Verticillium wilt is a soil-borne fungal disease that results in the yellowing, and eventual browning and death of foliage, particularly in branches closest to the soil. The wilt starts as yellow, V-shaped areas that narrow at the leaf margins. These yellow areas grow over time, turn brown, and then the leaf dies. Often, entire branches are infected.
Damage to Plants:
Despite the sickly appearance of verticillium wilt-infected branches, often the upper part of the plant will continue growing, though growth will be stunted. Tomatoes that are growing on infected branches will often drop before reaching maturity, or they will be sunburned because of the lack of shade that the foliage would have provided. Even on branches that are not showing signs of wilt, the tomatoes will be smaller than normal, and often develop yellow shoulders.
Verticillium wilt is the result of a fungus called Verticillium albo-atrum, and is present in most soils in the Northeast U.S., which tend to remain cool for long periods of time. It stays alive for long periods of time by living on the dying underground parts of infected plants. It often attacks, and multiplies, on the roots of common weeds such as ragweed and cocklebur.
Treatment and Prevention:
Once you notice signs of wilt on your plants, there's really nothing you can do to prevent further damage during the current growing season. There are two things to do to ensure crop health in the upcoming gardening season. First, don't plant tomatoes in the same spot for at least three years, but five years is even better. This will allow the fungus in the soil to reduce enough (without tomato roots or other Solanaecae family members to feed on) to make it safe to grow there again. Also, if Verticillium wilt has been a recurring problem in your garden, look for varieties that are wilt resistant. These will be labelled with a "V" somewhere on the plant tag or seed packet, near the variety name.