Abstract
Peronospora effusa is an obligate pathogen that causes downy mildew on spinach and is considered the most economically important disease of spinach. The objective of the current research was to assess genetic diversity of known historical races and isolates collected in 2014 from production fields in Yuma, Arizona and Salinas Valley, California. Candidate neutral single nucleotide polymorphisms (SNPs) were identified by comparing sequence data from reference isolates of known races of the pathogen collected in 2009 and 2010. Genotypes were assessed using targeted sequencing on genomic DNA extracted directly from infected plant tissue. Genotyping 26 historical and 167 contemporary samples at 46 SNP loci revealed 82 unique multi-locus genotypes. The unique genotypes clustered into five groups and the majority of isolates collected in 2014 were genetically closely related, regardless of source location. The historical samples, representing several races, showed greater genetic differentiation. Overall, the SNP data indicate much of the genotypic variation found within fields was produced during asexual development, whereas overall genetic diversity may be influenced by sexual recombination on broader geographical and temporal scales.
Highlights
Spinach (Spinacia oleracea) is believed to be native to the Middle East and has been cultivated for more than 1,300 years [1,2]
In the United States, spinach production increased from 200,000 tons in 1961 to 354,000 tons in 2012 [4]
Multiple laboratories run parallel tests and, if the results consistently indicate unique virulence, a new race number is assigned [13].The exact cause for the rapid emergence of new races is unknown, but is postulated to result from intense selective pressure associated with new spinach production practices [8]
Summary
Spinach (Spinacia oleracea) is believed to be native to the Middle East and has been cultivated for more than 1,300 years [1,2]. It became established in Europe in the 11th century and was introduced to the new world and southern hemisphere in the 16th century [3]. Global demand for spinach has increased substantially during recent decades. In 1961, world production of spinach was 2.96 million tons. By 2012, production increased seven fold to 21.66 million tons worldwide. In the United States, spinach production increased from 200,000 tons in 1961 to 354,000 tons in 2012 [4]. The increased demand can largely be attributed to the popularity of pre-packaged baby spinach leaves and the health benefits associated with spinach including vitamins C and A, lutein, iron, folic acid, and magnesium [6,7]
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