The spot is brighter on the upper leaf surface than on the lower leaf surface. The spots may turn brown or may remain yellow. During moist weather, the undersides of the spots are covered with a layer of fungus that can be white to purplish to almost black. The entire leaf dies quickly. Usually older leaves are affected first, then younger leaves.
Petioles remain green after the leaf blade dies.
Fruit are usually not affected, but muskmelons can be covered with the fungal growth. Prevention of Downy Mildew. Select sites with good drainage and air movement. Avoid overhead irrigation. Water early in the day to allow leaves to dry thoroughly.