What Causes White Spots on African Violets?

Powdery Mildew

• African violets are vulnerable to powdery mildew, which is a common fungal ailment that affects hundreds of trees, shrubs and herbaceous plants. Several groups of pathogens can cause this condition. White, powdery fungal colonies emerge on infected leaves and stems. Powdery mildew fungi feed by draining nutrients from the surface of foliage, but they do not infiltrate the internal tissues of their host. Excessive application of fertilizer and overly moist environments increase the chances of powdery mildew developing.

Botrytis

• African violets are susceptible to botrytis blight. The fungus can damage leaves, buds, stems and flowers. Gray-white or transparent spots appear on flowers and leaves infected with this disease. Damaged tissue eventually wilts and falls from the plant. Environments with high humidity levels, like greenhouses, foster the growth of botrytis fungi. According to the African Violet Society of America, botrytis blight can infect a plant through open wounds, but it also may attack healthy tissue.

Disease Management

• Basic management and preventative measures can help avoid an outbreak of fungal diseases on African violets. It is particularly important to deal with fungi promptly when a large number of plants are growing in a small area, as most of these diseases are highly contagious. African violets should be grown in areas with good air circulation. Balance their preference for humidity with the need to discourage fungal diseases by keeping the humidity at 60 percent or less. An infection of powdery mildew can be treated by pruning infected growth and applying fungicide to the infected plant and surrounding plants. However, a plant with a confirmed infection of botrytis should be removed immediately and all surrounding plants should be inspected for infection.

Cultivation Problems

• Fungal diseases are not the only possible cause of white spots on an African violet plant. The foliage of African violets may turn white in response to contact with cold water, according to Clemson University Cooperative Extension. Yellow spots may appear on the plant's foliage if it is exposed to direct sunlight throughout the day, although these spots do only superficial damage. Avoid spots caused by cold water by watering the soil at the base of the plant.