Late blight affects tomatoes, peppers, eggplant and some other vegetables. It appears as irregular, small pale to dark green water-soaked spots that are surrounded by a zone of yellowish tissue. Lesions may expand rapidly and become brown to purplish black. Fruit discoloration usually begins on the upper side and remain firm. If you suspect that a plant has this disease, have a knowledgeable garden member confirm it.
This is a serious disease so prompt action is necessary. Pull out the plant and dispose in a plastic bag to avoid spreading the disease to other nearby plants.
Source: http://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/lateblight.html .