Genetics of Southern Blight Resistance in Tomato

Southern blight caused by Sclerotium rolfsii Sacc. is a soil-borne fungal disease of a wide range of plant species occurring throughout tropical and sub-tropical regions. The main objective of this study was to identify molecular markers associated with resistance to southern blight in tomato.  A mapping population was generated from a cross between a susceptible tomato cultivar (Fla. 7776) and an accession of Solanum pimpinellifolium (PI 126932) with reported resistance to southern blight.  The parent lines, F1 and F2 individuals, and F3 families were assayed for southern blight resistance.  The plants were inoculated at an age of eight weeks by S .rolfsii grown on rye seeds. Disease severity was estimated by scoring individual plants on a visual scale of 0-4 with increasing severity, and overall survival was recorded.  The resistance of PI 126932 and susceptibility of Fla. 7776 was confirmed using the screening procedure.  The percent of surviving individuals increased from 10% in the susceptible parent to 90% in the resistant parent.  The F₁ individuals had resistance that was similar to the resistant parent.  An F₂ population involving 23 extreme resistant and 22 extreme susceptible plants was screened with molecular markers distributed throughout the genome.  A locus was found where PI 126932 alleles were significantly associated with the resistant phenotype. Other loci with modifying effects were also detected.  The markers associated with southern blight resistance were confirmed in F₃ families. It seems that the resistance is acting as a complex binary trait, presumably controlled by multiple genes and environmental factors. These marker-trait associations may help to isolate the genes conferring resistance, and will be useful in marker-assisted breeding for southern blight resistance.