Powdery mildew tends to plague indoor begonia plants during the autumn and winter months. The main symptom of powdery mildew on a begonia is a grayish-white powder coating the leaves of the plant. The mildew can be tough to stop, but treating the plant with a fungicide and improving its living conditions can help.
Isolate the begonia plant with powdery mildew. This fungus can spread to other plants.
Spray the begonia with a fungicide to stop the mildew from spreading. Read the directions before doing this, as some types of fungicide should only be applied outdoors.
Use a pair of gardening shears to cut away the infected parts of the plant. Cut off leaves where they meet the main stem, and discard them in an outdoor receptacle.
Move the treated begonia plant to a well-ventilated area, away from heat registers and radiators. Powdery mildew thrives in dry environments, which is why it is more common in indoor plants when the heat kicks on in the winter.
Water the begonia carefully. Never splash water onto the leaves and stem of the plant, as that can aggravate its condition.