Primary infection of onion downy mildew on onion seedlings caused by belowground Peronospora destructor inoculum containing germinable oospores

Abstract

AbstractThis study investigated the possible role of oospores in soil as a primary source of onion downy mildew caused by Peronospora destructor. Oospores were extracted from the debris of diseased leaves collected in spring and stored outdoors during the summer. The resulting oospore suspensions were used for germination assays and inoculation experiments in the laboratory. Germination of oospores buried in the soil in cups occurred at a rate of approximately 0.3%–1.3%. Oospore suspensions were dripped or injected into artificial culture soil in cups at sowing onion seeds or at the seedling stage. After raising these plants in a growth chamber for more than 40 days, approximately 20%–30% of the plants exhibited leaf chlorosis and distortion, reproducing the primary infection symptoms of onion downy mildew observed in commercial fields. The presence of P. destructor in symptomatic plants was confirmed by histological observations and species‐specific PCR. Thus, laboratory experiments have demonstrated for the first time that (i) P. destructor oospores that have oversummered outdoors germinate in soil and (ii) belowground inoculum containing germinable oospores causes primary infection. The results showed that P. destructor survives as oospores during host‐free periods and can be a primary inoculum source for downy mildew during the following growing season in autumn. This research on soil‐borne primary infection by P. destructor may help develop knowledge‐based disease management strategies for onion downy mildew, such as proper disposal of crop residues after harvest, effective crop rotation and chemical soil treatments.