Powdery mildew is a common problem with begonias, and it is easy to identify by the white to greyish spots that look similar to a cobweb growing on the leaves, stems or flowers. Begonias with powdery mildew can also be vulnerable to even more dangerous fungus that can eventually kill your plant. Left untreated it will turn into brown spots, disfigure your plant and compromise the health of your begonia. Try using a homemade fungicide for mildew rather than harsh chemicals.
Remove severely damaged leaves with sharp pruning shears. Remove no more than about half of the begonia leaves, if needed. If stems are seriously infected, remove them as well, but cut off only the three or four stems that are in the worst shape. Removing any more than that may send your begonia into shock.
Wipe off any remaining mildew with a soft cloth before treating. If your begonias are outside in a large area, wash them off with water from a hose to loosen the powdery mildew.
Mix 2 tbsp of neem oil with 1 1/2 tbsp of mild or natural dish-washing liquid in 1 gallon of water. According to the University of California Agriculture and Natural Resources, neem oil can kill the mildew that is present and protect the plant against further infection. Many commercially sold fungicides have neem oil as an active ingredient. Pour into the spray bottle and apply to the infected begonia.
Combine non-fat milk and water at a ratio of two parts milk diluted in four parts of water. Use only fat-free milk to prevent any unpleasant odours from milk sitting on your plants. The amino acids and salts are the important active ingredients; the fat is not necessary. The University of Washington Botanical Gardens says this homemade fungicide can also be used as a preventive measure against begonia mildew.
Spray the milk mixture once per week on the entire infected begonia. Make sure you spray the underside of the leaves as well.
Use this milk mixture for as long as wish. It will not harm your plant, and it is a natural deterrent to further infestations of mildew.
Prevent mildew by keeping your begonia in a well-ventilated area in the sun. Don't overcrowd or overwater your begonias; mildew is caused when the plant is not receiving enough air circulation and too much water or humidity.