Dude, I promise you that aint burn from MG. Start watering them with either a tsp per gal of Epsom Salts, or get them some Cal-Mag somehow. Thats either a P or an Mg isuue, either way that will deal with it. The dead brown tissue will not regenerate, but if you watch the new growth it should be fine. Not that I recommend watering with MG on the plant at anytime during flower, but MG CAN and has been used with great success if one knows what one is doing. CERTAINLY not by me, but Ive seen too many really good grows done with it.