It looks very similar to black leaf spot caused by Xanthomonas spp. There are many species that cause such disease: X. euvesicatora, X. perforans (for warm climate), X. vesicatoria, X. gardneri (for cooler weather), and less common X. campestris pv. raphani & X. arboricola. Correct identification of the species is possible by PCR, ELISA, or microbiological analysis of isolated bacteria. The disease can be seed-born.