Summer black stem and leaf spot

Symptoms of summer black stem and leaf spot usually appear after the alfalfa has grown a dense canopy. Defoliation from the base of the stem to the top is the most obvious symptom, but leaf spots usually appear first. Leaf spots begin as brown areas with a wavy margin. As spores on the surface of the spot are produced, the color of the spot appears gray or silvery. A diffuse yellow margin often surrounds the spot. Brown lesions also form on stems. High humidity (close to 100%) and temperatures ranging from 75 to 82°F favor disease development.