This is a fungal disease that appears as a white, powdery coating on the surface of the leaves. Poor air circulation and high humidity conditions may increase the problem. Powdery mildew doesn’t usually kill a plant, but it is unattractive and my cause distortion of the leaves or premature leaf drop. It will however interfere with photosynthesis by covering the surface area of the leaves. This can weaken the plant overall and make more susceptible to other diseases or add complications to an existing problem, such as insect or environmental issues.
Increase air circulation if possible and remove infected leaves. Spray with sulphur. If outdoors copper sulphate may be applied instead of sulphur.