Boy, it certainly looks like some sort of disease I'm sorry to say. Leaf speck, leaf spot. Hard to tell from the pic, but is it affecting the stems? Doesn't look like blight, although I wouldn't completely rule out early blight. I do see that your
plants are not mulched. Mulching tomato
plants is essential to aid in controlling diseases as many of them are soil-borne in part. Remove the diseased foliage and discard it. Mulch your
plants with straw, grass clippings, etc., and consider starting a preventative spraying program to thwart any further spread of whatever disease is manifesting itself in your garden.