It can be found anytime during the growing season but is most common in spring. During cool moist weather or when humidity is high, a fine grayish growth of spores is usually apparent on the underside of leaflets. The upper side of infected leaves is light green to yellow. Symptoms are usually restricted to portions of leaflets, but, if the infection is systemic, they may appear on entire leaves or sheets. Infected leaves are twisted and curled.
Plant growth is stunted.
Pernospora trifoliorum overwinters as resting spores in the crown of surviving plants and in plant debris. They are fragile and survive for several hours to a few days, depending on environmental conditions. Dispersal is primarily by wind and splashing rain. Spores fall on young, susceptible leaves and germinate in free water. The fungus produces large numbers of spores during periods of abundant moisture.