Powdery mildew affects indoor potted plants, including begonias. Leaves develop white spots that spread and join together. Although powdery mildew is rarely deadly, it is unsightly and may cause leaf loss and poor growth. Powdery mildew proliferates in cool weather and tends to prefer humid conditions. Sudden changes in light and humidity promote the release of spores, so avoid sudden climatic changes. Regular application of a chemical fungicide to both sides of the leaves is the most effective treatment after powdery mildew has been detected.
Triadimefon, which is marketed under the names Bayleton and Strike, is a fungicidal treatment for powdery mildew. In an experiment conducted at the University of California Davis, triadimefon proved the most effective fungicide tested, although it also suppressed the growth of the plants somewhat. There was no reduction in the number of blossoms produced. Triadimefon is a moderately toxic chemical to humans and animals and exerts little to no toxic effect on the environment.
Triforine is sold under the name Funginex and is an effective treatment for powdery mildew on begonias. In the study of powdery mildew treatments at UC Davis, triforine suppressed powdery mildew for 45 days, roughly half that of triadimefon, but plants treated with triforine grew the tallest of all plants in the study. Triforine is labeled as a highly toxic chemical and should be used with caution, though it creates little or no adverse effects on wildlife and the environment.
Propiconazole controls powdery mildew infections on begonias and may be purchased under the generic name Tilt. In the UC Davis study, propiconazole performed similarly to trifornine, although it did not result in the same noticeably greater growth. Propiconazole is moderately toxic and acts as a skin irritant.
Although fungicide application remains the most proven treatment for powdery mildew on begonias, the American Begonia Society also suggests removing affected leaves and parts of the plant to stop the spread of the fungus. However, since powdery mildew is invisible to the eye for the first few days of growth, this method may have limited effectiveness.