Verticillium dahliae and Verticillium albo-atrum
Verticillium Wilt is a common fungal disease of tomato and squash family crops. It prefers warm (not hot) and wet conditions. The fungus enters the plant through natural openings and wounds in the roots and grows up into the stem, where it blocks the supply of nutrients and water to the leaves. This causes the tops to wilt in sunny afternoons, though they recover when it cools down (this often starts to happen when plants begin bearing fruit). Only a part of the plant is infected initially, but it eventually spreads through the whole plant. The lower leaves usually turn yellow, die and drop. The stem often becomes discolored as with Fusarium Wilt, but this is usually a lighter color. Tomatoes don’t necessarily die from Verticillium Wilt, but it slows their growth and reduces yields. Peppers are commonly killed though. With potatoes the tops die off prematurely (it’s also known as early dying fungus), so the crop of tubers is reduced (these often show some discoloration and don’t store well).
If this disease is a persistent problem the easiest way to deal with it is to plant resistant varieties. Many modern tomato (and some potato) varieties are very resistant. Unfortunately many of the more interesting older heirlooms aren’t resistant. To minimize the effects of the disease you should keep the plants well fed and watered. Also keep the soil cool with mulch, as it prefers warm conditions. If any plants start to show symptoms of partial wilting, remove them immediately to reduce the spread of this disease. The spores overwinter on crop debris, so keep the beds clear. The spores can survive in the soil for up to 7 years.
Image: Howard F. Schwartz, Colorado State University, Bugwood.org