So no nutes so i assume it could be a deficiency.
Magnesium deficiency will exhibit a yellowing (which may turn brown) and interveinal chlorosis beginning in the older leaves. The older leaves will be the first to develop interveinal chlorosis. Starting at leaf margin or tip and progressing inward between the veins.
Now don't take this as being right wait until someone with more experience chimes in, it just a guess.:hippy::stoned: