What is this

This is a phosphorus (P) deficiency, not a potassium (K) deficiency: a potassium deficiency looks like there is a lighter coloured margin surrounding the leaf, which looks pretty cool.

That is potassium deficiency. You are in flowering and plant likely ate all the potassium and phosphorus you were feeding causing the ph to climb because it was not enough. Then it put you in lock out. Your ph in substrate is likely too high. Or your not getting good run off and ph is low from salt build up. Only you know this answer. 🌱

That is a problem is what that is. Your plant has been locked out of getting to any of the nutes it needs like calcium and magnesium - maybe even potassium. WHY they've been locked out is anybody's guess since you don't have a diary... but lockouts can result from too much nitrogen or a pH that's badly out of range. You need to keep the pH between 6.0 and 6.5 (6.2 is perfect) assuming you're growing in soil - which is another piece of information we don't have. Flush your plant ... take 3x the pot volume of pH'd water and run it through ... so if you're growing in a 3 gallon pot (another piece of information we don't have), you'd need to run 9 gallons of water that is pH'd to 6.2 through each pot. Then let it dry out. When it's dry, you can resume nutes but only at 1/2 strength for that first feed... but DON'T include any nitrogen heavy nutes or at least cut THEM to 1/4 strength and keep them there for the rest of the grow... plants in flower don't need as much nitrogen as they do when they're in veg... The only other problem I can see is that you don't have a diary. Diaries help us help you... Start one - even if you can only go back one or two weeks - start one. Join the community! Good luck...

It is likely a ph lockout as Rob says. If it were salts building, your pistols would be going brown by now. Check your runoff waters ph and flush her back in line if it is out to much, reapply your feed and watch new growth. Give Rob the point

hi, Looks like nutrient lock. I had the same problem couple of weeks ago. I payed particular attention to the Ph level ans re-calibrate my Phmeter, then feed her with a strict 6.5Ph during 3 watering. But nutrient lock could also come from the salt accumulated in the soil. In that case I would try to generously rince her once or twice and wait for 4-5 days to see if she's getting better. Wait an other comment to confirm or not what I say. I'm not an expert.