GMSgrowsanswered grow question 4 years ago This is caused from a lack of phosphorus but you need to know why. To diagnose check your runoff ph and ec levels. If ph is out fix that to flowering levels. If your ec is low and ph is ok then you need to add more flower nute, it's a deficiency. If your ec is high then its lockout starting and you need to just water for a while or flush if its really high. Cheers