Nutrient deficiencies can adversely affect grapevines, not only the growth of the vine itself, but also the fruit. Here in Poplarville, the soil pH is low. To give you an idea, in general this area is good for growing blueberries. Blueberries thrive in soil pH of 4.2-5.2, which is too low for most grapevines. Often in other regions I have seen Iron (Fe) deficiency but here, because of the low soil pH, I am seeing Magnesium (Mg) deficiency show up. The photo below shows what late season leaf symptoms look like. I don’t remember which vine this came from — some cultivars appear to tolerate the soil pH better than others.
So, what to do about it? Raise the soil pH with lime is the first thing. If that doesn’t alleviate the problem, then foliar or soil applied sprays may be necessary. My colleague at Cornell University, Hans Walter-Peterson, gives an excellent primer on Mg deficiency and offers suggestions for correcting it. You can find that here: http://www.growingproduce.com/uncategorized/managing-magnesium-in-grapes/. The eXtension grape community of practice has a good article on other potential disorders too — that article can be accessed here: https://www.extension.org/pages/31599/grapevine-problems:-leaf-spots-not-caused-by-insects-or-disease#.U-jgJfldV8E
You may be inclined to think, “It is after harvest, so I shouldn’t worry about it”. Nutrient deficiencies can lead to poor winter hardiness, overall vine stress, and other issues. It is best to correct the problem rather than to let it negatively affect the vine.