The fungus Fusarium oxysporum is very common in the soil, where the majority of the population lives as a saprophyte (a harmless organism that feeds on dead organic matter). However, there are also a large number of pathogenic strains, which are able to infect different plants.
The pathogenic strains are known as formae speciales, abbreviated to f.sp. For example, Fusarium wilt of Callistephus is caused by Fusarium oxysporum f.sp. callistephi.
Infection usually occurs through the roots. The xylem vessels are blocked due to a combination of fungal growth and the production by the plant of gums and gels in response to the attack. The fungus also produces toxins that are carried up in the water stream.
Fusarium wilts are often diseases of hot weather and warm soils, although some can be troublesome at lower temperatures. They can be introduced into a garden on infected seeds, cuttings and young plants. The fungus produces long-lived resting spores (chlamydospores) that contaminate the soil for several years.
There are also strains of Fusarium oxysporum that affect bulbous plants such as daffodils, onions, tulips, irises and lilies. Rather than causing typical wilt symptoms as described above, on these plants the symptom consists of a decay of the bulb itself (often from the base), so the disease is usually known as basal rot or bulb rot.