I only speculated at that point, having topped for 24 colas, that that specific main root system was strangled out or rotted . I begged for 17 weeks and in a mini hydro bucket, drought and uncontrollable light intensity and distance. It is possible that that leaf on that one was super close to the light in the tent and was a result of a prolonged light burn and reduced ability to take more? I kept guessing as to what it was. I never touched my nutes and kept trucking.