LAUSR

Cucumber (Cucumis sativus L.) is one of the most important vegetables cultivated in the world. It is widely cultivated and mostly grown under greenhouse conditions (Sallam et al. 2021). Cucumber has a long growth cycle and is particularly susceptible to bacterial diseases. In May 2021, bacterial leaf spot was found on cucumbers of the variety Lyuyou NO.3 in Hainan Province, China. In the early stage of the disease, the leaves showed small yellow-brown spots in the shape of water stains. When exposed to light, a yellow halo around the disease spots could be seen. In later stages, the lesions gradually become larger and more yellow. The leaf veins around the disease site also gradually turned yellow (Figure 2a). In serious cases, the whole leaf turned yellow, resulting in leaf death. We collected plants with the same symptoms from 25 different farms in Hainan Province. Five plants were selected from each farm by the classic five-point sampling method and three leaves were selected from each plant, for a total of 15 leaves collected from each farm. Then three leaves were randomly selected from the 15 leaves on each farm for isolation of the pathogen, and a total of 75 leaves were isolated. We found that the incidence of the disease was from 20% to 30% based on a diagnostic test, which conducted on 75 cucumber leaves samples suspected of same symptom of cucumber, collected from Hainan Province. Using microscopy, bacterial streaming was observed to tentatively identify the causal agent as a bacteria. Tissue isolation was used to isolate the responsible pathogens. A 5 mm × 5 mm sample of tissue at the junction of diseased and healthy sections was collected. First, the surface of the tissue was disinfected in a 75% ethanol solution for 30 sec; then it was soaked in 2% NaOCl for 5-7 min, and finally, it was washed thrice in sterile distilled water. The tissues were inoculated onto lysogen broth culture media (LB) and cultured in a 28℃ incubator for 2 days. Bacterial colonies that emerged from the tissues were cultured in LB. Four isolated colonies were selected for verification. The colonies of isolated from the diseased leaves of cucumber are round, egg yellow and slightly sticky (Figure 2c). The isolate named PA-1 was identified by PCR amplification and sequencing of the partial 16S rRNA gene with the primer 27F/1492R (Lane 1991) and gyrB gene (Li et al. 2019). Sequences were stored in GenBank with the accession numbers OK576932.1 (16S rRNA, PA-1) and OL978577 (gyrB); BLASTn was used to compare these with other GenBank sequences. Sequencing of the 16S rRNA gene showed that PA-1 had a sequence length of 1403bp, with 99.78% genetic similarity to Pantoea ananatis strain MZ007857.1. Sequencing of the gyrB gene showed that the sequence length of PA-1 was 1136bp, with 99.29% genetic similarity to P. ananatis strain MW981331.1. Then, a pathogenicity text was conducted to verify Koch's postulates, which was done by first inoculating P. ananatis into LB liquid medium (shake culture at 28°C, 180 r/min). The log phase cell was collected by centrifugation (5,000 r/min for 2 min at 4°C), and inoculated strains were resuspended in sterile water at OD600 = 0.5. The bacterial suspension was inoculated on healthy cucumber leaves with a syringe. The control was sterile water, which was injected onto healthy cucumber leaves using the same methodology. The plants were placed in a greenhouse with a diurnal temperature difference of 21- 27°C and were observed daily. After two weeks, all bacterial inoculated plants developed symptoms of shriveling and necrosis (Figure 2b), while the control group showed no symptoms. From the symptomatic plants, the pathogen was isolated again and identified by morphological and molecular characterization. The sequences of the isolates recovered from the inoculated experiment matched 100% the sequences of the isolate PA-1. Koch's postulates were completed by following the previously described method. To our knowledge, this is the first report of P. ananatis causing leaf spot of cucumber.