First Report of Neck Rot Caused by Botrytis aclada on Onion (Allium cepa L.) in Gansu Province, China.

Onion (Allium cepa L.) is one of the most important cultivated vegetable crops in many countries, including China (Hanci 2018; Steentjes et al. 2021). Gansu, in the northwest region of China, is a major area of onion production (Zhang et al. 2022). In October 2021, typical symptoms of neck rot were observed on stored onion bulbs (cv. Honghe) in Jiuquan, Gansu Province, China. Further surveys of 20 bags of onion bulbs randomly selected in a storage facility with bulbs harvested from 73 ha indicated that approximately 5% of the bulbs had typical neck rot symptoms. The neck of infected bulbs developed a water-soaked decay, with softened and discolored inner scales with white to gray mycelium in the neck. Infection usually began in the neck and sometimes spread through the entire bulb. In severely affected bulbs, the fleshy scales were span style="font-family:'Times New Roman'">decayed and the bulbs had shrunk, with black sclerotia between the rotting scales in the neck and shoulders of the bulb. Small pieces cut from the margins of lesions were surface-disinfested with 75% ethanol for 10 s, and 1% NaClO for 1 min, rinsed three times in sterile distilled water, dried on sterile filter paper, placed onto potato dextrose agar (PDA), and incubated at 20 ± 1℃ for 5 days in the dark. Ten pure cultures were obtained by single-spore isolation (one from each of 10 bulbs). All 10 isolates produced colonies that initially were white, and became gray to dark in color with gray mycelium that was covered with abundant conidia resembling Botrytis species when cultured on PDA at 20 ± 1℃ for 10 days in the dark. Conidia (n = 100 per isolate) were one-celled, ellipsoid or ovoid, beige to dark brown, and 5.6 to 14.8 × 3.9 to 8.5 µm. Sclerotia were not produced by any of the isolates on PDA after incubation at 20 ± 1°C for 30 days. The internal transcribed spacer (ITS) region of ribosomal DNA, glyceraldehyde-3-phosphate dehydrogenase (G3PDH) gene, heat shock protein 60 (HSP60) gene, and DNA-dependent RNA polymerase II core subunit (RPB2) gene of two representative isolates, JQ21AC03 and JQ21AC09, were amplified and sequenced with primers ITS1/ITS4, G3PDH-F/G3PDH-R, HSP60-F/HSP60-R, and RPB2-F/RPB2-R, respectively (Staats et al. 2005), and deposited in GenBank (ITS: OP604277 and OP604283; G3PDH: OP627512 and OP627515; HSP60: OP627513 and OP627516; and RPB2: OP627514 and OP627517). BLASTn analysis of the resulting sequences showed 98 to 100% similarity with those of Botrytis aclada. A maximum likelihood phylogenetic tree generated by combining the sequenced loci using MEGA11.0, clustered isolates JQ21AC03 and JQ21AC09 with B. aclada with 99% bootstrap support. Based on these results, isolates JQ21AC03 and JQ21AC09 were identified as B. aclada. Pathogenicity of the two isolates on the onion cv. Honghe was confirmed by inoculating 30 heathy onion bulbs per isolate with 100 µL of conidial suspension (1×105 conidia/ml), while control bulbs were inoculated with 100 µL sterile distilled water. All 30 bulbs inoculated with each isolates developed neck rot symptoms after 30 days incubation at 15 ± 1ºC in the dark, with symptoms identical to those observed in the original storage facility, whereas control bulbs remained symptomless. The pathogen was re-isolated from the symptomatic tissue of inoculated bulbs, fulfilling Koch's postulates. hang et al (2008) documented this pathogen causing bulb rot of onions in Wuhan, Hubei Province, 1,700 km from Gansu. To the best of our knowledge, this is the first report of B. aclada causing neck rot on onion bulbs in storage in Gansu Province, which is the main onion production region in China. Considering that onion is the main source of income for growers in Gansu, further studies will be required to understand the epidemiology of this disease and foster appropriate disease management measures to avert disease outbreaks in the future.