These are quite distinct and once you are familiar with them they can easily be distinguished from those of other leaf spot diseases. Lesions on the stem are long and range from a reddish brown to a chocolate brown. Sometimes infection of the stems is severe enough to kill the stem. In the central Great Plains, summer black stem rarely causes losses to the first cutting but can be a problem in the second and third cuttings. When the humidity approaches 100 percent in the plant canopy in July and August, plants become infected by spores produced on the previous crop residue.
Secondary spread occurs within the plant canopy on infected leaves and stems.
As with most leaf and stem diseases of alfalfa, losses are greatest if harvest is delayed until full bloom. Summer black stem and leaf spot can develop rapidly the final week before harvest. This disease is usually not severe enough by itself in Nebraska to cause economic loss. However, in the presence of other leaf diseases, the combined severity may cause substantial loss of forage. Summer black stem has not been identified in Wyoming.