Safer® Hydro FAQ: Why is there White Powder on Plant Leaves?
Powdery mildew is caused by several different strains of fungi that grow where humidity is high and air circulation is limited – the conditions in many grow rooms. Powdery mildew blocks light from reaching leaves and can siphon nutrients away from the plants.
To prevent the fungus from growing, try to keep the room's humidity level below 65% and set up fans to gently blow on your plants. Also, avoid watering or spraying leaves during the dark cycle, when moisture can linger before it evaporates. Treat an outbreak of powdery mildew by spraying affected areas with a fungicide that contains sulfur. If the outbreak is severe, you may need to repeat the application at 7- to 10-day intervals.