What is the cause of the spots and what can we do to overcome it?
White Spots on Parsley and Thyme Killing Plants
Certified GKH Gardening Expert
There are a few ways that you can deal with powdery mildew. I recommend using neem oil. Just make sure to spray OUT of direct light to avoid burning the plants. Another thing that will help is mixing half and half peroxide and water, and spraying with this as well as spraying the soil.
This will get rid of the problem when you do both of these together.