Downy mildew of alfalfa is a fungal disease caused by Peronospora trifoliorum. In New Mexico this disease can be troublesome on non-dormant alfalfa when environmental conditions favor disease development. Infected seedlings are weakened and become susceptible to winterkill. The disease begins as chlorotic spots on the leaflets. As the disease progresses, these chlorotic areas may enlarge and eventually cover the entire leaflet.
Infected leaflets become twisted and the margins curl down.
The tell-tale sign of downy mildew is the production of a white to grayish downy growth on the underside of infected leaflets. Downy mildew occurs during cool, moist periods in the spring or fall. The fungus sporulates at night with high humidity. The resulting spores are short-lived, surviving only a few hours to a few days depending on environmental conditions. Spread of the fungus is usually dependant on wind or splashing rain.