What is your relative humidty?
What are you using for air circulation?
In order to prevent powdery mildew, you have to keep these factors in check. Humidity above 60% can be a problem, and lack of air circulation will also contribute to the problem.
But now that you already have the problem, you can make a solution that should take care of it:
To control
powdery mildew on plants, mix together:
- 1 tablespoon of baking soda
- ½ teaspoon of liquid soap (Ivory or something pure)
- 1 gallon of water
Spray this mixture on the affected plants when the lights are off (otherwise it may burn the leaf surface). Also, as a precaution, if you can, move the affected plants away from the other healthy plants to reduce the chance of the mildew spreading. You can also try neem oil if you have it.
Good luck