With Captan you need to make sure the spray water is acidic. At a pH of 8 half the Captan is destroyed in 10 minutes. A pH of 5 is ideal with 5-7 being ok if your going to spray right after you mix the Captan into the water. You can use vinegar or citric acid to lower the pH. I use citric acid which is found with canning supplies in retail stores. I use a 1/4 teaspoon of citric acid in 2 gallons of spray water. The amount needed will vary with the pH of your tap water. Buy some pH paper and use it to check pH. For more info on spray water pH take a look at this-