I am not certain if this grape has a disease or a nutrient problem. It is a variety of Concord named "VanBuren", supposed to be very hardy in Northern climates. Or soil has a tendency to higher pH, so I was thinking of lowering it with sulfer prills. Any comments will be appreciated.
Given that it is the more distant parts of the plant that are coloured, I'd suggest nutrient deficiency. Iron, maybe?
Good call! It does look like severe chlorosis. I put on about 5g of iron chelate this morning with a slow deep, watering. We shall see what develops.