Thanks for your question.

Your grapes are experiencing a fungal infection from powdery mildew. The primary fungus involved is Erysiphe necator. It is very common in grapes. Unfortunately, our 2024 season has been a perfect storm for such an infection. Our June was very wet and now hot temperatures are upon us. While your berries do have an appearance of a powdery mildew infection, look around your grapes for one of more of these additional symptoms:

1). Besides the grapes themselves, is a white or grayish powdery coating present on the surfaces of the leaves and/or stems?

2). Do you observe any leaves that are distorted or curled?

3). Have you observed any berries that cracked, shriveled, or failed to ripen properly?

Any grape cluster having infected berries should be removed and discarded in the trash. The berries will be unfit for consumption. Fungicides can be used to control this infection, but in so doing legitimate concern can be raised regarding adverse effects upon humans. If you do use a fungicide, please carefully read the label regarding its use around products for human consumption. If you wanted to be super-cautious, avoid using any fungicide and only pick/use grape berries that show no sign of an infection.

See the following for further information on these and related points. Pay special attention to the proactive steps you need to take in 2024 to prevent a recurrence of this infection in 2025.

https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/agriculture-and-seafood/animal-and-crops/plant-health/grape_powdery_mildew.pdf

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

https://ask2.extension.org/kb/faq.php?id=873577

Good luck. Please get back with any additional, related questions. Thank you for using our forum.