Young, infected shoots and cluster stems may curl and are frequently covered with a white, sporulating, fungal mass. Berries on these clusters turn brown and eventually shrivel. Young berrries are highly susceptible. Cluster infections that occur later in the season cause berries to remain hard with a mottled light green to red appearance. Berries become less susceptible when mature.
Infected berries remain firm compared to healthy berries, which soften as they ripen.
Eventually, infected berries will drop. The downy mildew fungus overwinters as dormant spores in fallen leaves on the vineyard floor. The fungus first becomes active in the spring about 2-3 weeks before bloom. This fungus has two types of spores, both germinating to give rise to swimming spores. Water is necessary for the spores to swim and to infect, so outbreaks of the disease coincide with periods of wet weather.