Hello Jen

Based on your images - I believe this is powdery mildew versus downy mildew

Look at pages 130 through 132 of this publication - This will explain Powdery and Downy Mildew on Grapes - I WOULD SUGGEST YOU SAVE THIS PUBLICATION - LOTS OF GOOD INFORMATION ON GRAPES - It is at this link: http://plantpathology.ca.uky.edu/files/mw_home_fruit_productn_b591.pdf

Look at pages 24-26 on Fungicides and page 30 Grape Spray Guide of this publication - I WOULD SUGGEST YOU SAVE THIS PUBLICATION - LOTS OF GOOD INFORMATION ON GRAPES - It is at this link: https://www.extension.purdue.edu/extmedia/id/id-146-w.pdf

This is Powdery mildew - Uncinula necator - Michigan State University at this link: https://www.canr.msu.edu/uploads/files/Diseases_PDFs/PowderyMildew.pdf

This is Downy mildew - Plasmopara viticola - Michigan State University at this link: https://www.canr.msu.edu/uploads/files/Diseases_PDFs/DownyMildew.pdf

Below is some great information on growing grapes in the backyard - I provide 4 universities for your reading enjoyment:

Michigan State University: https://www.canr.msu.edu/news/growing_grapes_in_the_backyard

Penn State University: https://extension.psu.edu/backyard-grape-growing

Cornell University: https://extension.psu.edu/backyard-grape-growing

University of Minnesota: https://extension.umn.edu/fruit/growing-grapes-home-garden

Hope this helps!!