Canker Diseases

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.

Canker diseases cause losses of about 20 percent during the first season in plantations established with unrooted cottonwood cuttings. Cankers are most severe on poor sites and under conditions of environmental stress.

Septoria musiva Peck. is considered the pioneer organism. Fusarium solani (Mart) Snyder & Hans., Cytospora chrysosperma Fr., Phomopsis macrospora Kobayshi & Chiba, and Botryodiplodia theobromae Pat. usually invade the small Septoria cankers. Although these fungi are secondary in the ecological succession, they singly or collectively cause mortality.

Septoria stem canker on young cottonwood. Photo by USDA Forest Service Archives, USDA Forest Service

The four fungi are indigenous to most areas and can infect nursery stock by means of wind-borne spores. They overwinter as mycelia or spores on cottonwood cuttings stored for spring planting. No fruiting structures are associated with F. solani.

The other three fungi produce inconspicuous, tiny, flask-shaped fruiting structures, which protrude through the epidermis and produce numerous spores. When these infected cuttings are planted, they may leafout and appear healthy; but many die before developing a root system. Mortality is increased by enviornmental conditions that limit plant growth or cause plant stress.
Phomopsis macrospora producing tiny, yellow strings of spores.	Cytospora canker girlding 3-year-old cottonwood.

Inoculum enters older trees through wounds or insect borings. Cankers on stems of 2- and 3-year-old trees are easily detected. They usually develop in fall and by spring are 3 to 6 inches (7.5 to 15 cm) in diameter. On vigorous trees, cankers are arrested during the growing season and usually callus over. On slow growing trees, cankers can girdle the stem, and leaves above the cankers will wilt but do not drop until late summer. The stem below the cankers may remain green until fall when it dies back to the root crown. Often such trees develop root sprouts the following spring.

On trees older than 4 years, cankers usually develop in the crown. Yeast, bacteria, and other microorganisms quickly invade the sap flow from the cankers and cause a fermentation. Foresters refer to the unsightly wounds as “crud cankers.” Generally cankers do not girdle the tree, but wind breakage occurs at the wound.

Fusarium canker on 4-year-old cottonwood. Photo by USDA Forest Service Archives, USDA Forest Service

Extreme care should be taken to select only healthy, canker-free cuttings for planting. Proper storage and handling of cuttings will minimize losses. Cuttings should not be allowed to dry out and should be stored at 34 to 40º F until time of planting. Cultivation of l-year-old plants reduces competition for moisture and nutrition by weeds and thus reduces losses from cankers. Preliminary results show that improved clones may have resistance to Septoria, the pioneer organism in the canker complex.