Wow, those are nasty looking! I've never seen plants look that bad. Just because we don't know the reason, we can try to fix it. It's called reading your plants, and you've let this go too long without correction.
Flush. If you're in hydro, change the res. If you're in soil, flush generously.
Maybe even spray the foliage with plain water once a day. I don't think that damaged leaves will come back, but they might. What you have to decide is if you have enough room for new growth to develop before flowering. If not, then start over.

HTH

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