Certified GKH Gardening Expert
This brings to mind two possible issues. One would be powdery mildew, and the other would be aphids. Any pictures would help me to identify the issue. Luckily they can have the same treatment.
Neem oil is a good treatment for a broad range of issues, and can be purchased online, or in gardening centers. Just have care and do not spray in bright direct light. Wait for a cloudy day, or early morning.
Here is a collection of articles for more ways on controlling mildew, and other pests: https://www.gardeningknowhow.com/special/organic/