I had the same problem with bubblegum.
the answer to my problem was.
i cut the nutriants by half for a week and raised the light another couple of inches. after a week it started to cure.
Another thing it could be is dead salt build up,flush the system with fresh water for a day.Then start the process as normal.
mrxthethcman Reviewed by mrxthethcman on . severe yellowing in week 8... I have 3 plants in a flood and drain system...I use GH nutes according to their instructions...I had to pull a late male at 21 days and noticed the plant next to it yellowing...I suspected root shock, so I didnt worry...then it didnt recover in 2 weeks so I suspected magnesium was too low and added epson salts...no recovery and now the other 2 plants are yellowing...it seems that only the older top leaves are affected, so I suspect nitrogen is real issue, and am not gonna decrease the floragrow Rating: 5