Young shoots are often girdled and killed. Leaf lesions may enlarge and merge, killing large areas of the leaflets. The leaves turn yellow and often wither before dropping off. Stem and petiole lesions may enlarge, girdle, and blacken large areas near the base of the plant. The fungus may extend into the crown and upper root, causing a crown and root rot.
Affected stems are brittle and easily broken.
When the disease is severe, entire stems are blackened and killed. Large numbers of microscopic spores formed within the pycnidia, ooze out in cool, wet weather and are spread primarily by splashing water, but also by wind and insects. In cool, humid areas, seed pods may become infected and the fungus becomes seedborne as mycelium in the seed coat. Infection of new alfalfa shoots occurs as the shoots grow through the residue or stubble of a previous alfalfa crop. The first harvest is usually damaged the most.