Molecular phylogenetics and population genetics of Fusarium oxysporum f. sp. radicis-lycopersici and its management by silicon amendment

Fusarium oxysporum f. sp. radicis-lycopersici , the causal agent of Fusarium crown and root rot of tomato, is an important soilborne pathogen. The objectives of this study were to investigate the population genetics of the pathogen, to conduct phylogenetic and mating-type analyses, and to evaluate the effect of silicon (Si) on disease severity. Twenty seven microsatellite loci were acquired from a bioinformatics approach and a microsatellite enrichment procedure. Ten of these 27 microsatellites along with vegetative compatibility group (VCG) assays revealed migration of the pathogen among three main tomato-growing regions in Florida. Hendry County had a higher overall average gene diversity than Manatee and Collier Counties. However, the highest mean number pairwise differences and average gene diversity of either VCG 0094 or 0098 were exhibited in Collier County, suggesting that these two VCGs might have migrated from Collier County or other regions to Manatee and Hendry Counties. VCG 0098 probably diverged from VCG 0094 according to VCG and phylogenetic analyses. Although VCG 0094 is still predominant in Florida, VCG 0098 possesses a higher virulence and increased frequency, suggesting that VCG 0098 may be helpful for screening resistant tomato lines. The complete region of intergenic spacer (IGS) provided more phylogenetic resolution than translation elongation factor (EF-1alpha) and FOL185, a noncoding microsatellite locus. At least two evolutionary origins were revealed based on these three loci: VCGs 0094, 0098, and 0099 likely originated independent of the other VCGs. Each VCG carried a unique mating-type idiomorph but no perithecia were found after crossing isolates with opposite mating-type idiomorphs, suggesting that other factors may be required for sexual recombination. Si amendment significantly reduced disease severity of the stem due to delaying initial infection of roots and the basipetal movement of the pathogen from infected roots to stems. The increase in the Si content of roots correlated significantly with the reduction of disease severity. This study suggested restricting genotype flow of F. oxysporum f. sp. radicis-lycopersici for disease management, regularly monitoring the population structure of the pathogen, and further studying the application of Si fertilizers for controlling Fusarium crown and root rot in field-grown tomatoes.