First report of bacterial soft rot on onion caused by Dickeya sp. (ex Pectobacterium chrysanthemi) in Spain

In summer 2005, following heavy storms, soft rot symptoms were observed at the base of central leaves of onion plants (Allium cepa) in fields with drip irrigation in Zaragoza, northeastern Spain. The incidence was about 10% of plants affected. Longitudinal sections revealed that lesions developed downward, so that the inner layers of bulbs also appeared macerated. Isolations from diseased leaves and bulbs yielded pectolytic bacteria on crystal violet pectate (CVP) medium. Colonies were purified on King´s B medium and characterized. Isolates were Gram-negative rods, oxidase negative, facultatively anaerobic, degraded pectate and rotted potato slices. They grew at 37°C, were sensitive to erythromycin, positive for phosphatase, indole production and malonate utilization, while negative for acid production from trehalose and α-methyl glucoside. The bacteria were further identified by ELISA, PCR amplification (Nassar et al., 1996) and biovar discrimination tests (Ngwira & Samson, 1990) as biovar 3 of the species Pectobacterium chrysanthemi. Suspensions (10⁸ cells per ml) were injected on central leaves of onion plants, which were maintained at 28°C with uninoculated plants as controls. Soft rot symptoms identical to those observed in the field appeared on all inoculated plants within one or two days days after inoculation but not on control plants. A bacterium with identical characteristics to those described above was consistently re-isolated from rotted tissues of inoculated plants.

The transfer of P. chrysamthemi to a novel genus, Dickeya gen. nov. was recently proposed (Samson et al., 2005). Biochemical characteristics agree with two recently proposed new species in this genus, Dickeya zeae and D. dadantii (Samson et al., 2005). However, partial 16S rDNA sequence analysis showed 97% similarity with the type strains of D. zeae (CFBP 2052^T). Nevertheless, species identification within genus Dickeya is still difficult since only a limited number of strains of each species have been characterised fully. This is the first report of soft rot on onion in Spain caused by the bacterium formerly referred to as Pectobacterium chrysanthemi biovar 3, now proposed as Dickeya sp. (probably D. zeae).