Cultural characteristics, pathogenicity and genetic diversity of Fusarium oxysporum isolates from tobacco fields in Spain

Several formae speciales of Fusarium oxysporum are capable to produce disease in tobacco plants. Different authors have classified those isolates as a forma specialis or a race within on the basis of the severity of disease and host specificity. Fusarium wilt of tobacco plant in Extremadura (central Spain) tobacco fields have been recorded in the last years and F. oxysporum was isolated from symptomatic plants. The aim of our study was to characterize these F. oxysporum populations. For this purpose, the in vitro spore production and growth and the virulence (severity of disease) have been tested. Although all isolates behaved as pathogen, the virulence of isolates was different. The differences in growth could not be correlated with other characteristics but the two isolates with scarce spore production have also behaved as the weakest pathogen. We have analyzed intergenic spacer (IGS) region polymorphism of ribosomal DNA and random amplified polymorphic DNA (RAPD) markers to assess the genetic diversity within F. oxysporum isolates. These molecular analyses showed two major groups with different physiological capabilities that could reflect two different lineages. One group was characterized by medium–high sporulation, high virulence and the same IGS-RFLP pattern. The other group was more heterogeneous featuring low–medium sporulation and variable virulence and growth. This first experimental approach to pathogen population could be a good starting point for further studies including non-pathogenic isolates and a larger number of pathogen that could clarify if there are two or more genetic lineages.