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. The fungus requires water on the leaf surface for germination and infection. Infection can occur at temperatures between 40 and 85 degrees F, with peak fungal activity occurring at 65 degrees F. Only young, succulent tissue is susceptible to the fungus.
Infection occurs either by direct penetration of the leaflet surface or by entry through stomata.
Under favorable conditions, secondary infection cycles occur every five days. Spring seeding is recommended for areas where fall infections are common. Metalaxyl, a systemic fungicide, is registered as a seed treatment for alfalfa. Because downy mildew requires cool, moist conditions, it is usually only a problem on the first cutting of alfalfa. After the first cutting, environmental conditions limit the activity of this fungus, greatly reducing damage caused by the disease.