I can't tell from that picture. Personally I'd just do my normal 'try to save it routine'; Flush, then on last flush use anti-stress supplements, b vitamins, Epsom salt (or a tiny bit of cal-mag). Then make some foliar feed spray containing all the micronutrients you can, and spray the leaves a couple times a day.
Tbh it doesn't look like a very common deficiency, but perhaps one of the micro-nutrients. Like the guy above said maybe boron; but also check out calcium, magnesium, copper, etccc