My grapevines had numerous flowers this spring, most of which were fertilized and started to form berries, but a week later most of the tiny berries and flowers had disappeared and all I have left was stalks with weblike thread covering around the remaining portion of the bunches. I live in a rural area woodlot; zone 5A/4B. The 15 yr old vine is covering a small wooden archway and is protected from the prevailing west wind by a couple of 20 year old Gleditsia triacanthos var inermis. I have had very small quantities of white grapes from this vine in the past but have not had the quantities of flowering before this spring. I have mulched around the base with home made compost this growing season. Any ideas to promote more fruit please. Sorry, no photos. Regards Pat Nicholson
I can't determine the issue from your description. I would suggest talking with your County Extension Service.
The links below will help you.