Different species of Fusarium cause vascular wilt of lettuce: molecular and pathogenicity approach

Strains of Fusarium oxysporum species complex are causal agents of vascular wilt of lettuce. Destructive vascular wilts, leaf yellowing with necrosis, stunting and plant death are the most common symptoms of Fusarium wilt on lettuce. According to pathogenicity, the isolates causing this disease are considered as members of the forma specialis lactucae (FOL). The aim of this study was to characterize Fusarium species causing wilt of lettuce from Italy and other countries based on morphology, pathogenicity, and combined phylogenetic analysis of rpb2, tef-1α, cmdA and tub2. Eighteen Fusarium spp., isolated from lettuce, were selected as representative of the four known races of FOL. Results of multigene phylogeny showed that Fusarium strains of race 1 and 4 belong to F. curvatum, strains of race 2 to F. odoratissimum and strains of race 3 to Fusarium cugenangense. Pathogenicity was performed on five species to confirm the Koch’s postulates: lettuce cv. Romana Verde, lamb’s lettuce cv. Trophy, spinach cv. Zebu, wild rocket cv. Winter and cultivated rocket cv. Coltivata. Pathogenicity tests showed that all the studied isolates were pathogenic on lettuce. Moreover, isolates of FOL race 3 and race 4 were pathogenic not only on lettuce but also on lamb’s lettuce. Phylogenetically, race 4 is very close to race 1, but, by considering the host spectrum, it shows some similarities to race 3.