Certified GKH Gardening Expert
Is the white powdery in nature or more like bleached in appearance? This would help us to better determine the problem. It could be attributed to either powdery mildew or from water spots associated with overhead watering or splashing of the leaves. Limiting overhead watering will help with this water spots. The powdery mildew can be treated with neem oil, which is an effective and safe fungicide. It will also guard against many insect pests. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm