In the thickened center of fully developed spots, a tiny raised light brown disk-shaped fungus fruiting body forms on the upper leaf surface. This fruiting body is easily visible with a hand lens or reading glass. Large numbers of microscopic spores are shot into the air during cool to warm wet weather and are carried by wind currents and rainsplash to other plants. Thus, the disease may spread quickly throughout a field. When the spots are numerous, the leaflets soon turn yellow and fall off.
Premature defoliation reduces the quality and quantity of forage.
The causal fungus overwinters in fallen, undecayed leaves and leaf fragments on the soil surface. 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.