we have just one Bacchus vine in the garden which every year produces a few small bunches of grapes, which every year go black and withered after a few weeks - and it's not noble rot..........
Oh that's a great shame, do you prune off the ends of the branches which bear the tiny bunches? I chop off each branch leaving one or two leaves the pruned side of the tiny bunch. I expect you do this, but just in case - it might help.
