It looks like your grapes have symptoms of black rot, a fungal disease. Fungal spores are released during wet periods before bloom when new shoots first emerge. There is information here on our website about how to manage black rot disease. https://extension.umd.edu/hgic/plants/diseases-grapes
We refer grape growers to the spray schedule produced by Virginia Tech. This guide is updated regularly and provides a schedule for managing common problems in fruit production. Immunox and Mancozeb are two products that can control black rot effectively. Refer to section 3-18 in this guide for the spray schedule for grapes.
In addition, if you have a commercial vineyard, you may contact Extension Agent Joe Fiola for further assistance with grape production. His contact information can be found here: https://agnrfacultystaff.umd.edu/jfiola