Blight is a fungal disease that spreads when tomato leaves touch or stay close to damp soil. Spores splash up during watering or rain and infect the lowest leaves first, working upward if untreated.  To be clear, this disease lives and overwinters in the soil.  It normally starts at the bottom, the mature leaves, and works its way up.  If left to continue will cause lesions on the stem, which in a severe infestation can girdle the stem.  Eventually fruit develops brown leathery spots usually near the stem end

How to Fix It 

Keep leaves off the ground.  Aim for at least 6 inches of stem with no leaves, in a mature plant. but begin early by removing anything touching the soil.   Water splashing up from the soil onto the leaves is another way it is transmitted.  Water at the base rather than overhead.  Remove and destroy any spotted lower leaves as soon as you see them.  Mulching around the base also helps.  

Since blight is a fungus, fix it with antifungals.  Examples are a baking soda spray or Peroxide spray which alters the pH making the environment uninhabitable.  Also yogurt and milk sprays act as antifungal.