Asteraceae weeds may be an alternative host of Dickeya dianthicola, a causal agent of potato blackleg in Japan

An episode of potato blackleg caused by Dickeya dianthicola (Ddi) in a Japanese seed potato field in 2018 was suspected to have occurred by infection of Ddi from a contamination source in the field, rather than from infested seed, because of the local distribution of symptomatic plants. In this study, a contamination source and infection route in the field were identified by BIO-PCR (biological amplification and PCR, i.e., a combined method of semi-selective enrichment in a pectate-based medium and PCR detection), fingerprinting analysis of Ddi isolates by rep-PCR (repetitive sequence-based PCR), and field inoculation tests. We obtained 9 Ddi isolates from the rhizome or rhizosphere soil of 210 samples of weedy Asteraceae plants (including annual fleabane, butterbur, goldenrod, and dandelion) grown on a ridge near the field where symptomatic potato plants were found. Four Ddi isolates were also obtained from 15 samples of surface water that pooled on the ridge after heavy rains. Rep-PCR showed the same fingerprint pattern in isolates obtained from diseased potato stems and some of the isolates from annual fleabane, butterbur, and the surface water. Inoculation of seed tubers with Ddi isolates from annual fleabane or butterbur confirmed that they could cause blackleg infection in potato. From these results, we concluded that blackleg infection in the field may be caused by transmission of Ddi from infested Asteraceae weeds to potato plants through surface water flow.