First Report of Pectobacterium aroidearum Causing Soft Rot in Olecranon Honey Peach (Prunus persica) in China

Olecranon honey peach (Prunus persica L.) is a popular fruit tree cultivated in Guangdong Province of China. Due to its excellent economic values and popularity, it has recently been widely adopted and planted in several other southern Provinces and Autonomous Region in China, including Yunnan, Hunan, Jiangxi, Guizhou, and Guangxi. In Lianping County of Guangdong Province alone, the annual peach fruit production was about 78,800 tonnes (Xie et al. 2017). In July 2021, peach fruits showing soft rot symptoms were collected from an olecranon honey peach plantation in Lechang, Guangdong, China. Symptoms included tissue disintegration with bacterial oozes and rotting smells. To isolate the causal agent of soft rot in the peach fruits, the bacterial oozes from various rotted fruits were streaked on the modified YEB agar plate (Huang et al. 2021), and 21 bacterial colonies were selected for PCR amplification using the primers targeting the conserved region of 16S rDNA gene (Wei et al. 2020). A blastN analysis of the DNA sequences of the obtained PCR fragments in NCBI website indicated that 17 isolates named as ZL strains were potential bacterial species of Pectobacterium with about 99% similarity (Genbank accession number of ZL1: OK189602) to Pectobacterium aroidearum SCRI 109T (Genbank accession number: NR_159926). Three of them (ZL1, ZL2 and ZL3) were selected for assay of pathogenicity. The bacterial suspensions (10 μl, 1×106 CFU/ml) of strains ZL1, ZL2 and ZL3 were injected into olecranon honey peach fruits by using a syringe. A portion of peach fruits were similarly injected with sterile distilled water as the negative control. After 18 h incubation at 25 °C, the typical symptom of soft rot, i.e., tissue decay, became visible on the peach fruits inoculated with the bacterial suspensions. After inoculation for 42 h, bacterial oozes were exuded from rotting tissues. Peach fruits without injuries were also sprayed with the bacterial suspensions under the same conditions, but decay symptoms were not observed, suggesting that the bacterial infection needs the wounding or injuries. To fulfill the Koch's postulates, bacterial colonies were re-isolated from bacterial oozes, and their conserved region of 16S rDNA fragments were amplified and sequenced. Bioinformatics analysis of the DNA sequence data confirmed that all the isolated colonies were Pectobacterium strains. Using the Biolog Gen III system, the representative strain ZL1 was identified as Pectobacterium (SIM 0.56). Transmission electron microscopy analysis showed that the bacterial cells of strain ZL1 were rod-shaped with peripheral flagella. To further determine the species of ZL strains, eight housekeeping genes (acnA, gapA, icd, mdh, mtlD, pgi, proA and rpoS) were analyzed by the methods described previously (Nabhan et al. 2013). The amplified DNA sequences analyzed by the blastN program in NCBI showed that the sequences of eight housekeeping genes from strains ZL1, ZL2 and ZL3 were identical to each other (Genbank accession number: OK274248 to OK274255), and most of the gene sequences shared over 99% similarity to their counterparts in P. aroidearum L6 (Genbank accession number: NZ_CP065044) (Xu et al. 2021), except that the acnA and proA genes showed about 98% and 96% similarity respectively to the corresponding genes of P. aroidearum L6. In addition, the multi-locus sequence analysis (MLSA) using DNA sequences of above eight housekeeping genes showed that ZL strains were grouped with other P. aroidearum strains. Taken together, the results of molecular and biochemical assays confirmed that ZL strains isolated from olecranon honey peach fruits were P. aroidearum. To our knowledge, this is the first report of P. aroidearum causing soft rot disease in olecranon honey peach in China. P. aroidearum is a relatively newly described soft rot pathogen (Nabhan et al. 2013). More recently, the pathogen was found causing soft rot infections in lettuce, Chinese cabbage, pepper (Capsicum annuum) fruits, konjac, carrot and Syngonium podophyllum (Barroso et al. 2019; Moraes et al. 2020; Sun et al. 2019; Tang et al. 2020; Xu et al. 2021). The results of this study add a new plant species to the host range of P. aroidearum.