Are formae speciales pathogens really host specific? A broadened host specificity in Fusarium oxysporum f.sp. radicis-cucumerinum.

Phytopathogenic strains of Fusarium oxysporum Schlecht exhibit clear host specificity, which appears to be a persistent characteristic and a dependable base for the forma specialis system of these pathogens. Here, we report an altered host specificity of the F. oxysporum f.sp. radicis-cucumerinum strain V03-2 g (Forc V03-2 g) - a causative agent of cucumber root-rot, the clonal derivates of which acquired the ability to infect tomato plants. Since the clonal derivates of Forc V03-2 g with transformed host specificity preserved their ability to parasitize on cucumber plants, the changes that occurred can be classified as broadening of host specificity. To our knowledge, this is the first observation of pathogenicity changes in formae speciales of F. oxysporum. The clonal derivates acquired could be used to trace genetic determinants of the host specificity of phytopathogenic strains of F. oxysporum.