Septoria Leaf Spot (SLS), caused by the fungal pathogen Septoria lycopersici, is a highly destructive foliar disease affecting tomatoes. SLS is most severe in the Northeast USA and North Carolina during periods of high humidity and elevated temperatures, which can lead to catastrophic yield loss. No quantitative trait loci (QTL) associated with SLS resistance have been reported. Therefore, the objective of this study was to map the QTL related to SLS resistance in tomatoes. An F2:4 mapping population consisting of 189 individuals derived from NC123S (susceptible) x Wisconsin 55 (moderately resistant) was assessed under both field and greenhouse conditions through artificial inoculation with a spore concentration of 15.3 x 10^4/mL at the Mountain Horticultural Crops Research and Extension Center, Mills River, NC, and Mountain Research Station, Waynesville, NC. The population was genotyped using the SPET (single primer enrichment technology) Allegro targeted genotyping method. SPET-derived SNP (single nucleotide polymorphism) molecular markers were used to construct a linkage map spanning 3810.2 cM. QTL analysis identified 12 QTLs associated with SLS resistance, including two major effects and ten minor effects, typical for at least two environments identified across the genome, explaining phenotypic variation (R² value) ranging from 3.7% to 13.5%. These results demonstrate that the genetic control of SLS resistance is polygenic. This study may provide a foundation for understanding the genetics of SLS resistance and marker-assisted selection (MAS) for transferring SLS resistance genes into elite tomato breeding lines.
