I just treated my mag def, what you got is defently mag def looks just like mine, it starts on the older leaves and works its way to the younger ones, what i did since i dont use bottled water is, i collected rain water, boiled it and added hydrogen peroxide too it, let it cool too a warm temp, i then added the epsom salts with a mild organic kelp fert, because if your having a mag lockout other nutes are effected, i then flushed my plant many times with the water thats slightly over 7 ph level, some soils will go acidy by them selfs because of numerious things, which locks out mag, so i raised the ph of my soil abit and foliar fed it the solution as well as like i said flushed it many times, the warmish water stimulates the roots and the boiling with peroxide makes for sterile water, when i flushed the water fizzed and foam lots, the peroxide never hurt the roots or anything i did it mostly because im using recycled soil from my last grow, this will elminate hot spots, the next day my plant looked much perkyer and is now growing as usual after being locked out for a week, hope this helps.