First Report of Bacterial Leaf Spot on Muskmelon Caused by Pseudomonas syringae pv. syringae in China.

Muskmelon (Cucumis melo L.) is one of the most widely cultivated and economically important fruit crops in the world. In January 2023, muskmelon leaves of cultivar 'Sheng Gu' were observed with irregularly shaped spots in four nurseries in Wanxiang Village, Pudong District of Shanghai, China. Initial symptoms were irregular soaking on the leaves, which progressed to rotting and necrotic spots. The disease incidence of melon seedlings in different nurseries ranged from 10 to 25%. To isolate and identify the causal agent, the small pieces of lesion tissues (5×5 mm) from symptomatic leaves were sterilized in 75% ethanol for 30 s and rinsed three times with sterile water. Following that, tissues were crushed with sterile glass rod in a sterile 2.0 mL centrifuge tube containing 100 µl of sterile water. The suspension was serially diluted before being spread on Luria-Bertani (LB) medium. After 48 h of incubation at 28°C, the cream-colored bacterial colonies from the 10-4 dilution were tiny and purified by streaking on new LB plates. To confirm the species identity of the bacterial isolates, genomic DNA was extracted from four independent representative colonies from different diseased plants, and several conserved genes were amplified and sequenced, including the 16S rRNA gene with primers 27F/1492R, gyrB gene with primers gyrBFor2/gyrBRev2, and rpoD gene with primers rpoDFor2/rpoDRev2 (Lelliot et al. 1966; Murillo et al. 2011). The results showed that the four colonies were identical. Using BLAST analysis in GenBank, the 16S rDNA (accession no. OQ659765, 1,402 bp), the gyrB (accession no. OQ708618, 911 bp), and rpoD sequences (accession no. OQ708619, 798 bp) showed 99.86-100% homology with 99-100% coveage as the corresponding gene sequences in the P. syringae pv. syringae strain HS191 (accession no. CP006256.1). The bacterial isolate was designated as P. syringae pv. syringae strain PDTG. Phylogenetic tree analysis of 16S rDNA, gyrB and rpoD genes further verified that the bacteria isolate was in close proximity to P. syringae pv. syringae. Additionally, all four isolates were detected in PCR with P. syringae pv. syringae specific primers, PsyF/ PsyR (Borschinger et al. 2016; Guilbaud et al. 2016). Ten two weeks old healthy 'Sheng Gu' muskmelon seedlings were inoculated by spraying with a bacterial suspension of 108 CFU/ml, and ten additional healthy plants treated with sterilized water served as the control. The inoculated plants were maintained at 25°C and 75% relative humidity for 7 days in artificial climate room. Water-soaked rot, similar as those seen in the nurseries, appeared on leaves 7 days after inoculation (dai), while the leaves of control plants remained healthy. The bacteria were re-isolated from rot of inoculated leaves and confirmed as the original pathogen by PCR with the PsyF/ PsyR primers and the 16S rRNA gene sequences. To our knowledge, this is the first report of P. syringae pv. syringae causing bacterial leaf spot on muskmelon in China, and this report expands the host range of P. syringae pv. syringae.