Characterization of a Resistance Locus (Pfs-1) to the Spinach Downy Mildew Pathogen (Peronospora farinosa f. sp. spinaciae) and Development of a Molecular Marker Linked to Pfs-1

Abstract

Downy mildew is a destructive disease of spinach worldwide. There have been 10 races described since 1824, six of which have been identified in the past 10 years. Race identification is based on qualitative disease reactions on a set of diverse host differentials which include open-pollinated cultivars, contemporary hybrid cultivars, and older hybrid cultivars that are no longer produced. The development of a set of near-isogenic open-pollinated spinach lines (NILs), having different resistance loci in a susceptible and otherwise common genetic background, would facilitate identification of races of the downy mildew pathogen, provide a tool to better understand the genetics of resistance, and expedite the development of molecular markers linked to these disease resistance loci. To achieve this objective, the spinach cv. Viroflay, susceptible to race 6 of Peronospora farinosa f. sp. spinaciae, was used as the recurrent susceptible parent in crosses with the hybrid spinach cv. Lion, resistant to race 6. Resistant F(1) progeny were subsequently backcrossed to Viroflay four times with selection for race 6 resistance each time. Analysis of the segregation data showed that resistance was controlled by a single dominant gene, and the resistance locus was designated Pfs-1. By bulk segregant analysis, an amplified fragment length polymorphism (AFLP) marker (E-ACT/M-CTG) linked to Pfs-1 was identified and used to develop a co-dominant Sequence characterized amplified region (SCAR) marker. This SCAR marker, designated Dm-1, was closely linked ( approximately 1.7 cM) to the Pfs-1 locus and could discriminate among spinach genotypes that were homozygous resistant (Pfs-1Pfs-1), heterozygous resistant (Pfs-1pfs-1), or homozygous susceptible (pfs-1pfs-1) to race 6 within the original mapping population. Evaluation of a wide range of commercial spinach lines outside of the mapping population indicated that Dm-1 could effectively identify Pfs-1 resistant genotypes; the Dm-1 marker correctly predicted the disease resistance phenotype in 120 out of 123 lines tested. In addition, the NIL containing the Pfs-1 locus (Pfs-1Pfs-1) was resistant to multiple races of the downy mildew pathogen indicating Pfs-1 locus may contain a cluster of resistance genes.

Spinach: better management of downy mildew and white rust through genomics

“…Although improving resistance to downy mildew is a critical breeding objective (Morelock and Correll 2008), relatively few studies have been conducted on the genetic control of resistance to the downy mildew pathogen (Irish et al 2008;Morelock and Correll 2008;Correll et al 2007). …”

“…This locus was identified by developing a near isogenic line (NIL1) using the hybrid Lion as the resistant parent and Viroflay as the susceptible recurrent backcross parent (Irish et al 2008). The line went through four backcross generations, selection of plants resistant to race 6, and three subsequent selfings, before producing seed for evaluation.…”

“…A total of 6 loci has been hypothesized to control resistance to the 11 known races and 2 additional deviating strains (UA12 and UA13) of the spinach downy mildew pathogen Irish et al 2008) (Table 3). Although the first P. farinosa f. sp.…”

“…Although the first P. farinosa f. sp. spinaciae resistance locus described was designated Pfs-1 (Irish et al 2008) and other hypothesized loci have been given a "Pfs" acronym, this has caused confusion in the spinach community, as the races of the downy mildew pathogen are often designated Pf1, or Pfs1. Thus, we have adopted the convention used with the Arabidopsis downy mildew pathogen system and have renamed the loci involved in downy mildew resistance of spinach as "RPF" loci (for genes involved in the Resistance against Peronospora farinosa).…”

“…However, screening needs to be complemented with studies of the genetics of resistance as one cannot conclude which locus governs resistance in a particular accession to a given race as a locus can control resistance to multiple races (Irish et al 2008). For example, the RPF1 locus controls resistance to races 1-7, 9, 11, and UA13 (Irish et al 2008;Correll et al unpublished) whereas RPF2 controls resistance to races 1-10 of Pfs (Correll et al unpublished).…”