Downy mildew causes light green to yellow … Downy mildew causes light green to yellow angular spots on the upper surfaces of leaves. White fluffy growth of the fungus develops on the lower sides of these spots. With time these lesions turn brown and dry up. Older leaves are attacked first. Severely infected leaves may die. On rare occasions the pathogen can become systemic, causing dark discoloration of stem tissue.If downy mildew infects the cotyledons of young seedlings, the plants can die. Greenhouse grown lettuce transplants can also be infected. lettuce transplants can also be infected.