Rusty spots of a type i've not seen before please help

Hello!
First my apologies for this brief post i'm in a bit of a hurry today.

Not really new to growing.
latest plants are having a terrible time !!

They started off slow and stunted then they picked right up and seem to get better but...
They are still stunted... and they have these rusty spots....

Starts off on older fan leaves with chlorosis of the leaves then small spots in a pattern almost like somebody has held a sparkler to the leaves...

Then the leaves curl up at the tips and turn full yellow with brown edges then fall off.

I thought... mg defficiency, So I added epson salts... no luck there...

I have other available nutrients available but i'm not sure which to narrow it down to.

The soil is sunshine promix with 'seasoil' I know... could be better but i'm not the most skilled with my soil mixing yet.

I'm pretty sure this is a nutrient problem rather than an environmental issue... since this isn't my first dance.

What is throwing me off is the consistent pattern of the spots and none of the symptoms seem to quite match anything 100%

I will include some pictures...

Any help would be greatly appreciated !!! many thanks !!

looks like it could be downey mildew

Thanks for the replies. This gives me some ideas.
I never thought of the PH because it rarely changes in my grows.... but this was a slightly different mix so it is very possible.
I use RO water and low nutrients but in this case ill check the PH then following that up the nutrients a bit.
BTW the plants seem to be going into preflower quite early and they are still quite small... I hope its not too late to rescue this bunch.

Thanks again !

I'm having a similiar leaf spotting issue that started when my pH went out of wack. A pH problem can lock up various nutrients and give a mixed bag of apparent nutritional deficiencies. Check your pH, and once you've fixed that your plants should perk up. Some of the older growth may not recover or improve much, but the new growth should look better.