Leaf spot of Alfalfa

Appearance: Leaf spot diseases are first seen on the lower leaves and then develop or move up the plant. Common leaf spot produces small, circular (1-2 mm (1/16 in.)) leaf spots that are brown to black. These lesions rarely join together to form larger lesions. Lesions on the upper leaf surface often have a raised centre. Within these raised centres, the black fruiting bodies (bumps) are easily seen with a hand lens. To be sure, put some infected leaves into a plastic bag with wet paper towels, to help speed the production of these fruiting bodies. Infected leaves become yellow (chlorotic) and drop prematurely.
Disease Cycle: Cool, wet weather favours leaf spot development, so it is found primarily in the early cuttings (spring and early summer) and regrowth (fall). These fungi survive in infected leaves and on dead leaves found on the soil surface. Spores produced on living and dead leaves are spread through the air, where they infect new growth. Young leaves are the most susceptible to leaf spot diseases.