It could be Fusarium-
the difference between blight and Fusarium is:
Blight will have leaves that turn yellow/ brown, but there are brown spots on the leaves that have a brown circle around them, like a bulls eye. This is caused by high temps/humidity and not enough rain, or watering that is not consistent. you can add mulch, to help with the maintaining of moisture in the soil, and it can help prevent the spreading of blight. The tomatoes-I would still eat them.
Fusarium -is a fungi -the stems will turn brown, and that prevents the leaves from getting nourishment. The leaves turn yellow-and finally as they all die, the plant will die too. this plant should be pulled up and burned. Do not put it in the compost pile, don't continue the spores...just throw it out in the trash or burn it.
hope that helps.