There are some varieties which suffer very badly from botrytis or grey mould, Black Hamburg being one of the most vulnerable.
The main approach is to grow it without it becoming crowded and so letting lots of air circulate. The other is to spray regularly with a copper based fungicide like Bordeaux mixture.
I imagine you are growing this indoors, as they are less likely to be affected outside.
Grow as a single cordon and keep it pruned to relatively few fruiting sideshoots. Most people fail to prune vines adequately. Just look at how harshly professionals prune wine growing vines.