African violets (Saintpaulia ionantha) have an old-fashioned charm lined up on an east-facing windowsill, with their brilliant blue, purple or soft pink blooms rising from a rosette of slightly furry leaves. Not so charming, however, is the amount of work these houseplants can require to stay healthy and blooming. The plants must be watered and fertilized, repotted and groomed, and still are prone to plant maladies, such as powdery mildew, that can make them lose prized blooms and disfigure their foliage.
Powdery mildew is a sign that an Oidium fungus has infected your plant. There are multiple varieties of fungi that cause powdery mildew. Many are specific to certain plant families, while others thrive on many types of host plant. Powdery mildew, in whatever form, can affect plants indoors or out. It occurs when temperatures are from 60 to 80 degrees Fahrenheit, light is relatively low and air is still. The fungus grows on the surface of the plant when conditions are dry and warm, and produces white to gray powdery spores when humidity increases around the plant. Though it often doesn't kill a plant outright, powdery mildew decreases the plant's ability to photosynthesize, decreases its flower production and definitely makes the plant less attractive.
Powdery Mildew on African Violets
Erysiphe cichoracearum is the specific fungus that causes powdery mildew on African violets. It first presents as white powdery patches across the African violet's fuzzy leaves -- usually just one or two. The patches are uniform in distribution of the powdery substance. Flowers may have gray powder across them as well, or just look discolored. If the plant is just in bud, powdery mildew can grow on the surface of the bud stems. Leaves and flowers can become distorted and they, as well as any buds, can drop from the plant prematurely. Plants can die when the powdery mildew goes unchecked.
Cultural Powdery Mildew Treatment
Good cultural practices are a primary deterrent to powdery mildew. Place plants to allow for plenty of air circulation between plants and adequate sunlight. Also place them in an area where the temperature does not fluctuate more than 10 degrees Fahrenheit, especially from day to night, as the change increases humidity around the plant, which encourages the development of powdery mildew. A fan near the plants can stir the air to both improve air circulation and keep humidity down around the plant. Dispose of any foliage, flowers or buds with signs of powdery mildew and keep the plant isolated and in ideal conditions.
Chemical Powdery Mildew Treatment
When infected plants do not respond to cultural controls, chemical controls can be effective. Both the African Violet Society of America and commercial African violet nursery Optimara recommend misting the air immediately around affected plants with a household antibacterial spray, such as Lysol. Do not direct the spray on the plant itself because it is extremely cold and could damage the plant. Spraying with organic neem oil, jojoba oil or a mixture of 1 teaspoon of baking soda in 1 quart of water is effective on mild to moderate powdery mildew. Synthetic fungicides are usually only effective before the powdery mildew is evident.
- University of Vermont Extension: African Violets - Seven Steps to Success
- University of Illinois Extension: Powdery Mildew on Ornamentals
- University of California Integrated Pest Management Program: Powdery Mildew on Ornamentals
- Colorado State University Extension: Powdery Mildew
- African Violet Society of America: FAQ
- Optimara: Doctor Optimara - Powdery Mildew
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