is that under a wooden trellis? maybe the sun only hits in that stretch of the plant? if so, light burn is possible? (you'd know if they are acclimated to full son or not... also if east/east was perpendicular to a trellis, lol, probably be a more even sun throughout day)
easy stuff: pH? recent change in behaviour etc... rethink the last couple weeks to help eliminate possiblities.
if always low on plant, likely immobile, if a nutrient related issue. the worst one is kinda mid-plant, eh? Starts at tip...
Zinc def? but, i don't see other symptoms of that, i don't think
If it never approaches petiole, Manganese deficiency is a viable option
no spots, so that eliminates some. some of those look pretty advanced and should have shown that sort of thing by now i think. (like zn, mg... Fe goes margins to tip, i think... )
Manganese is my 2 cents, lol. check your labels to rule it out.