lol, it's not a "mag deficiency"... so many cannabis growers overuse magnesium... doesn't help that it in a lot of bottled nutrients (which many newer growers use). that is good that the large leaves are yellowing like that, in that current state of maturation... it means that there isn't too...
hey, donjac82... it looks like there's a tiny bit of the beginnings of P.M., unfortunately... if you nip it in the bud now, you'll probably stop most of it before it spreads too badly.
Those are Russet or Broad Mites man. You cannot see them with the naked eye. You can see the damage though. They are viewable with a 60x microscope...even a handheld one will work (as long as it goes up to 60x). They like it hot and dry. They are the bane of the cannabis and hemp existence...
It looks like it's really hot where you're at...no? The file: IMG_1916 is a good example of what heat burn looks like.
So, you can do 2 things. Add Epsom Salts (as your Mag. source) and also add SILICA . You can get it in liquid form...Bontanacre makes one called "Silica Blast" or something...