The last two years (which were also my first two years) trying to grow tomatoes, the leaves of my plants started getting yellow, spotted, and shriveling sometime in July. After doing a little research last year I determined they must be suffering from either blight or septoria leaf spot, and that this year I should start using a fungicide before it gets too hot and humid out.

Not knowing whether my plants are suffering from septoria leaf spot, early blight, or late blight, I'm not sure what kind of fungicide to buy. Can anyone help identify this and recommend what kind of fungicide I should use?

I've attached a few pictures that were taken in late July last year.