First Report of Bacterial Soft Rot of Konnyaku Caused by Dickeya dadantii in China.

Konnyaku (Amorphophallus rivieri Durieu) is grown in some rural areas of China as an important cash crop. In 2011, there was a serious outbreak of Konnyaku soft rot in Xuanwei District of Yunnan Province of China. The disease was characterized by partial or complete tuber rot. At its anaphase, the soft rot may move up the stem, causing the caudex to decay and the whole plant to collapse. If the stem is strong or big enough, the soft rot may develop on one side of the stem, leaving the other side healthy for several days. In this case, the stem does not collapse, and etiolation may be observed on the rotten tissue. In serious cases, up to 80% of the plants were infected. The disease is even more serious if Konnyaku is grown continuously in the same field for more than one year. At its worst, the disease can wipe out the whole crop. In 2012 and 2013, we isolated 46 strains of bacteria from 60 Konnyaku tuber samples with soft rot symptoms from Xuanwei District. All strains grew on CVP medium, and produced iridescent, cross-hatched translucent colonies in deep, cuplike depressions or pits. All strains were facultatively anaerobic, gram-negative, straight rod-shaped cells with peritrichous flagella. All strains were catalase-positive, but oxidase-negative. They were able to ferment glucose, reduce nitrate, produce β-galactosidase and H₂S, and they utilized L-arabinose, D-galactose, D-glucose, glycerol, D-mannose, D-ribose, and sucrose, but did not produce urease, or acid from adonitol. Pectobacterium carotovorum subsp. carotovorum (syn. Erwina carotovora subsp. carotovora) has been commonly accepted as the causal agent of Konnyaku soft rot in Japan and China (1,3). Our studies also confirmed that P. carotovorum subsp. carotovorum caused Konnyaku soft rot, but the colony morphology and physiological and biochemical characteristics of these bacteria differed greatly from those of P. carotovorum subsp. carotovorum and other pectolytic Pectobacterium species. They grew at 37°C, caused potato soft rot, produced acid from melibiose, citrate, raffinose, and lactose, but did not produce acid from sorbitol and arabitol. The strain also utilized malonate but not keto-methyl glucoside as the sole carbon source. All strains were positive for phosphatase. Forty-one of 46 strains were sensitive to erythromycin. Thirty-seven of 46 strains produced indole. All tests were conducted with P. carotovorum subsp. carotovorum standard strain C2 isolated from Chinese cabbage as a positive control. Healthy Konnyaku tubers were inoculated with suspensions of the strains with a concentration of 10⁸ CFU/ml in sterile water to confirm pathogenicity. After ~48 h, tuber rot symptoms were observed on all inoculated Konnyaku tubers. In comparison, there were no symptoms on tubers inoculated with sterile water. The bacterium was re-isolated from the infected Konnyaku tubers and identified as Dickeya dadantii, in accordance with Koch's postulates. All strains were confirmed by using the species-specific primers ERWFOR/CHRREV (2), which amplified a 450-bp DNA fragment by PCR assay. To our knowledge, this is the first report of Konnyaku soft rot caused by D. dadantii in China. References: (1) N. Hayashi. Gunma J. Agric. Res. A (Genera1) 5:25, 1988. (2) E. J. Smid et al. Plant Pathol. 44:1058, 1995. (3) J. Y. Tang et al. J. Yunnan Agric. Univ. 16:185, 2001.