It was recently discovered that infected tomato plants were sold in big box store garden centers throughout the Northeast. The plants are infected with the causal agent of late blight, Phytophthora infestans, a fungus-like brown alga that's closely related to diatoms and kelp. Tomatoes, potatoes and their relatives are extremely susceptible to this pathogen, which was responsible for the Irish Potato Famine. I saw this pathogen decimate potato fields when I interned at a fungicide company after college. The plants just melted into rotting, black piles.
If you have infected plants in your garden you should destroy them and consider safely applying a commercial fungicide. (Don't ever compost sick plants (or weeds), since their spores will often survive or multiple). The guys at our experimental farm have already started spraying our fields, which is a real shame. They usually don't have to spray until late in the summer, but we risk losing a year's worth of data without it. Our cool, wet, cloudy summers in Central New York are perfect for this disease.