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 by decaying dead organic matter). However it also has a large number of pathogenic strains, which are able to infect different plants.
Fusarium oxysporum f. sp. narcissi only infects Narcissus. It is found in the soil, even where Narcissus have not been grown previously, and can be found on the surface of bulbs even when these are apparently healthy. Infections occur through wounds or weaknesses, such as the point at which young roots grow out into the soil from the base of the bulb.
Neck rot occurs when infection enters around the neck, often after flowering has finished.
Infection is sometimes associated with attack by the stem nematode (eelworm) Ditylenchus dipsaci, but the two problems may also occur independently.