First Report of Xanthomonas sacchari Causing Bacterial Panicle Blight of Rice in the United States.

Abstract

Rice (Oryza sativa) plants showing severe leaf and panicle blight symptoms were observed and collected from four field plots in Louisiana in summer 2023. As the observed symptoms looked like bacterial leaf and panicle blight caused by Pantoea ananatis (Bruno et al. 2025) or bacterial panicle blight caused by Burkholderia glumae and B. gladioli (Nandakumar et al. 2009), we presumptively attempted to isolate one of these bacterial pathogens as the causal agent. Bacteria were isolated from the symptomatic seeds, which were surface sterilized, dehulled and ground in sterile distilled water using a sterile mortar and pestle. The homogenate was spread on the CCNT medium (Kawaradani et al. 2000) and LB agar supplemented with nitrofurantoin (50 µg/ml) and cycloheximide (50 µg/ml). After 72 h incubation at 41⁰C, round, smooth and faint, yellow-colored bacterial colonies appeared on both CCNT and LB agar plates. Unexpectedly, all six bacterial isolates from rice samples were initially identified as Xanthomonas sacchari based on PCR-amplified 16S rDNA sequences using the primers fD1 (5'-CCGAATTCGTCGACAACAGAGTTTGATCCTGGCTCAG-3')/rD1 (3'-CCCGGGATCCAAGCTTAAGGAGGTGATCCAGCC-5') (Weisburg et al. 1991), regardless of their sampling locations. Whole genome sequencing analyses (NCBI Accession ID: PRJNA1223440) performed through a hybrid sequencing approach using Oxford Nanopore sequencer & Illumina NextSeq2000 system (Plasmidsaurus Inc., California, USA) further confirmed that all six strains belong to X. sacchari. To satisfy Koch's postulates, each of the six strains LXP-1780, LXP-1783, LXP-1790, LXP-1791, LXP-1792, and LXP-1793 was confirmed for its pathogenicity to rice through two inoculation methods, using one-month-old plants (cv. Kitaake) at tillering stage; 1) pricking the stems with a sterile toothpick touched to the overnight grown bacterial culture on LB agar (~ 5-8 x 10⁵ bacterial cells per tip) and 2) clipping the leaves with scissors dipped in the bacterial suspension containing ~1 × 10⁸ bacterial cells per mL of sterile distilled water. This virulence assay was conducted twice with three replications for each strain, and rice plants inoculated with sterile distilled water were included as the negative control. By twenty-seven days after inoculation (DAI), the inoculated plants developed symptoms on leaves and panicles, which were similar to the symptoms initially observed in the field. Besides blight symptoms and water-soaked lesions in the leaves, the most noticeable symptom observed was the blackening or darkening of the leaf tips, which appeared as dark spots or streaks. In panicles, panicle blight-like symptoms, such as grain discoloration and darkening of rice hulls, were observed. The bacteria were reisolated from each artificially inoculated plant and their identity were validated to be X. sacchari. To our knowledge, this is the first report of rice disease caused by X. sacchari in the United States. This finding strongly suggests the emergence of this disease as a new potential threat to U.S. rice industry. Furthermore, the strains of X. sacchari and their whole genome sequence information obtained from this study will serve as a foundation for future studies on this newly emerging disease.