advice re grapes pls

We planted a vine 2 years ago, mostly for ornamental purposes and to cover a big expanse of bare fence, though I bought it from my local, trusty nurseryman and it is a dessert variety.  Indeed the grapes last year were lovely.

Loads of bunches of tiny (thus far) grapes this year but just the slighest touch and they're falling off in showers. 

Can anyone tell me why this is happening please?  Is it possibly the very hot, dry weather - we did soak it once or twice during the last 3 weeks.. or perhaps I should have thinned the bunches?

Any advice welcome ass to whether there's anything we can do to remedy this   Should have said, the vine is otherwise very healthy and vigorous.

Ah, thanks, that would probably make sense.  In its first year it obviously wasn't so big and didn't drop fruit then; this year it's rampant and we didn't prune as you've mentioned (or at all, actually) to encourage fruiting because its purpose was ornamental.   I suppose we need to decide next year whether we want lots of grapes that won't fall off or more leafy growth to cover the fence.

Thanks again

I love the fact they grow and cover things so quickly   Though another I planted last autumn in what you'd think a perfect spot against a very sunny fence - a 'vinifera', ornamental but unpalatable grapes I'm told - hasn't done anything at all.  It's just sulking at about the same height/spread as when planted.

I've literally hundreds of bunches on the other vine but, as I mentioned, the actual fruits on each are very sparse as most have dropped off