Grape Vine, Pest Disease

That looks like mealybug. Can you see any small white creatures that look like a woodlouse?

The puckering of the leaves suggests it is something actually 'eating' the leaves, so insect damage. Mildews tend to cause surface damage which leads to curling or eventually browning. Does it feel dry or slightly sticky/slimy, dry would suggest mildew, sticky/slimy I would still go with mealybug.

It certainly isn't going to hurt to spray with what you are suggesting so give it a go.