Agree with Alex, it could be black rot disease. To confirm the disease, we request you do some tests like
1. Look for discoloration (black) of the vascular tissue of leaf petiole.
2. Or you can do ooze test by cutting the infected leaves into small bits and put into water in a glass tube, if it is bacterial disease you will see the streaming of bacteria (image attached for your reference).
Bacterial streaming from cut tissue (ooze test)
Symptoms of black rot on cabbage leaf
Blackening of water-conducting tissues of the stem due to bacterial black rot
Oozing of bacteria from infected tissue