First Report of Bacterial dieback of Mango Caused by Dickeya fangzhongdai in China.

Mango (Mangifera indica L.), a globally significant tropical fruit, is a major economic crop in Hainan Province, China. In September 2024, serious dieback symptoms were observed in two mango plantations (plantation 1: 18.440450 °N, 109.112387 °E; plantation 2: 18.327334 °N, 109.418158 °E) following heavy rainfall during the typhoon season in Sanya City, Hainan Province. Disease incidence reached approximately 20% in plantation 1 (7 ha) and 50% in plantation 2 (13.5 ha). Initial symptoms included water-soaked interveinal lesions at leaf bases, which expanded into black necrotic streaks along veins, progressing to petioles and stems. Severe infections resulted in partial or complete branch dieback, accompanied by white milky exudates from necrotic lesions on stems, buds, and petioles. For pathogen isolation, one leaf, one fruit and one stem sample were collected from each plantation, totaling six samples. Diseased tissue segments (5 × 5 mm) were surface-sterilized in 70% ethanol for 1 minute, minced, and suspended in sterilized water. The resulting suspensions were streaked onto Luria-Bertani (LB) agar plates and incubated at 28 °C for 2 days. Small yellow-white bacterial colonies with irregular margins were predominantly observed across all six samples. Three representative isolates (MG2-2 from plantation 1, MG3-1 and MG3-2 from plantation 2) were subjected to further characterization. The isolated bacteria were rod-shaped, gram-negative, motile, facultatively anaerobic growth, and utilized D-glucose, sucrose, D-(+)-galactose, but not D-sorbitol and maltose (Li,et al., 2024). The total DNA of these bacterial cells was extracted and used to amplify the sequences of 16S rRNA (GenBank: PQ967981-PQ967983) using primers 27f and 1492r (Heuer et al., 1997). The 16S rRNA sequences exhibited 100% identity with Dickeya fangzhongdai strain B16 (CP087226) and 99.78% identity with the D. fangzhongdai strain JS5 (NR_151914). Additionally, the fusA, dnaX, gapA, gyrA, purA, recA, rplB, rpoB, rpoD, rpoS (PQ900325 to PQ900354) genes were amplified. A maximum-likelihood phylogenetic analysis based on the concatenated sequences of the 16S rRNA and the ten genes placed the three representative isolates within a clade comprising Dickeya fangzhongdai. Pathogenicity was confirmed by inoculating six 4-year-old mango seedling (cultivar: Jinhuang) with a bacterial suspension of strain MG2-2 (OD600 = 1.0). A 10 μL bacterial suspension was injected into the stems using a sterile disposable syringe, and 5 mL of the bacterial suspension was sprayed onto the leaves. Control plants inoculated with LB medium. Seven days post-inoculation, necrotic lesions with white milky gum were observed on the stems of the inoculated mango seedlings, and partial dieback symptoms appeared on the leaves, while controls remained asymptomatic. The bacterium was reisolated from the symptomatic stems and leaves, and the isolates exhibited the same cultural, physiological, biochemical characteristics as MG2-2. D. fangzhongdai has previously been reported as the causal agent of pear bleeding canker, taro soft rot, banana peduncle soft rot in China (Huang et al., 2021; Yang et al., 2022). To our knowledge, this is the first report of D. fangzhongdai causing dieback on mango in China.