FATAL-D is a Potassium Phosphonite combination with some plant extracts and very effective on Downy-Mildew and other Fungus. FATAL-D is very rich in phosphorus, and it fulfills plants Phosphorus need. FATAL-D is also very effective on Root Fungus. Downy mildew is cause by parasitic organisms that are more closely related to algae. Because it is closely related to algae, downy mildew needs water to survive and spread. It also needs cooler temperatures. You are most likely to see downy mildew in your plants where rainfall is frequent and temperatures stay cool.