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. Cultivars resistant to downy mildew have been developed and are the most economical means of control. However, varietal resistance is not well documented, so choosing a resistant cultivar is difficult. Fortunately, downy mildew is rarely of economic importance, and growers can manage it with cultural practices.