By Rogermay
United Kingdom 
I have been attempting to grow a dessert grape (Muscat d'Alexandria) under glass for 7 or 8 years, but have never had an edible crop.
The main problem is a grey/white mould on the grapes, which prevents the grapes from ripening.
I have treated the grapes (from first flowering) with Dithane, monthly. And between doses of Dithane I spay the vines (weekly) with plain water (both as suggested by a grape growing book I have). I keep the greenhouse frost fee over winter and ventillate freely both day & night after the frost has gone.
Anything else I could do, or should I dig it out & forget it?
Thanks,
Roger
23 Aug, 2015
Answers
Chances are its Botrytis Bunch Rot (Gray Mold) of Grapes Caused by the fungus Botrytis cinerea, it can occur anytime during the growing season, but grapes are most likely to be damaged near harvest, and the pathogen can overcome a cluster very quickly. Fungicides will not get rid of Botrytis once it has started in a cluster - If it were mine I'm afraid I would get rid of it - Burn it and Sterilize the soil around the vine or replace. There are a lot of sites that have information on this - I would read them 1st before you decide what to do with your vine - Just Google Botrytis Bunch Rot.
24 Aug, 2015
Try sending a Private Message to DavidS - he doesn't contribute on here very often but he is a very experienced vine grower. To do this the easiest way is to type Grapes into the search box top right of this page. The first item on the list that appears is one of his. Click on his avatar and you'll see a page with the option of sending him a private message. He will be notified by email that he has received it.
23 Aug, 2015