African violets are susceptible to powdery mildew, a white fungus that may be any number of species of fungus. It spreads across the leaves and petals of African violets when they are in an area with very low air circulation. This typically occurs when they are crowded with other plants or stuck in a corner. While they can topple next to a gusty window, an area with moderate ventilation and good spacing between plants -- and between the individual leaves -- will help prevent future problems with powdery mildew or other pests that thrive in low ventilation.
Ensure proper diagnosis of powdery mildew. Powdery mildew starts as powdery white or gray patches or circular areas, then spreads to the entire surface of the leaf or petal. Ambient dust would coat the entire surface of the plant evenly.
Correct cultural problems encouraging the growth of the fungus, such as growing the African violets in areas with cool, moist, stagnant air. Remove infected leaves that are weak or that have already shed, and space plants for better air circulation. Cut away such leaves with sharp, disinfected scissors. Move African violets to an area with better ventilation.
Apply a fungicide if the infection persists after cultural remedies have been applied. Powdery mildew, though primarily a cosmetic pest, also weakens the plant and makes it inefficient. Ensure that the fungicide you apply is effective against powdery mildew and is safe to use on African violets. This information should be on the label.
Clean all implements used on infected plants with rubbing alcohol and a clean cloth. This will help prevent the spread of powdery mildew.