Occurrence of Stem Blight Caused by Pseudomonas extremorientalis on Pinellia ternata in China.

Pinellia ternata is a perennial herbaceous plant, which tubers can be used for anti-inflammatory and has a significant position in Traditional Chinese Medicine (Marki et al. 1987). In April 2020, bacterial stem blight first occurred on P. ternata in Jingmen City (30°32'N, 111°51'E), Hubei Province, China. In the follow-up investigation, the disease also appeared in plantations of P. ternata in Qianjiang City, Tianmen City. Initial symptoms showed orange-red streak on the stem, then progressed into chlorotic and water-soaked lesions, which caused roots to be necrotic and leaves to stunting, fading, and wilting. In the end, the leaves withered, the stems rotted completely, and the incidence of plant collapse reached 20~30%. To isolate the plant pathogenic bacteria, twenty P. ternata plant samples with distinct chlorotic stem symptoms were obtained from two fields in Jingmen City. Symptomatic samples were cut to 1-cm-long pieces by sterile scalpel, then were streaked onto nutrient agar medium and grow at 28℃ for 48 h. Four pure typical aerobic, gram-negative bacteria were isolated by characterized with transparent, smooth, round, convex surfaces. The isolated colonies did not produce fluorescent pigments on King's B medium. In addition, the isolates were positive for nitrate reduction, arabinose, mannitol, D-ribose, sucrose, D-sorbitol, and were negative for gelatin liquefaction, rhamnose, D-glucose, D-melibiose. These characteristics were identified as Pseudomonas extremorientalis (Ivanova et al. 2002). One representative colony ZJH1 was selected randomly for further verification. The 16s rRNA, gyrB, and rpoD regions were obtained with primers 27F/1492R (Weisburg et al. 1991), gyrB-Fps/ gyrB-Rps, and rpoD-Fps/ rpoD-Rps, respectively (Sarkar and Guttman. 2004). These sequences were deposited in GenBank as accession nos. MT459234.1, MT469887.1 and MT469886.1, which revealed 99% homology with P. extremorientalis strain BS2774 (accession nos. LT629708.1). The pathogenicity of P. extremorientalis strain ZJH1 was confirmed by using 3-month-old, healthy, greenhouse-grown P. ternata plants. The stems were stabbed and inoculated 10 μL of the bacterial suspension (108 CFU / ml), inoculating the same amount of sterile water as a control, repeated 5 times for each treatment. The plants were cultivated in a greenhouse at 28 °C and a humidity of 80%. Three days later, the stems showed necrosis, followed by the withered leaves and died plants, whereas the control had no symptoms. P. extremorientalis were reisolated and verified again from symptomatic plants, which was consistent with Koch's postulates. This experiment was repeated thrice to get the same result. To our knowledge, this is the first report of bacterial stem blight caused by P. extremorientalis on P. ternata in China. Stem blight caused by P. extremorientalis poses a significant threat to yield and marketability of P. ternata. Further research on selecting resistant variety and effective chemical control is needed. References: Ivanova, E. P., et al. 2002. Int J Syst Evol Micr. 2113:2120. https://doi.org/10.1099/00207713-52-6-2113 Marki, T., et al. 1987. Planta Med. 53:412. Sarkar, S. F., Guttman, D. S. 2004. Appl. Environ. Microbiol. 70:1999. https://doi.org/10.1128/AEM.70.4.1999-2012.2004 Weisburg, W. G., et al. 1991. J. Bacteriol. 173:697. https://doi.org/10.1128/jb.173.2.697-703.1991 F. F. Wang and Y. J. You contributed equally to this work. The author(s) declare no conflict of interest. Funding: National Modern Agricultural Industrial Technology System (grant no. CARS-21), Technology R&D Program of Enshi Tujia and Miao Autonomous Prefecture (grant no. D20190015), Science Funds for Young Scholar of Institute of Chinese Herbal Medicines, Hubei Academy of Agricultural Sciences (grant no. 2019ZYCJJ01), Key R&D Program of Hubei Province (grant no. 2020BCA059), Key Technology R&D Projects of Hubei Agricultural Science and Technology Innovation Center (grant no. 2020-620-000-002-04).