Fusarium oxysporum

Fusarium oxysporum is a soil-dwelling fungus that causes what is known as Fusarium wilt. After the leaves turn yellow, the plant begins to wilt—often starting with the oldest leaves and initially on only one side. Eventually, the plant dies.

There are now resistant tomato varieties, since fungicides tend to harm the soil flora rather than control the fungus. Infected plants should be disposed of separately, as the fungus can survive for many years and will be reintroduced into the system if the compost is reused.