The GARDENA gardening expert
This really has not been a grape year for us, Mr. R. I have the same problem. Even my fungus-proof vines have numerous rotting grapes in the clusters. In the middle of July, I sprayed them with a copper preparation against mildew and saved them from a massive spreading of fungus. But I had the impression that it was already too late and numerous grapes were already damaged. I believe that the damage on the grape skin benefited the rotting. Even when grapes burst – a feast for flies and wasps. The bursting can be explained by mildew infestation and irregular watering of the vines: first, there was intermittent drought then ample rainfall. On the whole, the rain and moisture brought even the fungus-proof table grape varieties to their knees. Although, we must remember that fungus-proof only means “fungus tolerant” and not “fungus resistant”. The latter are guaranteed not to be infested, the former, “maybe not”.
My conclusion and at the same time, advice: I am going to treat my grapevines in the next May/June and a second time later with an environmentally-friendly fungicide (Net-Sulphur WG, Atempo Copper-FungusFree, both from Neudorff). This means for later: Depending on the maturation period of the variety, around the end of July/beginning of August because a waiting period of 35 days applies after spraying.