Cottonwood. Gen. Tech. Rep. SO-8. New Orleans, LA: U.S. Dept of Agriculture, Forest Service, Southern Forest Experiment Station. 41 p.

Septoria musiva Peck causes leaf spots as well as the cankers described earlier. The disease is common throughout the United States, parts of Canada, and Argentina. It is a serious threat to nurseries because it provides entry for other disease organisms. In plantations it reduces growth by causing premature defoliation.

Septoria musiva overwinters in fruiting bodies in fallen leaves or branches. In spring, during periods of high humidity, spores are shot into the air. They infect new leaves at bud break. Leaf spots develop 1 to 2 weeks later. Spots first appear as depressed black flecks. Under favorable moisture conditions, flecks increase in size. Spots merge on leaves with multiple infections, and as much as 50 percent of the leaf tissue can be affected. As the dead tissue dries, it fades to light tan or white in the center. Three or 4 weeks after initial infection, spore-producing pycnidia appear as small, black, inconspicuous flecks in the centers of leaf spots. Spores from these pycnidia spread the infection to other cottonwoods.

Septoria leaf spots Photo by Theodor D. Leininger, USDA Forest Service

Control measures would be economically justified in nurseries but probably not in plantations. After cottonwood cuttings are harvested from nursery beds, all debris should be removed or plowed under to destroy infected plant parts and to prevent new shoots from being infected in the spring. Native poplars in or near nurseries should be removed to prevent infection by airborne spores.