Like any foliar, you want to spray in indirect light. Dusk is typically good. If you are dealing with high humidity and Powdery Mildew is a concern, then spraying just before dawn is the better option. If it gets really humid at night, we would probably spray at dawn so that it dries quickly after being applied. You can also spray during the day as long as it is overcast and a little cloudy. The only hard rule is that direct light could cause it to burn. The second rule, especially for treating PM, is to try to get it dry as quickly as possible after spraying. If indoor, add dehumidifiers and fans. Air movement in a greenhouse helps, also.