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First Report of a New and Highly Virulent Race of Xanthomonas oryzae pv. oryzae, the Causal Agent of Bacterial Leaf Blight of Rice in Guangxi Province, China

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    Authors and Affiliations
    • X. L. Chen
    • Q. Yan
    • R. F. Li
    • K. H. Li
    • L. J. Gao , Institute of Plant Protection, Guangxi Academy of Agricultural Sciences, Guangxi Key Laboratory of Biology for Crop Diseases and Insect Pests, Nanning, Guangxi, 530007, China.

      Bacterial leaf blight (BLB), caused by Xanthomonas oryzae pv. oryzae, is one of the most widely distributed and devastating diseases of rice (Oryza sativa) in Asia. BLB can cause yield losses ranging from 20 to 30% and as high as 50% (Mew 1987). Typical BLB symptoms, including tannish gray or white lesions along the veins, were observed in the rice fields in Guangxi, China, in October 2013. Fifteen rice leaves with such symptoms were collected from the infected rice fields. These leaves were cut into approximately 1-cm pieces and homogenized in 9 ml of sterile water by grinding after 1% sodium hypochlorite solution treatment. Diluted homogenates were plated on peptone sucrose agar (PSA) and incubated at 28°C for 3 to 4 days. The nonflat, mucous colonies with yellow, round and smooth margins that developed on the plates were selected for further analysis (Dye et al. 1980). A multiplex polymerase chain reaction assay was performed to distinguish X. oryzae pv. oryzae from other X. oryzae pathovars using genomic DNA of the isolates as template with the primers and cycling conditions as described previously (Lang et al. 2010). Three of the isolates were used to conduct pathogenicity assays on differential rice varieties IR26, Java 14, Nanjing 15, Tetep, and Jingang 30 (Fang et al. 1990). The seeds were sown in plastic trays (50 × 40 × 15 cm) in the greenhouse under a 12-h photocycle at 25 to 28°C, and 30-day-old seedlings were transplanted with row spacing of 25 × 20 cm. Plants were clip-inoculated with a bacterial suspension of 3 × 108 CFU/ml at the booting stage and maintained at 100% humidity for 24 h. Thirty leaves were inoculated with each isolate or with sterilized water (as control). Disease severity was scored 20 days after inoculation by visual assessment of the percentage of the leaf area covered by lesions. Leaves with lesion area <25% and >25% were classified as resistant (R), and susceptible (S), respectively (Fang et al. 1990). The result showed that all the isolates were highly virulent on all the tested rice lines, but no symptoms were observed on water-treated leaves. Colonies recovered on PSA plate from inoculated leaves were identical to field isolates and confirmed by morphology together with PCR, which fulfilled Koch’s postulates. Furthermore, pathogenicity assays using these isolates were performed on several near-isogenic lines (NILs), including IRBB4 (Xa4), IRBB5 (xa5), IRBB7 (Xa7), IRBB13 (xa13), IRBB14 (Xa14), IRBB21 (Xa21), and CBB23 (Xa23). Consequently, all the isolates displayed SRRSRSS on the NILs in contrast to SSSSSRR for the Philippine strain PXO99A. The assays were repeated three times with similar results. Previous studies revealed that CBB23 derived from a cross between a wild rice Oryza rifipogon accession (RBB16) and a susceptible indica rice variety Jingang 30 conferred high resistance to all naturally occurring X. oryzae pv. oryae races tested (Wang et al. 2014). Nevertheless, our research proved that our isolates were highly virulent on many rice lines, including CBB23. To our knowledge, this is the first report of a new and highly virulent race of X. oryzae pv. oryae causing BLB of rice in Guangxi province, China, where it could potentially be destructive under conducive conditions.

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      This work was supported by the Natural Science Foundation of Guangxi (2015GXNSFBA139106).