Botrytis blight - fungal disease generally caused by dead leaves and spent flowers being left on the plant, too much humidity or poor ventilation. Crown, stem or root rot - caused by overwatering, especially during the winter months when plants are dormant and do not need much moisture. Fungi which usually develops when water is allowed to remain on the leaves. Cold water can also be a cause of spotting. Use room temperature water for misting and watering, and make sure the foliage dries before night.
Powdery mildew is an airborne fungal disease.
African violets and Begonias are particularly succeptible. Good cultural practices will eliminate many diseases and other houseplant problems. Many problems may be halted by removing damaged parts of the plant if they are detected early enough. However vigilance is necessary afterward to make sure that you have completely eliminated the problem. I will be dealing with insect and disease problems in depth, in the near future.