|De Los Reyes, Benildo|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2008
Publication Date: 8/1/2008
Citation: Irish, B.M., Correll, J.C., Feng, C., De Los Reyes, B.G. 2008. Characterization of a resistance locus, Pfs-1, to Peronospora farinosa f. sp. spinaciae, and development of a marker linked to Pfs-1. Phytopathology. 98:894-900. Interpretive Summary: Downy mildew is one of the most destructive diseases of spinach worldwide with ten races having been reported to date. Race identification is difficult because it is based on using disease reaction of commercial cultivars to tell races apart. Problems with this scheme include disease resistance in different genetic backgrounds and hybrids becoming unavailable. Downy mildew is an obligate (cannot be cultured) plant pathogen therefore maintaining reference race isolates is burdensome. In an effort to begin addressing some of these issues, a set of open-pollinated spinach lines in a similar genetic background (near-isogenic) were developed. In addition, the segregating population during near-isogenic line development was used to identify molecular markers closely associated with disease resistance. The development of open pollinated near-isogenic lines will reduce reliance on commercial hybrid cultivars currently used as differentials for race identification. Also, molecular markers developed were shown to be valuable and could be utilized as a selection tool to expedite identification of resistant plant material circumventing the use of the pathogen altogether.
Technical Abstract: Downy mildew is one of the most destructive diseases of spinach worldwide with ten races having been reported to date. Identification of races is based on disease reactions on a set of host differentials composed of a combination of hybrids and open pollinated cultivars. Problems with this scheme include resistance genes in different genetic backgrounds, hybrids becoming unavailable, and the difficulty in maintaining obligate plant pathogens. A set of resistant near-isogenic lines in the same genetic background would be highly desirable for race characterizations as well as for identification of molecular markers linked to disease resistance. To begin developing such a set, backcross progenies of a race-6 susceptible recurrent parent Viroflay and a race-6 resistant donor parent Lion were generated. Data through four backcross generations showed that resistance to race-6 segregated consistent with a single dominant gene model and the locus was designated Pfs-1. By bulk segregant analysis, the AFLP marker (ACT/CTG) was found to be tightly linked to the Pfs-1 locus. This AFLP marker was used to develop a co-dominant PCR-based screening method via Sequence Characterized Amplified Region (SCAR). The results showed that SCAR marker Dm-1 was tightly linked (1.7 cM genetic distance) to the Pfs-1 locus and could effectively discriminate spinach genotypes that were homozygous resistant (RR), heterozygous resistant (Rr), and homozygous susceptible (rr). Further evaluation of a wide range of commercial material indicated that Dm-1 was robust and can be used as a selection tool to expedite identification of Pfs-1 resistant genotypes.