There's probably not a lot you can do about it this late in the growing season, but you can control the wilt from spreading to most of the plant. I use a fungicidal soap and/or a copper solution. A good source for these sprays is the Gardens Alive website.
First, though, remove all the yellow and brown leaves and stems from the plants and the soil, and throw them in the garbage (not the compost heap). Wilts are a soilborne fungus, and you don't want to spread it in your compost. Keep the garden weeded and mulched.
Next season, look for plants that are rated VF -- resistant to fusarium and verticillium wilt. Make sure the plants are well caged or staked, and remove the first two or three leaf stems closest to the ground for air circulation.