The symptoms that develop on tomato leaves, stems and fruit are quite dramatic, and are very obvious to the naked eye. The infected areas on leaves appear to be water-soaked, varying in size from a nickel up to a quarter, often beginning at leaf tips or edges. They proliferate when the foliage has been exposed to watering, rainfall, or heavy overnight dews. If these infected areas dry out quickly, they may appear lime-green or beige in color.
The edge of the water-soaked area, either on the top or bottom of the leaf surface, will be covered with white fungal growth (mycelium) that contains the spore inoculum (visible with a hand lens). Spores are easily blown to surrounding areas and infect plants and even weed species, in the plant family Solanaceae (the black nightshade family).
Brown to almost black lesions appear on infected stems, and the same lesions will develop on fruit, either directly on the infected plants, or a few days after they are sitting on your kitchen counters
Be aware that there are several other common, but less serious, diseases of tomato and potato. If the infected area has a yellow border and is occurring on the bottom of the plant, it is likely due to either Early Blight or Septoria Leaf Spot. These two diseases are found in home gardens most every year in the northeast, but they rarely kill the plants, and they don't spread long distances.
Please inspect your tomato and potato plants on a daily basis. If Late Blight symptoms are already appearing on plants in your garden, these plants should be removed immediately and put in a plastic bag for disposal. Don't just put the removed plants in a compost pile as spores will still spread from this debris. Your neighbors, not to mention commercial growers, will appreciate your taking this action immediately.