Presented with the wide-ranging occurrence of
F. oxysporum strains that are nonpathogenic, it is reasonable to conclude that certain pathogenic forms were descended from originally nonpathogenic ancestors. Given the association of these fungi with plant roots, a form that is able to grow beyond the cortex and into the xylem could exploit this ability and hopefully gain an advantage over fungi that are restricted to the cortex. The progression of a fungus into vascular tissue may elicit an immediate host response, successfully restricting the invader; or an otherwise ineffective or delayed response, reducing the vital water-conducting capacity and induce wilting. On the other hand, the plant might be able to tolerate limited growth of the fungus within xylem vessels, preceded by an endophytic association. In this case, any further changes in the host or parasite could disturb the relationship, in a way that fungal activities or a host response would result in the generation of disease symptoms.
Pathogenic strains of
F. oxysporum have been studied for more than 100 years. The
host range of these fungi is broad and includes animals, ranging from
arthropods to humans, as well as plants, including a range of both
gymnosperms and
angiosperms. While collectively, plant pathogenic
F. oxysporum strains have a broad host range, individual isolates usually cause disease only in a narrow range of plant species. This observation has led to the idea of "special form" or
forma specialis in
F. oxysporum.
Formae speciales have been defined as "…an informal rank in Classification… used for parasitic fungi characterized from a physiological standpoint (e.g. by the ability to cause disease in particular hosts) but scarcely or not at all from a
morphological standpoint." Exhaustive host range studies have been conducted for relatively few
formae speciales of
F. oxysporum. For more information on
Fusarium oxysporum as a plant pathogen, see
Fusarium wilt and
Koa wilt. Different strains of
F. oxysporum have been used for the purpose of producing nanomaterials (especially
Silver nanoparticles). =="Agent Green" in Colombia==