Fusarium solani species complex

The Fusarium solani species complex (FSSC) is a group of filamentous fungi in the division Ascomycota, family Nectriaceae. Members are common soil-inhabiting molds. Members of this group are implicated in plant diseases as well as in serious human diseases such as fungal keratitis. This species complex includes at least 50 species. They were originally all considered a singular species Fusarium solani on the basis of morphology, until genetic methods that were able to differentiate between them were developed. Each of them has genetic characteristics that justify their distinction from another, reflecting adaptations to different hosts and habits. For example, the actual Fusarium solani is not associated with bean root rot. Nevertheless, due to the difficulty of distinguishing them without using genetic methods, non-taxonomic sources tend to still call all of them F. solani, sometimes with additional infraspecific designations such as forma and forma specialis. There exists a "lumper and splitter" problem surrounding the genus and FSSC. Neocosmospora in the modern splitter view of Lombard et al. (2015) is equivalent to the FSSC in scope. Species in the genus are saprobes (processing of decayed (dead or waste) organic matter), endophytes (a fungus that lives within a plant for at least part of its life cycle), and plant and animal pathogens and they are commonly found in soil, plant debris, living plant material, air and water (Sandoval-Denis et al. 2019; Guarnaccia et al. 2021).

History and taxonomy

The genus Fusarium was described in 1809 by Link. In the 1930s, Wollenweber and Reinking organized the genus Fusarium into sections, including Martiella and Ventricosum, which were collapsed together by Snyder and Hansen in the 1940s to form a single species, Fusarium solani; one of nine Fusarium species they recognized based on morphological features. "F. solani" in this historical, morphological classification actually constitutes a species complex consisting of multiple, closely related and morphologically poorly distinguishable, "cryptic" species with characteristic genetic differences. This species complex includes the sexual species, Nectria haematococca, in the family Nectriaceae (phylum Ascomycota). There is a "lumper and splitter" problem surrounding the anamorphic genus Fusarium, which affects the taxonomy of the genus. The "lumper" view holds that there should be a large Fusarium that would include all of this complex. It is more similar to the traditional taxonomy used by disease specialists and has more acceptance among them. The "splitter" view holds that the existing finer splits based on teleomorphic morphology should be respected and revised. In such a view, genus Neocosmospora should be adjusted to all of the species complex. Both views are based on molecular phylogeny, with the trees largely agreeing with each other; the question is merely where to draw the boundaries on the trees. As of March 2026:

MycoBank appears to mostly follow the splitter view, with Neocosmospora species names generally being accepted over the Fusarium names. Species Fungorum appears to largely either prefer the Fusarium over the Neocosmospora name or accept both.

Phylogeny Internal phylogeny, three loci (tef1, rpb2, and rDNA), maximum likelihood:

Notes and abbreviations:

"FSSC C." is the FSSC clade numbering [FSSC (number)] is a historical numbering used in the early stages of differentiating FSSC species (M) indicates medically important species Data derived from TreeBase study S27101, tree Tr129542, which does not fully match the paper's Figure 2. External relations (phylogeny of Netriaceae clade IX), using the "splitter" genus concepts:

For more extensive synonymy lists, consult the two rival nomenclature papers.

Taxonomy Neocosmospora was established by Smith in 1899. As accepted by Species Fungorum;

Growth and morphology Like other species in its genus, FSSC produces colonies that are white and cottony. However, instead of developing a pink or violet centre like most Fusarium species, FSSC becomes blue-green or bluish brown. On the underside, they may be pale, tea-with-milk-brown, or red-brown. However, some clinical isolates have been blue-green or ink-blue on the underside. FSSC colonies are low-floccose, loose, slimy, and sporadic. When grown on potato dextrose agar (PDA), this fungus grows rapidly, but not as rapidly as Fusarium oxysporum. In PDA, FSSC colonies reach a diameter of 64–70 mm in 7 days. FSSC has aerial hyphae that give rise to conidiophores laterally. The conidiophores branch into thin, elongated monophialides that produce conidia. Phialides that produce macroconidia are shorter than those that produce microconidia. The macroconidia produced by FSSC are slightly curved, hyaline, and broad, often aggregating in fascicles. Typically the macroconidia of this species have 3 septa but may have as many as 4–5. Microconidia have thickened basal cells and tapered, rounded apical cells. However, some FSSC isolates have pointed, rather than rounded, macroconidia. Microconidia are oval or cylindrical, hyaline, and smooth. Some microconidia may be curved. Microconidia typically lack septa, but occasionally they may have up to two. Fusarium solani also forms chlamydospores most commonly under suboptimal growth conditions. These may be produced in pairs or individually. They are abundant, have rough walls, and are 6-11 μm. FSSC chlamydospores are also brown and round.

Ecology FSSC is found in soil worldwide. However, a given species within the complex may not be as widespread and may not have the same ecology as others in the complex. In general, as a soil fungus, FSSC is associated with the roots of plants and may be found as deep in the ground as 80 cm. It is frequently isolated in tropic, subtropic, and temperate locations, and less frequently isolated from alpine habitats. The pH of soil does not have a significant effect on FSSC, however, soil fumigation causes an increase in occurrence. FSSC is typically sensitive to soil fungicides. FSSC has been found in ponds, rivers, sewage facilities, and water pipes. It has also been found in larvae and adults of the picnic beetle, is a symbiote of the ambrosia beetle.

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