Characterization of Fusarium oxysporum isolates from Ethiopia using SSR, AFLPs and DNA sequence analyses

Fusarium oxysporum is a common soil-borne fungal species, which is mainly saprophytic. Pathogenic strains, however, cause wilts, crown rots and root rots in a wide range of crop species, often limiting production. As a species, F. oxysporum probably causes more economic damage to agricultural crops than any other pathogen. The species is also implicated in post-harvest rots of some tropical crops such as apio (Arracacia xanthorrhiza), yams (Discorea spp.), and taro (Colocasia esculenta).

Although pathogenic members of F. oxysporum cause destructive vascular wilts in many crops, individual pathogenic strains have limited host ranges. Consequently, the species has been subdivided into formae speciales, each of which affects a particular host or group of hosts. Given the differences among formae speciales in terms of pathogenicity, the ability to distinguish between them assumes economic as well as scientific importance. Identification based on pathogenicity is, however, an expensive and labor intensive task. Moreover, results of pathogenicity tests are often inconclusive as they are affected by a range of factors including the environment and genetic make-up of the host.

Very little work has been done on F. oxysporum in Ethiopian agriculture, despite the significance of this species. To shed some light on the diversity of F. oxysporum in Ethiopia, we studied 32 isolates obtained from agricultural soils. Isolates and/or sequences representing different formae speciales were also obtained from other sources and used here for comparative purposes. We employed DNA-based methods including analyses of DNA sequence data, amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) markers. All the three methods grouped 30 of the Ethiopian isolates into one of the three clades that had previously been described for this species complex (O’Donnell et al., 1998. PNAS, 95: 2044-2049). The remaining two isolates resided in two other clades.

The genetic diversity among the Ethiopian isolates was also low. This finding is consistent with the traditional agricultural system in Ethiopia that relies heavily on local crop varieties (Figs. 1 and 2). This would imply limited opportunities for the introduction of new strains of the fungus via infected seeds. Lack of resolution according to host among the formae speciales confirmed that pathogenicity of isolates does not necessarily correlate with groups based on genetic relationships.

Fig. 1. Tilling using the traditional oxen-drawn plow.	Fig. 2. Traditional way of sowing wheat. Farmers use landraces as seeds.

Related Publications

• Bogale, M., B.D. Wingfield, M.J. Wingfield, and E.T. Steenkamp (2005). Simple sequence repeat markers for species in the Fusarium oxysporum complex. Molecular Ecology Notes 5: 622-624.