Can you help me save my grapes?

The best grapes I managed were Black Hamburgh (cuttings from the Hampton Court grape vines) and Muscat of Alexandria. The main problems occur as the fruit becomes ripe. Damp cold nights will soon start mildew/botrytis.
Vents must be left open on warm nights, damping down should stop. If outside, put a clear polytarp over the top of the vine, leaving plenty of air underneath. Now is the time to clean up where your vines grow. Paint any brickwork white. Pick up any dead leaves etc. use a fungicide on the floor, if vines are under glass wash the glass with vinegar.

I agree Bob and suggest a treatment with Bordeaux mixture (available at garden centres) wouldn't go amiss either.

John.

dont confuse the slight grey bloom with mould. The bloom will rub off easily. my kids thought it was mould.
if it is, ventilation will be a good answer.

Sorry Seaburngirl but I totally disagree. It sounds like it could be Guignardia bidwellii or downy mildew, which is a fungal disease. If you do a bit of research on line you will see that it is quite serious and many experts advise you to cut out and burn infected parts of the vine. We can't be certain without a photo that that is what it is but if I had fruit dying off like that I would certainly take more action than opening the vents.

John

Agreeing with John here, this could well be the downy mildew as he mentioned, but as he also says, we can't be certain without a photo or seeing the fruit, so we can only give advice on what we can deduce from the question and our own experience. You never wipe bloom off, it is the natural finish of any fruit, doing this will damage your fruit. We used Gishurst compound to wash down the rods at this time of year.

I know you dont rub the bloom off but as you have said without a photo it is difficult to be 100% certain. i also didnt mean ventilation to cure but to prevent it in the first place.

i assume all leaves etc are not left in the greenhouse to contaminate next years growth.

are you a mycologist by any chance Doctorbob1, your knowledge of fungal species is spot on?