Fusarium diseases of tomato plants (Solanum esculentum) are known to be mainly caused by two formae speciales of the morphospecies Fusarium oxysporum, F. oxysporum f. sp. radicis-lycopersici (FORL), responsible for crown and root rot, and F. oxysporum f. sp. lycopersici (FOL), responsible for a vascular wilt disease. Members of the species complex F. solani may cause some crown and root rots of solanaceous plants such as potato and tomato under tropical conditions (Romberg & Davis, 2007; Blancard et al., 2009) and some other species may be potentially responsible for pre-emergence damping off of various crops including tomato (Palmero et al., 2009). In contrast to these species, formae speciales FORL and FOL have strict host specificity. Within FOL, three races have been identified, differentiated by their specific pathogenicity to different cultivars carrying specific resistance genes (Grattidge & O’Brien, 1982). However, no races are known within FORL. Both formae speciales display a considerable genetic diversity. Like many formae speciales in F. oxysporum, FORL and FOL are composed of several vegetative compatibility groups (VCG) and are known to be polyphyletic (Kistler, 1997; O’Donnell et al., 2009). To date, nine VCGs have been identified among FORL isolates (VCG 0090 to 0094 and 0096 to 0099) and five VCGs among FOL isolates (0030 to 0033 and 0035) (Katan, 1999; Cai et al., 2003). The two formae speciales can be identified and distinguished by time-consuming pathogenicity tests on tomato and by the disease symptoms. VCG can also help identify formae speciales but this method requires obtaining mutants and pairing them with reference strains until a heterokaryon is detected, which is also very time-consuming (Correll et al., 1987). However, VCG grouping remains the reference method to characterize pathogenic strains of F. oxysporum because it makes it possible to compare new isolates with reference strains and previously collected isolates. Recently, a rapid molecular method has been proposed to identify FOL, based on the PCR amplification of specific markers corresponding to the SIX genes (secreted in xylem) (van der Does et al., 2008; Lievens et al., 2009). Genetic diversity within formae speciales can be effectively characterized by targeting different DNA regions, the ribosomal intergenic spacer (IGS) being one of the most polymorphic (Lori et al., 2004; Abo et al., 2005; O’Donnell et al., 2009).
Fusarium diseases of tomato have caused severe damage in tomato growing areas all over the world. During the past decade, diseases caused by FOL have decreased in European countries due to the availability of resistant cultivars. However, resistance genes can be overcome, resulting in the emergence of new races. Only tolerant cultivars are available for FORL but the disease can be effectively prevented using grafted material (Blancard et al., 2009). However, this strategy is not always used and fusarium disease due to FORL is still present in many Mediterranean countries including Turkey (Can et al., 2004) and Malta (Porta-Puglia & Mifsud, 2005), where it was recently reported for the first time.
Tomato is an important and common crop in North Africa. Contrary to the decline in several other countries, fusarium diseases are still frequent in tomato-growing areas in Tunisia (Hibar et al., 2007). Similarly, an outbreak of fusarium diseases of tomato was recently discovered in Algeria (N. Hamini-Kadar, University of Oran, Algeria, personal communication). However, nothing is known about the pathogen(s) responsible for this outbreak of disease. This study was therefore initiated to determine: (i) whether the pathogens are diverse or are composed of a limited number of genotypes; and (ii) whether they are clonally related to other known strains of FOL and FORL found in neighbouring countries or are newly emergent pathogenic forms. These questions were addressed by characterizing the genetic diversity of the fungal isolates responsible for fusarium diseases in Algeria, and by comparing them with isolates originating from other Mediterranean countries.