Genetics of the Partial Resistance Against Race 2 of Verticillium Dahliae in Lettuce

Lettuce (Lactuca sativa L.) production on the Coastal California is threatened by Verticillium wilt, a soil borne fungal disease caused by Verticillium dahliae that diminishes yield and quality. Two races of V. dahliae were identified on lettuce, race 1 and race 2. Complete resistance to race 1 is conferred by a single dominant gene, but only partial resistance (PR) to race 2 is known. The objective of this research was to determine the inheritance of PR in 99 F₃ families derived from a cross between PI171674 and PI204707 with PR. Plants were inoculated with race 2 isolate VdLs17 in two greenhouse experiments with three replications. The parents and the susceptible cv. Salinas were replicated 11 times. Disease Severity (DS) was rated on a scale of 0 (no root discoloration) to 5 (complete root discoloration and wilting lower leaves). Disease Incidence (DI) was calculated as the proportion of symptomatic plants. The Percentage of Foliar Wilting (PFW) and Days to First Flower (DFF) were also evaluated for each plant. Families were genotyped using the Genotyping-By-Sequencing approach based on digestion with the restriction enzyme AvaII and SNPs were identified using TASSEL 3.0 GBS-pipeline. A genetic map was constructed using JoinMap and QTL analysis was conducted using Composite Interval Mapping feature of QTLNetwork. Both parents (PI171674, DI = 18% and PI204707, DI = 52%) had significantly less disease than Salinas (DI = 71%). In the F₃ families, DI ranged 4% - 70%. Significant differences among F₃ families for DI, DS and DFF were detected, but no families with resistance significantly higher or lower than both parents were found. Three QTLs were identified for DI, DS and PFW on linkage groups (LG) 5, 6, and 8 explaining 4%, 12%, and 6% of the phenotypic variation (R²), respectively. Environmentally specific QTL were detected on LG 3, 4, and 5, demonstrating the occurrence of QTL x environment interactions. DFF had a large effect QTL in LG7 (R²= 30%) and a smaller QTL in LG4 (R²= 4%) that were not linked with resistance QTLs. The presence of more than one QTL indicates that several genes control PR which is strongly influenced by the environment. Large numbers of progeny need to be screened to identify favorable allele combinations using breeding methods that deal with quantitative inherited traits such as Single Seed Descent in order to develop resistant cultivars.