that looks to me like a magnesium deficiency. did it start on the lower part of the plant? are the growing shoots a very light green or almost white? if it is a MG deficiency you can try epson salts at a rate of 1 tablespoon per gallon of water, a faster way is to use a foliar spray but since you are into flowering now i would discourage that type of application. a low N reading is what you want at this stage of flowering, i would also discourage raising your N level.