Answer: The white powdery coating on your African violet leaves is probably powdery mildew, a fungal disease. The conditions that promote this disease are dry roots and a humid environment around the plant. The fungal spores are spread by splashing water and in the air. Keep your plants adequately watered (from below, avoid watering the soil from the top). You can decrease the humidity by improving air circulation and spacing the plants further apart. Remove the affected leaves, and you may even need to dispose of the plant so the disease doesn't spread to the others. As a last resort, you can dust the plant with a sulphur fungicide. Check your local garden center, and follow the directions carefully.
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