Downy mildew of spinach (Spinacia oleracea), caused by Peronospora effusa, is a production constraint on production worldwide, including in California, where the majority of U.S. spinach is Corresponding author: S. J. Klosterman; E-mail address: Steve.Klosterman@ars.usda.gov.
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Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts grown. The aim of this study was to develop a real-time quantitative polymerase chain reaction (qPCR) assay for detection of airborne inoculum of P. effusa in California. Among oomycete ribosomal DNA (rDNA) sequences examined for assay development, the highest nucleotide sequence identity was observed between rDNA sequences of P. effusa and P. schachtii, the cause of downy mildew on sugar beet and Swiss chard in the leaf beet group (Beta vulgaris subsp. vulgaris). Single-nucleotide polymorphisms were detected between P. effusa and P. schachtii in the 18S rDNA regions for design of P. effusa-and P. schachtii-specific TaqMan probes and reverseprimers. An allele-specific probe and primer amplification method was applied to determine the frequency of both P. effusa and P. schachtii rDNA target sequences in pooled DNA samples, enabling quantification of rDNA of P. effusa from impaction spore trap samples collected from spinach production fields. The rDNA copy numbers of P. effusa were, on average, ≈3,300-fold higher from trap samples collected near an infected field compared with those levels recorded at a site without a nearby spinach field. In combination with disease-conducive weather forecasting, application of the assays may be helpful to time fungicide applications for disease management.Downy mildews are widespread and destructive plant diseases caused by obligate oomycete pathogens. These pathogens produce large quantities of short-lived, asexual spores on host plant leaves which become airborne to cause secondary plant infections. Downy mildew on spinach (Spinacia oleracea), caused by Peronospora effusa (13), is a major disease constraint on production worldwide (16) and, in particular, in California and Arizona, where ≈85% of U.S. fresh-market spinach is produced (2). In the cool coastal Salinas Valley, CA, a region which produces roughly half of the fresh-market spinach grown in the state, downy mildew outbreaks are common. Although spinach downy mildew can usually be controlled with fungicide applications in conventional production, the disease can be devastating for organic spinach production, in which fungicide use is excluded. This, in turn, can accentuate inoculum availability for downy mildew epidemics in conventional production fields. The appearance of new races of P. effusa, which has occurred more frequently in recent years (16,18), has complicated efforts aimed at breeding host resistance to downy mildew. The rapid appearance of these new races also emphasizes a need for alternative disease management practices.Early and accurate warnings of the increases in airborne inoculum of plant pathogens can help manage diseas...