ABSTRACT We conducted a 3‐year field study to determine whether downy mildew‐infested onion residues incorporated into soil serve as primary inoculum under field conditions. We also clarified the timing of appearance and symptom characteristics of plants exhibiting primary infection. Pathogen‐free onion seedlings were transplanted into the main field in late November. Plants exhibiting chlorosis and down‐curled leaves, diagnostic of primary infection, occurred only in plots with incorporated infested residues. None appeared in untreated controls, demonstrating soilborne primary infection derived from incorporated residues. Cumulative incidence in residue plots reached approximately 2%–6% across years, with symptomatic plants appearing intermittently from mid–late February to mid–late March. In the 2 years with leaf‐emergence records, symptoms first appeared mainly during 6th–7th leaf emergence after transplanting three‐leaf‐stage seedlings. Sporangia were usually first detected on the 4th–5th leaves. Across successive symptomatic leaves, sporangia were detected in the basal–middle regions of the lowest leaf and tip–middle regions of the uppermost leaf, revealing a positional pattern. We demonstrated that these sites corresponded to leaf‐curl regions. These results constitute the first field‐based evidence, supported by appropriate controls, that infested residues can trigger soilborne primary infection by Peronospora destructor . This reinforces the value of measures such as crop rotation and summer flooding. The timing and growth stage of plants showing primary infection symptoms can inform the initiation of prophylactic fungicide applications against secondary spread. The characteristic symptom patterns documented for primarily infected plants can serve as field indicators for their early detection and removal.
Watanabe et al. (Fri,) studied this question.
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