First Report of Red Core Disease of Astragalus membranaceus Caused by Pantoea agglomerans in China

Astragalus membranaceus (Fisch.) Bge. (Fabaceae), an important medicinal plant widely cultivated in China, is valued for its roots, which are extensively used in traditional medicine. In August 2023, a red core disease was observed in commercial A. membranaceus fields in Sanying Town, Guyuan City, Ningxia Hui Autonomous Region, China (36°32′N, 106°24′E). Affected plants showed no visible symptoms on the root surface, whereas internal tissues exhibited distinct reddish-brown discoloration. The disease occurred throughout the growing season. In a surveyed field of approximately 1 ha, disease incidence reached about 40%, with a disease severity of approximately 70%. Symptomatic roots were collected and cut into 5-cm segments. Root pieces were surface-sterilized in 75% ethanol for 1 min, followed by 1.5% sodium hypochlorite for 3 min, rinsed three times with sterile distilled water, and aseptically split longitudinally. Tissues were placed onto Luria–Bertani (LB) agar and incubated at 37°C for 24 h. Four bacterial isolates were consistently obtained. A representative isolate, designated EMM901, was selected for further characterization. Colonies on LB agar were circular, yellow, and mucoid. Cells were rod-shaped, Gram-negative, and measured approximately 0.40–0.52 × 1.12–1.69 μm. Biochemical tests showed positive reactions for glucose utilization, cellulase activity, yellow pigment production, and growth at 37°C, whereas methyl red reaction, gelatin liquefaction, and Gram staining were negative. Based on morphological and biochemical characteristics, the isolate was preliminarily identified as Pantoea sp. For molecular identification, the 16S rRNA gene and a partial gyrB gene of strain EMM901 were amplified and sequenced. The resulting sequences were deposited in GenBank under accession numbers PZ055100 (16S rRNA gene) and PX789892 (partial gyrB gene). Phylogenetic analyses based on the 16S rRNA and gyrB gene sequences showed that strain EMM901 clustered with reference strains of Pantoea agglomerans in GenBank, confirming its taxonomic identity. Pathogenicity was assessed using a needle-inoculation assay. A bacterial suspension of EMM901 (1×10⁸ CFU/mL) was injected into healthy, unwounded roots of A. membranaceus seedlings. Control plants were inoculated with sterile distilled water. Inoculated seedlings were maintained in a greenhouse at 25 °C with 65–70% relative humidity. Typical red core symptoms identical to those observed in the field developed in inoculated plants within 7–10 days, whereas control plants remained symptomless. The pathogenicity test was repeated twice with consistent results. The bacterium was re-isolated from symptomatic roots (Fig. 3), and its morphological traits and DNA sequences were identical to those of the original isolate, thereby fulfilling Koch’s postulates. Pantoea agglomerans has been reported to cause diseases on various plant hosts (Fan et al. 2022; Zhang et al. 2022; Yan et al. 2024; Sepúlveda et al. 2023). To our knowledge, this is the first report of red core disease of A. membranaceus caused by Pantoea agglomerans in China. Identification of the causal agent provides a basis for disease diagnosis and management for Astragalus production.