The disease is primarily a problem on the first cutting and 
in fall-seeded stands. The causal organism is Peronospora 
trifoliorum de Bary. The fungus has been divided into 
pathogenic races based on cultivar reaction. It is an 
obligate parasite that is pathogenic on Medicago spp. The 
fungus overwinters as mycelium in systemically infected 
plants, residing in crown buds, the cortex of crown 
branches and in surviving shoots. Researches also suspect 
that oospores found in crop residue may serve as 
overwintering structures. The infection cycle is initiated 
in the spring when conidia are formed from the mycelium in 
infected shoots and systemically infected spring growth. 
The spores are produced in the dark when the humidity is 
near 100%. Spores are wind blown or water splashed to 
susceptible young leaf tissue. Older tissue is resistant to 
infection. Spores germinate within in a temperature range 
from 4oC to 29oC, with 18oC being optimum. Free moisture 
must be present for germination to occur. Germ tubes form 
an appressorium and commonly invade the host directly 
through the epidermis. The fungus may also penetrate 
through stomata. Conidia are formed on newly infected 
tissue with infection cycles occurring every 5 days under 
favorable conditions. Oospores may also be produced in 
infected tissue.