Keep them fairly dry, especially during the winter months when they are dormant and do not need much moisture. Leaf spots on houseplants can be caused by bacteria, fungi or poor growing conditions. In the case of fungi, the spores are spread in the air and water. Brown or yellow spots, sometimes with a wet looking centre, appear on the foliage. This infection occurs when water is allowed to remain on the leaves.
Cold water can also be a cause of spotting.
Powdery mildew Plants infected with this fungus disease become coated with a powdery-white mould. It differs from grey mould in color and lacks the fuzzy growths associated with grey mould. The spores are spread by air currents and do not require water to germinate. In the early stages, powdery mildew spreads very slowly and, if noticed, can be controlled fairly easily. African violets and begonias are often affected by powdery mildew.