Hey bro,
I have the same problem on my plants {one strain good, the other one (more sativa) burned}. I agree the answer of my growmies, but for me it's a mistake to cut the leaves "infected" before you see the real improvement of you plant health (after flushing)
Wait a stabilisation, and at this moment, cut the dead leaves ;)
Why ? Because if the problem is not solved, you will kill all the new leaves, the plant need a way to eject the waste of nutrients.