It is not known to be seedborne. As indicated, disease attacks are most damaging on susceptible varieties during prolonged periods of cool, wet weather in spring, early summer, and autumn. The incidence of disease decreases during dry, warm summers. Spring black stem and leaf spot. All above-ground parts of the plant are attacked by the fungus Phoma medicaginis var.
Numerous spots develop on the lower leaves, petioles, and stems in early spring.
These spots are small, dark brown to black, and irregular. 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.