Thank you for attaching the image to your inquiry. The plants are volunteers of forget-me-knots, Mysotis species. They’re charming annuals which grow well during cool weather. They were commonly used in old-fashioned gardens. (The early stages of powdery mildew on the leaves confirm that ID.)

So, right now, the bottom line is that you might as well pull out all such affected plants. (Discard in the trash.) If you have had forget-me-knots in the garden, you will still have plenty more seeds in the soil which will germinate this winter, grow and bloom to provide an excellent show next spring.