It's just 1 plant out if the 5. I believe the problem is because its been so hot the last week in my area, 98-102 degree, and because its the biggest in the garden, that the combo of heat and switching to flowering, the 2 feedings a day weren't cutting it for this purp urk. So I added another fan into the garden and added 2 more 20 minute floodings to the timer. I also noticed tgat this purp urk's rockwool cube was way lighter them the others. The last picture when she was healthy was only 3 days ago, the other picture is of the whole garden yesterday.
Got bugs? Fungus gnat larva will eat your roots and a bad infestation will do that. Not saying thats it but it's wort ruling out. Look for little flys mostly around the roots.