This will be my first harvest of concords

My vines have tons of beautiful green clusters, but a small number of the grapes are turning purply-brown and then shriveling.

It's not a huge amount .. . yet! A friend who has several years more experience growing grapes says hers are doing the same and she doesn't know why.

Any advice on what it is and what to do about it? She guesses it may be a fungus from all the wet weather we've had lately

TIA