Powdery mildew, or Oidium begoniae, appears on begonia leaves as a white powder or powdery looking spots. It tends to occur most commonly when conditions are more humid in late summer and early fall, and it's encouraged by warm days followed by cool nights. Although mildew is ugly and can disfigure your begonia, the chances are good that the plant won't die. Most fungus infestations cannot be treated with homemade products, requiring chemical applications instead. The good news is that mildew is the exception to that rule. You can easily and effectively remedy the condition with your own harmless fungicidal homebrew. Begin treating as soon as you detect the presence of mildew.
Pick any heavily infested leaves from the begonia and destroy them. Don't add them to your compost because the mildew spores they carry can easily spread to healthy plants
Dissolve 4 tbsp. baking soda in a gallon of warm water. Add 1 tbsp. horticultural oil and mix well. Fill a plastic spray bottle with the fungicidal solution.
Spray all surfaces of the entire plant, and pay particular attention to the undersides of leaves. These areas often go unnoticed and mildew will typically flourish there. Repeat applications weekly until all signs of the infection have subsided. It's best to continue treating as long as atmospheric conditions are most humid. You probably won't need to do this during dry weather.
Thin and prune excessively full areas of begonias to help provide excellent air circulation. Dense crowded foliage facilitates growth of powdery mildew.
Remove and destroy severely infected plants to prevent the fungus from spreading to healthy plants.