The begonia is a perennial and a member of the Begoniaceae family. Home gardeners often place this plant in their landscape because it produces green foliage and lovely flowers. Although begonias are generally healthy, they are susceptible to mildew development, which can affect the health and appearance of the plant.
Begonias commonly develop a disease known as powdery mildew, which is characterized by white or gray mildew that looks similar to talcum powder. Caused by fungi, powdery mildew affects almost all types of plants if the weather conditions are favorable for disease development. This disease is most active during temperatures of 60 to 80 degrees Fahrenheit, and is encouraged by high humidity. Powdery mildew is least active during the hottest part of the summer.
Powdery mildew is often seen in plants that are most susceptible to the disease. Herbaceous plants such as phlox, Reiger begonias, delphiniums, zinnias and dahlias are especially susceptible to powdery mildew. Woody ornamentals such as English oaks, flowering crabapples, lilac, privet, roses and dogwood are also at an increased risk of powdery mildew development.
Begonias with powdery mildew develop a powdery coating on leaves, stems, buds and flowers. The uppersides of leaves are usually affected first, followed by the undersides. Young, tender shoots are first affected by this disease. Begonia leaves with powdery mildew often curl, twist, yellow and drop prematurely. New shoots and flowers may be distorted, as well. Powdery mildew does not usually kill the host plant, but the plant often becomes unsightly.
Implement cultural controls before using chemical controls. Provide adequate spacing for plants in flower beds and gardens, as powdery mildew is more common in damp, crowded areas. If possible, purchase powdery mildew-resistant plants. Ask your local garden center which plants are considered resistant to the disease. Keep your plants in weed-free and avoid watering plants with overhead watering systems. Always water your begonias early in the day, so they dry out completely before dark. If cultural controls fail, contact your local extension office for advice on fungicide sprays for the treatment of powdery mildew.