Evaluation and <scp>QTL</scp> mapping of resistance to powdery mildew in lettuce

Abstract

Lettuce ( L actuca sativa ) is the major leafy vegetable that is susceptible to powdery mildew disease under greenhouse and field conditions. Quantitative trait loci ( QTL s) for resistance to powdery mildew under greenhouse conditions were mapped in an interspecific population derived from a cross between susceptible L . sativa cultivar S alinas and the highly susceptible L . serriola accession UC 96 US 23. Four significant QTL s were detected on linkage groups LG 1 ( pm‐1.1 ), LG 2 ( pm‐2.1 and pm‐2.2 ) and LG 7 ( pm‐7.1 ), each explaining between 35 to 42% of the phenotypic variation. The four QTL s are not located in the documented hotspots of lettuce resistance genes. Alleles for the disease resistance at the four QTL s originated from both parents (two from each), demonstrating that even highly susceptible accessions may provide alleles for resistance to powdery mildew. These QTL s appeared to operate during limited periods of time. Results of the field trials with F 2:3 and F 3:4 families derived from a S oraya (moderately resistant) × S alinas cross demonstrated effective transfer of resistance to powdery mildew in this material. An integrated rating approach was used to estimate relative levels of resistance in 80 cultivars and accessions tested in a total of 23 field and greenhouse experiments. Generally, very low resistance was observed in crisphead‐type lettuces, while the highest relative resistance was recorded in leaf and butterhead types. Comparison of two disease assessment methods (percentage rating and the area under the disease progress steps, AUDPS ) for detection of QTL s shows that the two approaches complement each other.