The spores of the fungus ... are often present in the soil, and small outbreaks are not uncommon in August and September. But the cool, wet weather in June and the aggressively infectious nature of the pathogen have combined to produce what Martin A. Draper, a senior plant pathologist at the United States Department of Agriculture, described as an “explosive” rate of infection.
The article says that William Fry, a plant pathology professor at Cornell, has been genetically tracking the fungus and says its spread is due in part to "hundreds of thousands" of plants bought at Walmarts, Kmarts, Home Depots, and Lowe's—all supplied with seedlings by Bonnie Plants. Bonnie Plants has since recalled remaining seedlings. [What to look for, after the jump.]
What to Look For
A strain of the P. infestans fungus was responsible for the Ireland's Great Potato Famine starting in 1845. According to Cornell's Integrated Pest Management Program, tomato fruit affected with late blight "causes a firm, dark, greasy-looking lesion from which the fungal spore producing structures emerge under humid conditions."
On leaves and stems, look for:
Black lesions appear within 3 to 7 days of infection of leaves. Under humid conditions, delicate, whitish fungal spore producing structures are produced at the edge of the lesion, particularly on the underside of the leaf. Lesions turn brown when they dry up,and are often surrounded by a halo of gray-green tissue.
If your plants are affected, the Times article says, you should pull it from the ground, seal it in a plastic bag, and throw it in the trash. For the love of God, whatever you do, DON'T COMPOST IT!