We had an early warning of the late blight in early July when the Washington Post ran this article. Late blight, as the name implies, is typically a problem later in the growing season after most of the tomatoes have been harvested.
The ample rain, mild temperature and high humidity of early summer was an environment favorable for the development of Phytophthora infestans, also known as late blight. Add the increase in backyard gardening and infected tomato plants being sold at big box stores and you get tomato Trojan horses spreading late blight throughout the Northeast.
Late blight also affects other members of the nightshade family such as potatoes (and is the same fungal disease that decimated the potato crop in Ireland leading to widespread famine in the 1840s).
Read more in this recent NY Times op-ed piece – You Say Tomato, I Say Agricultural Disaster.