Molecular characterization through IGS sequencing of formae speciales of Fusarium oxysporum pathogenic on lamb's lettuce.

Abstract

Twenty-nine strains of Fusarium oxysporum isolated from wilted lamb’s lettuce plants ( Valerianella olitoria ) along with eight ATCC reference strains were examined for differences in the nucleotide sequences of the ribosomal DNA (rDNA) intergenic spacer (IGS) region, which is about 2.5 kb long in these strains. A phylogenetic (neighbour-joining) analysis was performed on the strains and identified four phylogenetic groups, I, II, III, and IV. Most F. oxysporum isolates recovered from wilted lamb’s lettuce plants cultivated in northern Italy clustered in group I and were very similar to F. oxysporum f. sp. raphani . All the isolates, including the 8 control strains, were tested for pathogenicity on lamb’s lettuce cv. Trophy cultivated in the glasshouse. Most isolates from northern Italy were pathogenic on lamb’s lettuce, displaying varying degrees of virulence, and their IGS sequence were similar to the forma specialis raphani . However, four isolates were not pathogenic on lamb’s lettuce. Among the control strains, six showed moderate virulence; these belonged to the formae speciales raphani and conglutinans ; and two, belonging to the forma specialis matthioli (ATCC16602 and ATCC16603) were not pathogenic on lamb’s lettuce. In conclusion, analysis of the IGS sequences indicated that the isolates studied had different origins, and that phylogeny and pathogenicity were related; non-pathogenic isolates differed genetically from isolates that were poorly, moderately or highly virulent. To the best of our knowledge, this is the first report on the differences between formae speciales of F. oxysporum on lamb’s lettuce plants as determined by IGS sequence analysis.