This disease is especially severe under greenhouse conditions where begonias are propagated. Growers should be sure to start with disease-free cuttings, use a sterilized medium and keep the growing area free of any type of weak or decaying plant material that might serve as a food source for the fungus. Brown spots appear on foliage and reduce plant vigor. Most leaf spotting fungi thrive under high moisture conditions. Change the location of potted plants if conditions are overly wet and use appropriate protectant fungicides.
Affected leaves have a white powdery substance on the upper surfaces.
This problem is occasional in nature but may occur when environmental conditions are ideal for disease development. Small blister-like spots appear on leaves. These become clear with age and may run together to form larger spots. Affected leaves may shed prematurely. Chemical control may be only partially effective.