Powdery mildew is a fungal disease that is evident as a white or grey coating on the leaves of plants. Although many plants can handle a powdery mildew infestation, the fungus can cause serious injury to begonias. According to Cornell University, powdery mildew can affect a begonia's growth and cause leaf drop. Powdery mildew starts as small white spots, eventually growing to become a solid mass. If you discover powdery mildew on your begonias, take immediate measures to prevent further spread of the mildew and damage to the plants.
Remove all parts of the begonia that have powder mildew symptoms by cutting them off with scissors. This includes removing leaves, stems and blooms that are affected. Pick up any leaves or blooms that have dropped from the begonia.
Dispose of the cuttings and shed leaves by burning them or putting them in a garbage bag. Do not leave them on the ground near the begonia.
Spray all areas of the begonia with a fungicide. Use a fungicide listed for use on begonias and follow label directions exactly. For indoor begonias, choose a fungicide that contains potassium bicarbonate, Bacillus subtilis or neem oil.
Use the fungicide as often as the product directions recommend, and continue to use it until all evidence of powdery mildew is gone.
Prevent further outbreaks of powdery mildew by thinning out the begonia so that it has plenty of air flow. Trim back any plants growing near the begonia that may be crowding it or blocking the air flow.