 for like a month or two months? I would, and then next year when your lilacs come out, depending on the weather, I'd start spraying them again because once powdery mildew takes hold of it, it's really hard to get rid of. So I would do a couple of sprays right away in the spring and then watch them. Well, in the directions, it looked like it was about a one and a half teaspoons for four gallons of water.