need help diagnosing problem

Greetings
I cannot figure out what this problem is. It is on my vegging plant and it is also, to a greater degree , on my flowering plant. On the vegging plant I am in soil mix, the same I always use(my own blend) and I am watering with "Grow Big"(foxfarms) 1 tsp./ gallon H2O+ and 1 tsp. kelp.
As the problem progresses the "rust spots" spread and the leaf dries up , you can crumble them in your hand.
It is not a infestation, I have looked with microscope.
Anyone else ever had this problem?

this is early stage


this is late stage

looks like either...nute burn, cal or mag def. , but...you cant be for sure, have to have someone else take a look at it, second opinion

Thanks for the input NEO, I was leaning towards Mag. def., is epson salts the cure ?

ive seen this from fluctuating humidity had some bad swings in rh when the season changed had what looks like the same issue got a humdifier cleared up hope it helps