Its either the most severe K def I have ever seen, but more than that it looks like a fungus, but after dealing with the shit (leaf fungus) for 6 months everything starts to look like it.
Lets rule out K def. first
Makle sure Ph is straight, above 5.2 hydro, and above 6 soil. k is most soluable. Just about all nute programs provide adequate levels of K for cannabis. so if u follow a program like general hydro, or whatever and ur ph is in check its prob not K def.
If it is fungi, once it gets that bad it hard to control, in its early stages its easily contained with proper environment conditions. h2o2 in feed water will help sterilize the water, especially in a recirculating system.
I have been trying daily applications of h2o2 diluted to 1% in ro water, sprayed right on the plants top to bottom, I used it as strong as 5% with no negative effects on a test plant. I have heard good things about h2o2 beating plant fungus, but it takes a miracle along with it tons of persistence, and nearly perfect plant conditions until it is completely eliminated.. Keep me posted if u figure out or need some more help trouble shooting.