Lactuca sativa plant resistant to Bremia lactucae, wherein the Bremia resistance locus is obtainable from the genome of a wild Lactuca plant, particularly from the genome of Lactuca saligna, and wherein the Bremia resistance locus is a qualitative resistance locus located on chromosome 8 and wherein said allele at the qualitative trait locus in the L. sativa genome contributing to resistance to Bremia lactucae, is obtainable from a plant which has the genetic background of Lactuca sativa line LSA - (1306/SAT x SAT) - 37 - 1 - 3:1, representative seed of which is deposited at NCIMB under Accession No. NCIMB 41625, or from a progeny or an ancestor thereof comprising said qualitative trait locus; wherein the Bremia lactucae resistance locus is genetically linked to at least one marker locus, which co-segregates with the Bremia resistance trait and can be identified in a PCR reaction by amplification of a DNA fragment with a pair of PCR oligonucleotide primers selected from a. primer pair 1 represented by a forward primer of SEQ ID NO: 1 and a reverse primer of SEQ ID NO: 2, b. primer pair 2 represented by a forward primer of SEQ ID NO: 3 and a reverse primer of SEQ ID NO: 4, c. primer pair 3 represented by a forward primer of SEQ ID NO: 5 and a reverse primer of SEQ ID NO: 6; d. primer pair 4 represented by a forward primer of SEQ ID NO: 7 and a reverse primer of SEQ ID NO: 8; and e. primer pair 5 represented by a forward primer of SEQ ID NO: 9 and a reverse primer of SEQ ID NO: 10; and wherein primer pair 1 and primer pair 2 each amplifies an SSR fragment which co-segregates with the Bremia resistance locus, wherein a. primer pair 1 amplifies an SSR fragment of 247 bp; and/or b. primer pair 2 amplifies an SSR fragment of 465 bp; and/or c. primer pair 3 amplifies a DNA fragment within the 247 bp SSR fragment comprising an SNP which segregates with the Bremia resistance locus, particularly a SNP represented by an C for A nucleotide exchange at position 272 in the SSR sequence; and/or d. primer pair 4 and primer pair 5 each amplifies a DNA fragment within the 465 bp SSR fragment, particularly a SNP represented by a G for A nucleotide exchange at position 430 and a T for G nucleotide exchange at position 305 in said SSR fragment, respectively. a. primer pair 1 represented by a forward primer of SEQ ID NO: 1 and a reverse primer of SEQ ID NO: 2, b. primer pair 2 represented by a forward primer of SEQ ID NO: 3 and a reverse primer of SEQ ID NO: 4, c. primer pair 3 represented by a forward primer of SEQ ID NO: 5 and a reverse primer of SEQ ID NO: 6; d. primer pair 4 represented by a forward primer of SEQ ID NO: 7 and a reverse primer of SEQ ID NO: 8; and e. primer pair 5 represented by a forward primer of SEQ ID NO: 9 and a reverse primer of SEQ ID NO: 10; a. primer pair 1 amplifies an SSR fragment of 247 bp; and/or b. primer pair 2 amplifies an SSR fragment of 465 bp; and/or c. primer pair 3 amplifies a DNA fragment within the 247 bp SSR fragment comprising an SNP which segregates with the Bremia resistance locus, particularly a SNP represented by an C for A nucleotide exchange at position 272 in the SSR sequence; and/or d. primer pair 4 and primer pair 5 each amplifies a DNA fragment within the 465 bp SSR fragment, particularly a SNP represented by a G for A nucleotide exchange at position 430 and a T for G nucleotide exchange at position 305 in said SSR fragment, respectively. <EOS>