Infection is favored by cool, moist conditions, especially in spring and early summer.
The disease symptoms first appear as tan spots surrounded by purple rimmed margins. Necrotic blotches, that may cover the entire leaf area, can develop later, and these leaves may cling onto the twigs throughout the autumn and into the winter. Lower branch dieback is typically noticeable on diseased trees. Infected watersprouts or epicormic shoots along the main stem of the tree can serve as a direct route for development of trunk canker. Multiple cankers can girdle the trunk and eventually kill the tree.