The disease occurs throughout the United States and Canada. These swellings continue to enlarge through the next growing season to form a canker or knot. Branch tissue distal to newly formed knots may wilt or fail to break dormancy the next spring, and twigs may bend sharply at the knot due to the one-sided nature of the swelling. Well-developed knots are black, roughened, and spindle shaped. Although the average knot caused by Apiosporina is 4 to 12 inches in length, the swellings can enlarge yearly and grow each season closer to the base of the branch.
Knots eventually girdle affected branches, and tissue distal to the knot dies.
Occasionally, the fungus will grow into the main trunk, girdling the tree and killing it. For infection to occur, wet periods 6 or more hours in duration are necessary, and temperatures of 70 to 75 o F are optimal for disease development. Although the vast majority of infections caused by Apiosporina occur on green shoots soon after budbreak, the fungus can also invade larger branches and limbs through wounds. After penetrating host tissue, the fungus grows for several months throughout the cortex, cambium, and xylem before symptoms become evident. Eventually the branches begin to swell in response to growth-regulating chemicals produced by the fungus.