The fungus enters through wounds caused by winter injury, pruning cuts, insect punctures or mechanical injuries. The infected bark tissue exudes gum. The tissue dries out and breaks away as it is killed. The fungus overwinters in live bark at the edge of a canker. Cankered areas may or may not be surrounded by a roll of callus .
Cankers enlarge yearly until the limb or trunk is completely girdled.
Gum production is almost always associated with cankers. The fungus becomes active in the spring as the tree becomes active. However, as tree growth slows, the fungus invasion may move beyond the callus barrier. Spread of Cytospora Canker Small pimple-sized fruiting bodies are produced in the injured tissue under the bark and these produce spores which exude in droplets or in tendrils. Studies show spores are carried primarily by splashing rain, pruning tools, or insects.