First Report of Fusarium solani Species Complex Causing Vascular Wilt on Cauliflower in China.

Cauliflower (Brassica oleracea var. botrytis L.), which belongs to the family Cruciferae, is a cool-season vegetable with green leaves around a large hard white head of flowers. China is the leading cauliflower and broccoli producing country in the world, with approximately 10.71 MT production (FAOSTAT 2019). During September 2018 to July 2019, wilting symptoms were observed on cauliflower in several commercial fields, with approximately 45% to 65% disease incidence in Shen county (115°48'E, 35°98N) of Liaocheng city, Shandong province, China. Plant stunting, leaves yellowing and wilting, and dark brown, hollow appearance of vascular stem tissues were the symptoms prominently observed. To isolate the causal organism, nine symptomatic tissues were collected and cut into small pieces (5 × 5 mm), disinfected in 75% ethanol for 30 s, rinsed three times in sterile water, transferred onto potato dextrose agar (PDA) medium. The plates were then incubated in air-conditioned room at 26°C with an artificial 12 h light-dark cycle provided by incandescent lamp. In total, 15 single-spore isolates were obtained and morphological characterization of 15 isolates was done on both PDA and carnation leaf agar (CLA; Leslie and Summerell 2006). The mycelia on PDA were initially white, fluffy, later becoming brown, and the underside of the colonies were light brown. Typical macroconidia were abundant on CLA. Macroconidia were hyaline, slightly curved, one to five septa, both ends were smooth, measuring 3.7 to 6.4 μm × 23.7 to 38.1 μm (n = 40). Microconidia were oval to cylindrical, hyaline, zero to one septate, measuring 2.0 to 4.1 × 4.3 to 10.3 μm (n = 40). Chlamydospores were terminal or intercalary, solitary or in pairs, globose to oval, thick wall, smooth or rough, 6.3 to 9.8 μm. Based on morphological characteristics, all of the 15 isolates were identified as Fusarium solani (Leslie and Summerell 2006). The isolates were further identified based on PCR amplification. The ITS, mtSSU, EF-1α and RPB2 genes were amplified using primers ITS1/ITS4, NSM1/NSM2 (Li et al. 1994), EF-1 Ha/EF-2Tb (O'Donnell et al. 1998) and RPB2-5F2/fRPB2-7cR (O'Donnell et al. 2008). BLAST analysis showed that 15 isolates were highly similar to F. solani species complex, with 100% similarity for ITS (AB470904.1), mtSSU (KF125009.1), EF-1α (KF372878.1), and RPB2 (MK048113.1), respectively. The sequences of isolate HYC1410080102 had been deposited in GenBank with accessions MT378292.1 for ITS, MT383122.1 for mtSSU, OK595059.1 for EF-1α and OK595060.1 for RPB2, respectively. Pathogenicity of the 15 isolates were conducted on 4-true-leaf seedlings cv. Jinsong by dipping the roots into a conidial suspension (107 conidia/mL) for 10 min. The conidial was prepared from 7-day old cultures grown on CLA at 26°C and suspended in sterilized water. Control plants were dipped in sterile distilled water. All treated seedlings were planted in 5.0 cm diameter plastic pots containing pasteurized soil matrix. Then the plants were kept in a greenhouse at 15°C (night)/26°C (day) and 80%RH with natural daylight. Twelve days later, brown lesions appeared on stem bases in all inoculated cauliflowers, and finally, the plants wilted, similar to those observed in the field. The control plants remained healthy. Re-isolation of the infected tissues showed same morphological characteristics of F. solani as the original isolates, which were verified using PCR. To our knowledge, this is the first report of F. solani causing cauliflower wilt in China and the world (Farr and Rossman 2021). F. solani is a destructive pathogen with a broad host range worldwide and is responsible for significant crop losses, prevention and control measures should be considered.