Cankers are infections of shoots and branches that start as round to oval discolored areas where the bark has been killed. As the infection progresses cankers girdle the stem resulting in wilt and death of any leaves above the canker. Branches infected with canker-causing fungi often have discolored bark that may be cracked or oozing gums or dark-colored sap. Spore-producing structures often emerge along the margins or on the face of the cankered area. Examples of diseases that result in both leaf spots and cankers in Minnesota include Septoria leaf spot and canker of poplar and Septoria leaf spot and canker of dogwood.
Information source: https://extension.umn.edu
A smooth patch is a condition in which the rough outer layer of bark is decomposed or sloughs off in response to colonization by a fungus. Trees with smooth patch have areas of smooth, light-colored bark that are sunken in compared to surrounding bark.
Patches may form clear oval to circular depressions in the bark or may grow together resulting in larger irregular areas of smooth bark along the trunk and branches. In Minnesota, a smooth patch is commonly found on white oak trees, but maple, ash, willow, linden, and other trees may be affected as well. The fungi Aleurodiscus spp. and Dendrothele spp. cause smooth patch. These fungi do not invade the living portion of the tree and therefore are not harmful to the health of the tree.
Information source: https://extension.umn.edu