Fungal Problem On Cabbage

There are several fungal problems of cabbage and without culturing the tissue in a lab it's hard to say which one is harming your plants. A good pdf handout is available here: nhb.gov.in/vegetable/cabbage/cab002.pdf

If these were my plants I'd get rid of all that are affected. Any plants that are still OK I'd spray with either copper fungicide or Actinovate - both organic treatments. I'd also be sure I was only watering deeply once a week - frequent splashing of water on foliage can lead to any number of fungal problems. (Don't know if this applies to you but the worst way to water is frequently by hand, getting the plants wet as well as the ground. Better to use a soaker hose or sprinkler once a week for a long time.) Actinovate also makes a product to put on the soil - it's a natural bacteria that out-competes fungi. In the space where the cabbage was, there is time to plant Kale seeds, carrots, radishes or any lettuce.