Fungi that inhabit the soil and infect the plant through the roots cause both Verticillium wilt and Fusarium wilt. Damage results from the pathogen invading the water-conduction tissues (xylem) of the plant. Although symptoms of these two diseases are similar, Fusarium wilt tends to occur around midsummer when air and soil temperatures are highest. Often one side of the plant or branch will turn yellow and if sliced open, the xylem tissue in the stem will appear reddish brown. Symptoms of Verticillium wilt are similar, except the wilting is not restricted to one side of the plant of branch and this wilt tends to occur in late spring.
In all three of these cases, affected plants should be immediately removed from the garden.