This has the appearance of early leaf blight, but it could be one of several diseases including leaf spot. If it is early blight application of a copper fungicide will slow the spread of the disease.
Viral diseases are usually spread either from seed with the disease present (always purchase seed from a reliable seller) or from thrips that are carriers. Not much else can be done at this point if it is a viral infection.
Fungal disease can be reduced by eliminating spread of the fungus by sanitation practices, cleaning up debris, leaves etc. and by a mulching program to keep soil from splashing up on the plant. Water the soil not the plant (no overhead watering). Sometimes all the right things are done and you will still get blights or wilts or spots or rots. In my opinion, a mulching program and good garden sanitation will be most beneficial. Good luck.