Alternaria leaf spot I would say.
You can still grow that plant but be upon it like a hawk: as a first thing try avoiding overhead watering, wash your hands after touching affected plants and if the disease grows worse immediately buy a chlorothalonil-based fungicide and spray the plants with it every 10 days to contain the disease. Suspend treatments at least one week before harvesting.
Now the bad news: avoid growing other brassica in that area for the next three years. Alternaria is a soil-borne pathogen and can remain active in the ground for a long time.
