Spring black stem and leaf spot (Phoma medicaginis) of alfalfa is commonly found in fields in the United States. Although it is not a common problem each year, it can cause severe damage when it does occur. Due to the cool, wet spring season we are having, it is showing up this year here in South Dakota.
Figure 1. Spring black stem and leaf spot symptoms on alfalfa.
Spring black stem and leaf spot affects the stems and leaves. In severe cases, it can also infect the crown and upper root of the plant. Small black to dark brown spots develop on the lower leaves, petioles and stems in the spring (Fig. 1). Young shoots can be completely girdled and killed. As the disease progresses, infected leaves turn yellow and drop prematurely.
This disease overwinters as pycnidia on the old stems and the fallen leaves from the previous season. In the spring, these pycnidia ooze spores that can be spread by water splash primarily, but wind and insects can also carry the spores. The new growth can become infected as it emerges and pushes through last year’s old growth where the pycnidia have overwintered. The cool, wet spring weather provides the perfect environment for the infection to occur. Usually the first cutting is the most damaged by this disease because the temperatures and moisture typically decrease as we get into the summer months. During the fall if there are cool, moist periods, the disease can build back up again. This will provide inoculum for the spring crop and possibly for that last cutting of alfalfa for the season.
Early cutting of the alfalfa, if spring black stem and leaf spot are found in the field, can be beneficial as it tends to reduce the amount of leaf loss. There are no resistant varieties available. Seed produced under humid conditions can carry the fungus on the seed coat, so it is important to plant only certified seed that has been produced in arid areas.