DISEASES OF ALFALFA - SPRING BLACK STEM

Spring black stem is caused by the fungus Phoma medicaginis, the same fungus that can cause crown rot in alfalfa. When the fungus infects stems and leaves, it leads to small black spots (see photo). These spots can enlarge and eventually merge to form irregular black areas that can cover large sections of the leaf or stem. Heavily infected leaves may yellow, then fall off. Pathogen and disease development is favored by cool, moist conditions in the spring (thus the name of the disease). A second wave of disease often occurs in the fall when cool, moist conditions return. This disease can lead to reductions in both yield and forage quality.

Spring black stem is somewhat difficult to manage because the pathogen survives readily in the crowns and stubble of living alfalfa plants. Spores of the pathogen are easily moved by splashing water, wind and insects. Some moderately resistant alfalfa varieties are available and many alfalfa breeding companies are attempting to develop varieties that are more resistant to the disease. When spring black stem is severe, early cutting may be the best management option available to growers in order to avoid yield loss through defoliation.