We need a photo to i.d. the problem but is sounds like downy mildew...

Both indoor and outdoor grapes suffer from fungal diseases which affect leaves and fruit. The three top grape diseases are downy mildew, powdery mildew, and grey mould.

Downy mildew
Yellowish patches occur on the upper surfaces of leaves, often limited by veins, corresponding to whitish patches of fungus growth on the lower surfaces

Powdery mildew
White powdery fungal growth occurs on leaves, stems and fruits
Sometimes growth of the powdery mildew fungus is sparse and the symptom is a grey or purplish discolouration of the leaves
Fruit may become split or may drop prematurely if infected by powdery mildew
Fruits damaged by powdery mildew are often then infected by the grey mould pathogen and then rot, or become mummified

Grey mould
Fruit becomes soft and brown, especially as it starts to ripen
A fluffy grey mould may appear on the affected fruits, particularly when humidity is high

Non-chemical control
Clean up and destroy fallen leaves to reduce the amount of downy mildew pathogen surviving the winter
Mulch plants each season to avoid drought stress, which predisposes them to powdery mildew infection, and make sure vines are adequately watered helping to prevent this problem
Avoid overhead watering as this increases humidity around the foliage which makes the plants more prone to powdery and downy mildews
Keep vines under glass well ventilated
Prune overcrowded shoots to improve air circulation around the foliage
Consider heating the glasshouse in dull weather to avoid excessive humidity

I wouldn't recommend heating th greenhouse simply keep it well ventilated.

Hi, welcome to GoY, if you go to https://www.rhs.uk/advice/profile?pid=569 you will find everything Triffidkiller has said, plus lots more information on your choice of treatments, Derek.