where are the leaves worst, and where did the yellowing start-- bottom up, or top down? At this stage, some bottom up is normal as the plant drains the lower fans for what mobile nutes it can, particularly N,... usually with N defc. the leaves don't spot up or start to die back until well advanced,... If it starting at the top, moist likely suspect is Fe, but S and sometimes Zn can do similar things,... Go to the Defc. Pic Depot in the sticky's section here, above the this patient posting section, and look at defc. pics to try and find a match,... this late in bloom, and w/o pics, it's a hard call to make about what to do, vs. how long you have until harvest,...