Host Range and Molecular Characterization of the Four Races of <i>Fusarium oxysporum</i> f. sp. <i>lactucae</i>

Vascular wilt of lettuce is caused by strains of the <i>Fusarium oxysporum</i> species complex. According to their biological activity, these strains are members of the <i>forma specialis lactucae</i> (FOL). The aim of this study was to characterize strains of <i>F. oxysporum</i> isolated from lettuce in Italy and other countries, belonging to the four currently known races. Strains were analyzed based on pathogenicity on different hosts and a combined phylogenetic analysis of <i>rpb2</i>, <i>tef-1α</i>, <i>cmdA</i>, and <i>tub2</i> genes. For uncharacterized strains, the race was determined through pathogenicity tests with a set of differential lettuce cultivars for races 1 and 4. To assess the host range, pathogenicity tests were performed on five species: lettuce (<i>Lactuca sativa</i>), lamb's lettuce (<i>Valerianella locusta</i>), spinach (<i>Spinacia oleracea</i>), wild rocket (<i>Diplotaxis tenuifolia</i>), and cultivated rocket (<i>Eruca sativa</i>). Most FOL strains were pathogenic on lettuce with different levels of disease severity; in particular, FOL races 3 and 4 showed the highest level of virulence on lettuce. FOL races 3 and 4 were pathogenic both on lettuce and lamb's lettuce, whereas races 1 and 2 were pathogenic only on lettuce. Furthermore, this study identified the first occurrence of FOL race 4 in Italy to 2016, as the strain FOL 1/16 was assigned to race 4. Results of multigene phylogenetic analysis showed that FOL races 1 and 4 cluster with the species <i>F. curvatum</i>, strains of race 2 with <i>F. curvatum</i> and <i>F. odoratissimum</i>, and strains of race 3 with <i>F. cugenangense</i>. Phylogenetically, race 4 is very close to race 1, but it showed more similarities to race 3 for host range and virulence. This study shows for the first time a broader host range for races 3 and 4 of FOL. The results demonstrate that phylogenetic analysis permits the separation of some races of FOL, but phylogenetic classification alone is insufficient for conclusive separation of races; therefore, it should be accompanied by biological assays based on pathogenicity.