Real-Time PCR Detection of the Onion Downy Mildew Pathogen Peronospora destructor From Symptomless Onion Seedlings...

An outbreak of downy mildew disease of onion, caused by Peronospora destructor, in Japan in 2016 necessitated a reevaluation of the primary inoculum sources to optimize disease management. Detection of the P. destructor pathogen in plants with asymptomatic infection and in soil would guide the application of fungicides according to the extent of infection before disease development. Here, we detected P. destructor in both plants and soil using newly developed primer sets (Pd ITS and Pd ITS 614) by both conventional and real-time PCR. Validation by real-time PCR with Pd ITS 614 showed that P. destructor DNA was amplified from symptomless seedlings at 3.7 × 10² to 1.0 × 10⁰ conidium cells/50 mg leaf tissue, suggesting the detection of asymptomatic infection. Real-time PCR with Pd ITS amplified pathogen DNA from field soils at 1.6 × 10³ to 8.3 × 10¹ oospore cells/g of soil. This real-time PCR assay provides a useful tool for identifying and quantifying inoculum sources, which may be the foundation of the design of integrated disease management strategies.