Colonies are usually fast growing, pale or brightly colored (depending on the species) and may or may not have a cottony aerial mycelium. The color of the thallus varies from whitish to yellow, brownish, pink, reddish or lilac shades. Species of Fusarium typically produce both macro- and microconidia from slender phialides. Macroconidia are hyaline, two- to several-celled, fusiform- to sickle-shaped, mostly with an elongated apical cell and pedicellate basal cell. Microconidia are 1- to 2-celled, hyaline, pyriform, fusiform to ovoid, straight or curved. Chlamydoconidia may be present or absent.
Cultures of F. oxysporum showing purple pigmentation and F. subglutinans showing pink pigmentation.
Identification of Fusarium species is often difficult due to the variability between isolates (e.g. in shape and size of conidia and colony color) and because features that are required are not always well developed (eg. the absence of macroconidia in some isolates after subculture). The important characteristics used in the identification of Fusarium species are as follows. Note, sporulation may need to be induced in some isolates and a good slide culture is essential.
1. A pure culture, obtained if possible from a single conidium or hyphal tip isolation.
2. Colony growth diameters on potato dextrose agar and/or potato sucrose agar after incubation in the dark for 4 days at 25C.
3. Culture pigmentation on potato dextrose agar and/or potato sucrose agar after incubation for 10-14 days with a daily exposure to light.
4. Microscopic morphology including shape of the macroconidia; presence or absence of microconidia; shape and mode of formation of microconidia; nature of the conidiogenous cell bearing microconidia; and presence or absence of chlamydoconidia.
Most Fusarium species are soil fungi and have a world-wide distribution. Some are plant pathogens causing root and stem rot, vascular wilt or fruit rot. Other species cause storage rot and are important mycotoxin producers. Several species, notably F. oxysporum, F. solani and F. moniliforme, are recognized as being pathogenic to man and animals causing mycotic keratitis, onychomycosis and hyalohyphomycosis, especially in burn victims and bone marrow transplant patients.
Clinical manifestations of hyalohyphomycosis caused by Fusarium; include cutaneous and subcutaneous infections, endophthalmitis, osteomyelitis, and arthritis following traumatic implantation. Peritonitis has also been reported in patients on continuous ambulatory peritoneal dialysis (CAPD). Disseminated infections are similar to disseminated aspergillosis, however fungemia and ulcerated skin lesions are often more pronounced. The typical patient is granulocytopenic and receiving broad-spectrum antibiotics for unexplained fever.
Booth, C. 1977. Fusarium: laboratory guide to the identification of the major species. Commonwealth Mycological Institute, Kew, Surrey, England.
Burgess, L.W., and C.M. Liddell. 1983. Laboratory manual for Fusarium research. Fusarium Research Laboratory, Department of Plant Pathology and Agricultural Entomology. The University of Sydney, Australia.