First Report of Bacterial Fruit Blotch of Melon Caused by Paracidovorax citrulli in Guangxi Province, China

Résumé fourni par la source

Melon is an important horticultural crop. It has a long history of cultivation in China, which is the country with the largest production and consumption of melon. In March 2023, at the Modern Agricultural Experiment Station in Guangxi, China (23°15'26" N, 108°27'47" E), we observed bacterial fruit blotch symptoms on about 10% of 'Zhongtian No. 9' melon plants in an 8 ha field. Symptoms were mainly on two-week-old cotyledons. Leaves were collected from five sites. Infected tissues were sterilized with 75% ethanol, cut into 4 × 4 mm pieces, and ground in ddH₂O. The suspension was spread on KB agar plates and incubated at 28℃ for 2 days. Thirty single colonies were selected and streaked on new KB agar plates. The colonies were milky white, round, smooth, raised, opaque, and 1-2 mm in diameter. Three single colonies were isolated and sequenced for 16S rRNA gene. BLAST analysis showed >99% nucleotide sequence homology with Paracidovorax citrulli ICMP 7500 (NR_041758) and representative strain M6 (CP029373). One strain, named GXM1, had its 16S rRNA gene sequence deposited in NCBI with accession number PQ461200. Specific primers of P. citrulli BFB/BFB1 and BFB/BFB2 (Yang et al. 2019) were used for further identification, the result showed that the strain belonged to P. citrull group I. The whole genome of GXM1 was sequenced and assembled and submitted to NCBI under accession number GCA_042610265.1, with 99.3% genome-wide similarity to the P. citrulli model strain ICMP7500 (NR_041758.1) (based on dDDH analysis; Meier-Kolthoff et al. 2013) and 99.69% (based on ANI analysis; Richter et al. 2015). In the Biolog Microbial Identification System, Version 4.2 (Biolog Inc., Hayward, CA), GXM1 were similar with a match probability of 77.5% to P. citrulli. P.citrulli can trigger hypersensitive response in Nicotiana tabacum (Zhang et al., 2018). The GXM1 bacterial suspension (3×108 CFU·mL-1) was injection into 4-week-old N. tabacum leaves. The results showed typical HR after 24 h post injection. GXM1 was inoculated into nine watermelon and nine melon plants (10⁶ CFU·mL⁻¹). A 10 mM MgCl₂ solution served as the negative control, and the strain AAC00-1 (NC_008752.1) as the positive control, each inoculated into nine plants. After inoculation, plants were maintained at 75% relative humidity and 28℃ for 2 days. Plants inoculated with GXM1 and AAC00-1 exhibited typical bacterial fruit blotch (BFB) symptoms: irregular brown necrotic spots with faint chlorotic halos. The negative control plants remained symptomless and healthy. The pathogenicity tests were repeated three times with the similar results. Bacteria isolated from diseased melon leaves formed creamy white, circular, smooth, convex, opaque colonies on KB agar, consistent with the original isolate. Sequencing of the 16S rRNA genes of three colonies and BLAST analysis showed nucleotide sequence homology exceeding 99% with the type strain P. citrulli ICMP 7500. Based on pathogenic, biochemical, and molecular characteristics, the isolates from melon plants in Guangxi province (China) were identified as Paracidovorax citrulli. This is the first report of Paracidovorax citrulli infecting melon in Guangxi. Considering favorable environmental conditions, the pathogen can cause significant loses in melon production in Guangxi province (China). It is necessary to take phytosanitary measures to prevent spread of the pathogen.

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