The white material on the foliage of your African violets is probably powdery mildew. Powdery mildew is a fungal disease that is common on indoor plants, such as African violets, begonias, and poinsettias. Outbreaks of powdery mildew on houseplants typically occur in winter or early spring.
If only a few leaves have powdery mildew, pinch them off and discard them to keep the fungus from spreading. High relative humidities and poor air circulation favor powdery mildew development on houseplants. Altering the indoor environment is the best way to control powdery mildew on houseplants. Air circulation can be improved and humidity levels lowered by increasing the spacing between plants and running a small fan in the room.