That looks like a nutrient burn - Nitrogen deficiency should start at the lower leaves because the plant is consuming the nutrients from the lower leaves, moving them up to the new growth above. That is why the lower fan leaves yellow and drop as bloom progresses - the plant is consuming itself while it focuses on nurturing its buds and, it hopes and we don't, its seeds. Burning occurs from the top down because transpiration causes water to be lost from the leaves while the salts remain behind - the further from the roots and the closer to the lights the worse it will get.

You need to flush the hell out of that plant.