Sk1nRanswered grow question 4 years ago Having had the same I done some research and asked a similar question.
The research said it could be fungal, if lower leaves are dying and rust comes off on your hands
Nute defiances - again lower leaves dying and leaves curling
Or nute burn.
When I posted on here they seen I was in week 10 and said it was cal mag leaving the plant.
Not sure if this will help just my experience! Hope it does though
I believe it's cal mag leaving the plant as I've reduced it in the feed as per my remo food chart