There does not seem to be a way to detect when a cotton crop has died.
Even when the cotton crop has clearly died due to water stress, smi at 0.1 and lost all its leaves, and the biomass has stopped increasing etc. but the roots will still keep on growing and the plant_status still remains alive.
It looks like ozcot only has two states for plant_status (alive and out).
This tends to cause problems with all your manager rules that use plant_status = 'alive' in their if statements.
Perry mentioned that you have to manually write your own rules to kill the crop. I think there is also a defoliant manager command.
Ozcot does not die of its own natual causes due to water stress.It would be good to come up with some sort of natural death of cotton.