Host Range and Molecular Characterization of the Four Races of Fusarium oxysporum f. sp. lactucae.

Abstract

Vascular wilt of lettuce is caused by strains of the Fusarium oxysporum species complex. According to their biological activity, these strains are members of the forma specialis lactucae (FOL). The aim of this study was to characterize strains of F. oxysporum 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 rpb2, tef-1α, cmdA, and tub2 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 (Lactuca sativa), lamb's lettuce (Valerianella locusta), spinach (Spinacia oleracea), wild rocket (Diplotaxis tenuifolia), and cultivated rocket (Eruca sativa). 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 F. curvatum, strains of race 2 with F. curvatum and F. odoratissimum, and strains of race 3 with F. cugenangense. 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.