If I were to venture a guess, I would assume this is a lower fan and that is Phosphorus deficiency. Temps? pH? Is this hydro or soil?
What Homestead said.. mild Phosphorous and Potassium deficiencies are common in mid to late flower and not a big concern.. they strip lower leaves for nutrients which is why some random leaves will have it at the lower regions.. try using a PK boost once a week in ur feed to supplement cal-mag if you use it as it can rise EC! Sent from my iPhone using Tapatalk
It looks like a lack of light to me, was it taken from beneath the canopy? Making a solid diagnosis from a single leaf is near impossible though. The whole plant needs to be seen