Since 2006, stem and root rot of sweetpotato has been observed in fields at a number of sweetpotato-production areas in Huidong, Haifeng, Puning, and Zhanjiang counties and Guangzhou City in Guangdong Province of China. Initially, the leaves turn yellow, and a black, water-soaked rot occurs on the bottom of the stems that gradually extends to the top of the stems. Finally, the entire plant collapses and dies. Bacteria were consistently isolated from stems of diseased seedlings by streaking on nutrient agar. Twelve representative isolates were chosen for further characterization. All strains grew at 37°C, were gram negative, facultatively anaerobic, and rod shaped with peritrichous flagella. The strains were negative for oxidase and positive for catalase and tryptophanase (indole production) They fermented glucose, reduced nitrates to nitrites, degraded pectate, produced phosphatase and lecithinase, and utilized citrate, tartrate, malonate, glucose, sucrose, fructose, and maltose, but not trehalose and lactose. These characteristics were similar to those of Erwinia chrysanthemi (Pectobacterium chrysanthemi) (1). PCR was performed on the 16S rDNA gene from isolate H12 (1,503 bp; GenBank Accession No. GU252371) with primers 27f (5′-GAGAGTTTGATCCTGGCTCAG-3′) and reverse primer (5′-GGCTACCTTGTTACGACTTC-3′). Subsequently, PCR products were sequenced. Results of sequence analysis showed the sequence of isolated strain H12 was 99% identical to that of E. chrysanthemi, 99% identical to that of type strain CFBP 1269T of Dickeya dadantii (Accession No. AF520707), and 98% identical to that of type strain CFBP 1200T of D. dianthicola (Accession No. AF520708). Recently, E. chrysanthemi was transferred to Dickeya gen. nov., but it was difficult to identify the species within the genus Dickeya. Seedlings (20 to 30 cm) were planted in 10-cm-diameter plastic pots containing sterilized field soil at room temperature. Four days later, five stem tops of sweetpotato were injected with a bacteria suspension (108 CFU/ml) of approximately 100 μl to fulfill Koch's postulates. Five control plants were inoculated with sterile distilled water. The experiment was conducted three times. All plants were incubated in a chamber at 30°C with high humidity. One to two days after inoculation, symptoms were observed in all inoculated plants and appeared to be identical to those observed in the field. No symptoms were noted on the control plants. The bacterium was reisolated from symptomatic stems of sweetpotato plants. This pathogen was previously reported on sweetpotato in the United States in 1974 (2). To our knowledge, this is the first report of a Dickeya sp. (E. chrysanthemi) causing bacterial stem and root rot of sweetpotato in China.
References: (1) D. J. Brenner et al. Page 670 in: Bergey's Manual of Systematic Bacteriology. 2nd ed. Springer, New York, 2005. (2) N. W. Schaad et al. Phytopathology 67:302, 1977.
