Powdery mildew on herbaceous annuals is a real problem for modern gardeners. Begonias are susceptible to mildew fungi and must be treated for symptoms in order to cure the plant and stop the mildew from spreading to other plants.
As the name aptly describes, powdery mildew appears on the begonia as a white, spotty fungal attack in small swatches across the leaves, stems and sometimes buds. These swatches will enlarge, lacing together to form a felted, leathery surface, gradually curling and killing the foliage and sometimes, the plant itself. The overall appearance of an infected plant in the later stages will reveal a general withering with yellow to brown leaves and deformed flowers.
Powdery mildew fungi belongs to the Erisiphaceae family, a large group with various species that can affect singular plants or groups of plants, depending on the strain. The dangerous element to powdery mildew is its ability to remain on the infected plant over winter, returning to attack the plant and its neighbors in the spring. Mildew can also be spread in the wind, through spores, and by water. Damp, humid conditions are favored by the disease and will rapidly propagate the illness among other garden or house plants.
As with most plant-borne illnesses, begonia mildew can be largely avoided through proper plant maintenance and preventive measures. Begin when planning your garden space, allowing plenty of room between plants for proper air circulation. Adequate space also allows you to see infected foliage early for prompt removal and disposal. Take care to remove dead foliage from under plants, as it provides a potential breeding ground for disease. Also choose disease-resistant varieties when planting to reduce risk. For indoor begonias, remove and destroy dead material from the base of the plant and use fungicides as per manufacturer directions (taking care not to expose humans or animals) to control infections. The University of Georgia Cooperative Extension master gardeners suggest that although there are currently no helpful insects that remove begonia mildew known at present, fungicides may provide a level of disease control if the symptoms are noticed early and application is swift. Once infection is widespread or late in the growing season, fungicides are not recommended.