Cankers are formed by the interaction between a host and pathogen. The pathogen grows within the wood and the host tree tries to contain the growth. Cankers can take months (or years) to enlarge enough to girdle twigs, branches, or trunks. These areas of trunk and/or branch tissues that have been killed by diseases, insects or non-living agents, (such as fire), become cankers, posing the same danger as a bark or wood crack. Cankers weaken the tree trunk or branch because the solid cylinder of sapwood strength has been broken. Decay is often associated with cankers. Cankers cause failures more often in younger tree trunks, as younger trunks are more supple and flexible. If there is a weak spot due to a canker, the tree trunk or branch is most likely to break at that point.

Cankers are usually oval to elongate but can vary considerably in size and shape. Typically, they appear as localized, sunken, slightly discolored, brown-to-reddish lesions on the bark of trunks and branches, or as injured areas on smaller twigs. The bark often splits between the diseased and the healthy tissue, and sometimes it may ooze sap or moisture. The inner bark turns black and sometimes gives off a foul odor. The newest leaves are usually the first to show decline symptoms. Leaves may appear smaller than normal, pale green to yellow or brown, often curled and sparse. As the fungal pathogen invades bark and sapwood, the vascular system becomes blocked or dies, causing wilting and dieback to occur.