Downy Mildew on Roses Description: Downy mildew affects roses throughout the U.S. Infection begins on young plant growth. Leaves develop dark purplish to dark brown spots. Symptoms may resemble burns caused by pesticide toxicity. Humid, cool conditions favor downy mildew infection. Solution: If downy mildew has been a problem in the past, begin spraying Soap-Shield® Fungicidal Soap when new spring growth emerges. Repeat every 7-10 days for as long as needed to control disease.