i dont think it is Mg as i cant really see yellowing with green veins. and the burns arn't in the intervenual regions.

you know what it ;might be a disease. as it looks like the dead parts are causing the yellowing. Not yellowing causing the dead.

as there are burns surrounded with green. hmmm.

as well the dead spots are not uniform and dont follow a pattern. Which i would exspect from a def. hmmm very difficult.