Clade-Specific Biosurveillance of Pseudoperonospora cubensis Using Spore Traps for Precision Disease Management of Cucurbit Downy Mildew

Pseudoperonospora cubensis is an obligate oomycete and cause of cucurbit downy mildew (CDM), the most destructive foliar disease affecting cucurbit hosts. Annual epidemics develop throughout the United States as windborne sporangia travel great distances and survive prolonged exposure to solar radiation. Recent genomic evidence suggests that P. cubensis isolates display host adaptation based on their respective Glade. Early detection is key for fungicide application timing, and identification of the host-adapted Glade provides information on the risk of infection for specific cucurbit crops. In this study, a multiplex quantitative PCR assay was developed based on species- and clade-specific nuclear genomic markers. The assay detected as few as 10 sporangia or DNA at 100 fg/ml for both clades and was validated in the field by deploying rotorod spore samplers in cucurbit sentinel plots located at two research stations in North Carolina. Using this assay, sporangia DNA was detected in spore trap sampling rods before signs of P. cubensis or CDM symptoms were observed in the sentinel plots. Both Glade 1 and Glade 2 DNA were detected in late-season cucumber and watermelon plots but only Glade 2 DNA was detected in the early-season cucumber plots. These results will significantly improve disease management of CDM by monitoring inoculum levels to determine the cucurbit crops at risk of infection throughout each growing season.

Automated summary

Pseudoperonospora cubensis is the cause of cucurbit downy mildew, with annual epidemics in the United States due to windborne sporangia traveling long distances and surviving prolonged exposure to sunlight. A multiplex quantitative PCR assay has been developed to detect the pathogen early on and improve disease management.