Soil-borne fusarium fungi strip away the plants' fine feeder roots, so they are no longer able to keep leaves and fruits supplied with moisture and nutrients. As the disease progresses, the fungi clog up the plant's vascular system, so it is further handicapped in its struggle to stay alive.
Grow resistant varieties if fusarium is known to be present in soils in your area. If you want to grow susceptible heirloom varieties, grow them in containers filled with bagged soil.
Pull up affected plants discard them in the trash, or in a very active compost pile.