My friend has small vineyard of cab Franc grapes. A few of his vines have leaves are turning red. I was hoping someone could let me know what is causing this and if we should be pulling the vines out so they don’t infect others.
It looks more like a nutrient deficiency than a virus. The top pic fits magnesium deficiency with the redness starting at the edges and making wedge shapes towards the center. Mg deficiency can cause pale leaves too, though the red veins are a little unusual. A soil test would be a good idea before anything else as high K can displace Mg and pH can complicate uptake.
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Looks like potassium or magnesium deficiency. sul-po-mag/k-mag might help for next season. I don’t know if adding it to soil this late will help. The soil pH may be off for that variety, and causing the potash not to uptake. A quick search says they prefer a pH of 7-8.
A foliar feed of kelp might be a solution. Maybe reducing the number of grape bunches could be a short-term fix.
A soil test will give you more certainty.
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It does look like a mineral deficiency. I have potassium deficiencies and had similar problems that cleared up with Sulphate of Potash applied monthly at 2 lbs per 1000 sq. ft. Much of the US has potassium deficiencies. Other parts have phosphorus and some have both. Either do a soil test or leaf petiole analysis or both. If you guess and guess wrong you will make things worse. Boron is short in many soils and can cause problems as well.