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Identification of New Races and Deviating Strains of the Spinach Downy Mildew Pathogen Peronospora farinosa f. sp.
Chunda Feng1, James C Correll1, Katherine E Kammeijer2
1Department of Plant Pathology, University of Arkansas, Fayetteville, AR 72701.
Abstract:
Spinach downy mildew disease, caused by the obligate pathogen Peronospora farinosa f. sp. spinaciae, is the most economically important spinach (Spinacia oleracea) disease. New races of this pathogen have been emerging at a rapid rate over the last 15 years. This is likely due to production changes, particularly in California, such as high-density plantings and year-round spinach production. As of 2004, 10 races of P. farinosa f. sp. spinaciae had been identified, and the spinach resistance locus RPF2 provided resistance to races 1 to 10. Based on disease reactions on a set of spinach differentials containing six hypothesized resistance loci (RPF1-RPF6), races 11, 12, 13, and 14 of P. farinosa f. sp. spinaciae were characterized based on samples collected in the past 5 years as part of this study. Race 11, identified in 2008, could overcome the resistance of spinach cultivars resistant to races 1 to 10. Spinach resistance loci RPF1, RPF3, and RPF6 provided resistance to race 11. Race 12 was identified in 2009 and could overcome the resistances of the RPF1 and RPF2 loci. The RPF3 locus was effective against race 12. Race 13 was identified in 2010 and could overcome the resistance imparted by the RPF2 and RPF3 loci, whereas the RPF1 locus was effective against race 13. Race 14 was similar to race 12 and caused identical disease responses on the standard differentials but could be distinguished from race 12 by its ability to cause disease on a number of newly released cultivars, including 'Pigeon', 'Cello', and 'Celesta'. Five novel strains of P. farinosa f. sp. spinaciae were also identified. For example, isolate UA4711 of the pathogen, collected from Spain in 2011, was able to overcome the resistance imparted by the RPF1 and RPF3 loci, while RPF2 and RPF4 were effective against this strain. A total of 116 spinach cultivars, including 103 commercial lines and 13 differential cultivars, were evaluated for resistance to race 10 and the newly designated races 11, 12, 13, and 14.
Insights
New races of spinach downy mildew pathogen (Peronospora farinosa f. sp. spinaciae) are emerging, overcoming existing resistance genes. This study characterizes races 11-14 and novel strains, identifying effective resistance loci for spinach cultivars.
Area of Science:
- Plant Pathology
- Crop Science
- Genetics
Background:
- Spinach downy mildew, caused by Peronospora farinosa f. sp. spinaciae, is a major economic threat to spinach (Spinacia oleracea) production.
- Rapid emergence of new pathogen races over the last 15 years, potentially linked to intensive production practices.
- Previous resistance locus RPF2 was effective against races 1-10 identified by 2004.
Purpose of the Study:
- Characterize newly identified races (11, 12, 13, and 14) of spinach downy mildew pathogen.
- Identify effective resistance loci against these emerging races and novel strains.
- Evaluate resistance of commercial and differential spinach cultivars against prevalent pathogen races.
Main Methods:
- Disease reactions were assessed on spinach differential sets containing six resistance loci (RPF1-RPF6).
- Pathogen samples collected over five years were used for race characterization.
- 116 spinach cultivars (103 commercial, 13 differential) were tested against races 10-14.
Main Results:
- Races 11, 12, 13, and 14 were characterized, with each overcoming specific resistance loci.
- Race 11 overcame RPF2 resistance; RPF1, RPF3, and RPF6 were effective.
- Race 12 and 14 overcame RPF1 and RPF2; RPF3 was effective. Race 13 overcame RPF2 and RPF3; RPF1 was effective.
- Five novel strains were identified, including one from Spain (UA4711) overcoming RPF1 and RPF3.
Conclusions:
- Continuous monitoring and characterization of spinach downy mildew pathogen races are crucial.
- Identification of effective resistance loci (RPF1, RPF3, RPF6) aids in developing resistant spinach cultivars.
- Emerging races necessitate ongoing breeding efforts to manage downy mildew in spinach.
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