First Report of <i>Pectobacterium colocasium</i> Causing Soft Rot of <i>Amorphophallus konjac</i> in China

Konjac (Amorphophallus konjac) belongs to the Tennantaceae family and is an important cash crop grown in the southern provinces of China(Zhang et al. 2022). In July 2024, severe soft rot symptoms were observed in konjac grown in Baoting County and Qionghai City, Hainan Province, with an incidence of about 15%. Infected stem was brown with internal rotting, watery spots and foul odor; leaves turned yellow and wilted. Lesions first appear in the stem dark green oil stains like small spots, diameter of about 1 - 3 mm, after the rapid spread of tissue rot, sap flow, and finally the stem completely rotted and collapsed. Plant fell to the ground, the plant died within 15 days. Infected stem samples were soaked in 75% ethanol for 60 seconds, then rinsed three times with sterile distilled water to sterilize the surface and finally were cut into small pieces and ground in sterile water. The extracts were plated on Luria–Bertani (LB) plates and incubated at 28°C for 24 hours to obtain single colonies(Wu et al. 2023). The bacteria were milky white, translucent, oval rod-shaped (average 15 x 5 µm)and gram negative. The bacterial gDNA was extracted using the EasyPure Bacteria Genomic DNA Kit (TransGen Biotech, Beijing, China). The 16S rDNA region was amplified using the universal primer 27F/1492R (Weisburg et al. 1991) and specific primers published by Handique et al. (2022) were used to amplify the three housekeeping genes proA, mdh and gapA. We sequenced the 78 isolated strains. The rate of isolation from symptomatic plants was 26.92%. Sequence data from the strain BT10 and QH15 have been deposited in GenBank under accession numbers PQ444076, PQ456305, PQ456306 and PQ456307; PV759729, PV769088, PV769087 and PV769086. BLASTn results showed that these sequences were 99.93%, 97.07%, 99.03% and 99.80% similar to P. colocasium strain LJ1. Phylogenetic tree of 16S rDNA sequences and three housekeeping genes analyzed by multilocus sequence analysis using MEGA11 showed that strain BT10 and QH15 clustered with Pectobacterium colocasium. Pathogenicity was tested on three - month -old A.Konjac seedlings by loading 100µl bacterial suspension (108 CFU/ml, in water) into 200 µL pipette suction head, then the tip was inserted and injected into the upper parts of the stem of three healthy konjac plants. Control seedlings received sterile water and Klebsiella oxytoca.The seedlings were grown at 25°C and 50% relative humidity (each of six plants). After 7 days, inoculated seedlings showed soft rot symptoms similar to those initially observed in the field, while control plants remained asymptomatic. Using the same PCR primers as described above, the strain re-isolated from inoculated stems was identified as P. colocasium. The pathogenicity test was repeated three times with the same results, thus fulfilling Koch’s postulates.Pectobacterium colocasium has been reported as a pathogen of taro(Zhou et al. 2022). To our knowledge, this is the first report of P. colocasium causing soft rot of konjac, thus extending the host range of the pathogen.