First Report of <i>Plectosphaerella cucumerina</i> Causing Wilt in Cucumber (<i>Cucumis sativus</i>) in China

Cucumber (<i>Cucumis sativus</i> L.) is an important vegetable. China's annual output exceeds 50 million tons, accounting for more than 40% of the world total. In Nov 2021, a wilt disease was observed on cucumber plants in Beiluo Town, Shouguang, Weifang City, Shandong Province, China (36°88'N, 118°74'E). The incidence across the greenhouses (≈0.13 ha) was approximately 25%. Leaves of plants in the field turned yellow and wilted. Finally, the whole plant withered and died. The base of the main vine stem shrank and the epidermis cracked longitudinally. Tissue fragments from infected stem bases (5 mm × 5 mm) were surface disinfected with 0.1% mercuric chloride solution for 30 s, followed by 75% ethanol for 30 s, and then washed three times with ddH₂O. These sections were then placed onto PDA and incubated at 25 ± 2°C for five days. Twelve strains (similar colony morphology) belonging to fungal species were obtained from 10 disease samples. On PDA, the colonies appeared off-white or light-orange, occasionally with radiating wrinkles, a damp surface, and defined margins. The conidiophores were solitary, unbranched or rarely irregularly branched, transparent, smooth-surfaced, and thin-walled. Sporulation cells (bottle-stem type autotrophic hyphae) extended with a transparent, smooth surface, a slightly wider base, and a pointed top. Typically, they were solitary upright bottle-necks, 6.3-24.3 μm × 1.4-3.3 μm (n = 30). The conidia aggregated into a head-shaped, oval, shaped, or round shape with narrow ends and a wide middle, transparent, smooth-surfaced, with thin walls and 0-1 septa, containing oil globules. The size of conidia ranged from 4.0-8.9 μm × 2.0-4.0 μm. These morphological characteristics were similar to those of <i>Plectosphaerella</i> sp. At the same time, the internal transcriptional spacer (ITS), and the large subunit (LSU) of the 28S rRNA gene were amplified and sequenced using the primer pairs ITS1/ITS4 and LR5/LROR (Giraldo et al., 2019). BLASTn analysis, the obtained sequences of the ITS amplicon (OR994322), and the LSU domain amplicon (PP092994) showed 99.80% (507/508 bp) and 97.21% (872/897 bp) identity to sequences of <i>P. cucumerina</i> isolates OP881967.1 and MK143394.1, respectively, in the NCBI database. A multilocus phylogenetic tree constructed by MEGA 11 software showed that the representative isolates were assigned to the group of <i>P. cucumerina</i>. To confirm their pathogenicity, 10 ml of spore suspension (105 spores/ml) were poured onto the rhizomes of seedlings (1-2 leaf stage; 20 plants per strain). Another 20 seedlings treated with sterile distilled water were used as the control. Each trial was performed three times. Seedlings inoculated with <i>P. cucumerina</i> began to show symptoms on the 5th day, which continued until the 21st day after inoculation. The symptoms were similar to those observed in the diseased plants in the field. The control seedlings showed no symptoms. <i>P. cucumerina</i> was re-isolated from inoculated diseased seedlings with 100% homology, whereas no fungal growth was observed from the control seedlings. The results indicate that <i>P. cucumerina</i> was the pathogen of cucumber blight. So far, More than 10 species of plants, including <i>Brassica oleracea</i> and <i>Foeniculum vulgare</i>, are known hosts of <i>P. cucumerina</i> in China (Li et al.2017, Su, et al. 2017). This is the first report of <i>P. cucumerina</i> causing wilt on <i>C. sativus</i> L. (Lei et al., 2017). Our study laid a critical foundation for the prevention and control of this disease on cucumbers in China.