This looks like it is either a fungal or bacterial disease. I think the most likely candidates are Pseudomonas syringae (bacteria) which causes "bacterial brown spot" or Xanthomonas campestris (bacteria) which causes "common blight". Pseudomonas can be spread by contaminated tools and also in irrigation water and it usually comes from other plant hosts, where it grows on leaf surfaces. Xanthomonas can come in on the seeds, but can also come from other plants, tools, or be spread by humans touching infected plants and then touching healthy plants. I think if you google those options you will be able to find some photos and compare to the damage you are seeing on your plants.