Introgression of Septoria Leaf Spot Resistance from Wild Solanum Species into Cultivated Tomato via Ovule Rescue, Bridging Backcrosses, and Identification of Candidate Resistance Loci.

Septoria leaf spot (SLS), caused by Septoria lycopersici Speg., is an important and increasingly prevalent foliar disease of tomato (Solanum lycopersicum L.) with no commercially resistant cultivars available. We screened 22 cultivated and wild accessions against a prevalent S. lycopersici isolate (WV21-1) and identified S. peruvianum LA2744 and S. corneliomulleri LA1910 as highly resistant (score 1 on a 1-to-5 scale), with S. arcanum LA1984 as resistant (score 2), whereas cultivated accessions scored 4 to 5. Using an optimized ovule rescue protocol with a 25-to-35-day rescue window after pollination, we recovered five confirmed interspecific F1 hybrids from 34,475 cultured ovules. Hybridity was validated with a 24-marker genome-wide cleaved amplified polymorphic sequence (CAPS) panel, which also identified two self-escapes. Most F1 plants exhibited post-zygotic incompatibility, preventing direct backcrossing. Using of cv. Micro-Tom as a bridging parent overcame this barrier, yielding approximately 17 BC1 plants per 100 pollinations. Segregating populations displayed continuous SLS score distributions and high broad-sense heritability (H2=0.80-0.84), consistent with quantitative resistance. Composite interval mapping identified putative resistance-associated regions. In the S.arcanum-derived BC1 (n=169), a candidate locus on chromosome 8, with the wild allele conferring resistance, was significant in the multiple QTL model (P=0.003; 4.9% variance) but did not exceed the genome-wide threshold. In the S. peruvianum-derived ΨF2 (n=354), no locus reached significance. Beyond mapping, this study delivers an integrated, reproducible pipeline (donor screening, species-specific ovule rescue, low-cost CAPS-based hybrid validation, and Micro-Tom bridging) that converts wild SLS resistance into validated, backcross-ready material and nominates chromosome 8 for fine-mapping.