Alternaria Leaf and Stem Blight

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.

Leaf and stem blight caused by Alternuria tenuis Nees occurs in the United States, Canada, and Mexico. It was first reported on cottonwood in North Dakota in 1918. It now appears to be an important disease in nurseries and plantations. Losses among unrooted, green-tip cuttings in mist beds can be as high as 65 to 95 percent.

The fungus overwinters as mycelia on plant debris. In spring wind-borne spores are carried to new leaves; they germinate within 1 to 2 hours when relative humidity is 100 percent and temperature is between 40 and 95 degress F. The spore can penetrate epidermal tissue of young leaves and stems, but it usually enters through leaf margins and insect wounds, turning tissue brown. Within 5 to 7 days, mycelia have formed, and new spores are being produced. The infested area now appears as a sooty, irregularly shaped blotch.

Alternaria leaf blight.

Rotation and sanitation of nursery planting beds will reduce incidence of this disease by eliminating fungus which overwinters on plant debris. Early spring cultivation of stool beds to turn-under plant debris will greatly reduce subsequent infections.