Abstract
Flue-cured tobacco (Nicotiana tabacum L.) is a significant cash crop globally. In August 2022, necrotic lesions on stem associated with root rot and wilting were observed on flue-cured tobacco (Cv. Yunyan 87) in fields located in Banxin village (27.95N,109.60E) of Fenghuang county in Xiangxi Autonomous Prefecture, Hunan Province, China. The affected and damaged area of tobacco is approximately 10 hectares, with adisease incidence of 60%. Lots of small black speckling within the lower stem of the affected plant, vascular tissue changed to black, dry rot, and looked like charcoal breezes. Small pieces were cut from healthy and diseased tissues, surface sterilized with 5% NaClO for 3 min and 75% ethanol for 1 min, rinsed with sterile distilled water and air-drying, incubated on oat medium incubated at 28℃ for five days. These isolates grew fast and produced typical black microsclerotia. The morphological were septate hyphae and microsclerotia. The microsclerotia were black and regularly round, with a 42.5 - 92.9 μm diameter. These morphological features were consistent with Macrophomina phaseolina (Smith and Wyllie 1999). The internal transcribed spacer (ITS) rDNA and translation elongation factor 1-α (TEF1-α) genes of three representative isolates were amplified and sequenced using the primers ITS1/ITS4 and EF1-728F/EF2R (Machado et al. 2019). Our resulting sequences (GenBank accessions OR435093, OR435101, OR435102 for ITS; OR891780, OR891781 and OR891782 for EF1-α) showed 99-100% similarity with M. phaseolina by NCBI blast. Phylogenetic analysis was conducted using MEGA-X software with the NJ method. The combined sequences grouped with isolates to M. phaseolina with 100% bootstrap support. The strain XF22 has been sent to the China General Microbiological Culture Collection Center (CGMCC3.25349). Pathogenicity tests were conducted by inoculating potted plants (six plants per isolate, three times) from 45 day-old tobacco seedlings cv. Yunyan 87. Stems were randomly gently scratched with sterile needles, and a 5 mm agar disc with mycelium of the pathogen was attached to the surface of each wound, with a sterilized agar disc as control. Inoculated seedlings were incubated in growth chambers at 26℃ and 60% RH with a 12 h photoperiod/day. After ten days, symptoms that brown or black lesions on the inoculated lesions were dotted with numerous black, hard microsclerotia similar to those naturally occurring on the diseased plants, but not on the control plants. The same pathogen was re-isolated consistently, fulfilling Koch's postulates. Based on morphological, molecular, and pathogenicity test results, these isolates were identified as M. phaseolina. Charcoal rot of tobacco, caused by M. phaseolina was previously found in Guangxi in 1989 (Zhu et al. 2002), while this is the first report of M. phaseolina causing charcoal rot on flue-cured tobacco in Hunan, China. We speculate that the planting area is influenced by the preceding crop sesame. The soil carries M. phaseolina, which can cause stem rot of sesame, leading to the occurrence of tobacco charcoal rot. Our results indicated that charcoal rot caused by M. phaseolina is a new threat to flue-cured tobacco production and lue-cured tobacco might be acting as a reservoir and spreading this pathogen to other economically crops in China.
Published Version
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