First Report of Charcoal Rot Caused by Macrophomina phaseolina of Sweet Potato in Southern China
Authors: Tian Tian , Lin Xiangsheng , Tang Fajiang , Ding Xiawei , Wang Zhang-Ying , Liang Xuelian , Huang LiFei 
Published: January 2025

Sweet potato (Ipomoea batatas (L.) Lam) is a major food crop that is cultivated in southern China (Huang et al. 2020). In February 2024, a survey was conducted in three warehouses of Suixi County (21°3′ N, 109°50′ E), Dianbai County (21°44′ N, 111°6′ E) and Chengmai County (19°57′ N,109°56′ E). Symptoms similar to charcoal rot were found on the storage roots of sweet potato. An inspection of 1,000 kg of sweet potatoes in each of the three warehouses showed that 5.5% (Suixi), 6.8% (Dianbai), and 8% (Chengmai) of the sweet potatoes were affected respectively. The disease began as a firm, moist rot that was reddish brown to brown and initially restricted to the cortex of the storage root. As the decay progressed, the pathogen crossed the vascular cambium, and the pith progressively rotted. Two distinct zones become apparent within the infected tissue. The leading edge continued as an area of reddish-brown decay, and an area of black tissue developed behind the area of active decay. The infected areas often encompassed the entire root, which eventually dried and became hard and mummified (Clark et al. 2013). Three sick sweet potatoes were selected from each warehouse for separation experiment，the early lesions of sweet potato storage root (3 mm × 3 mm) were isolated. Diseased segments were disinfected with 75% ethanol for 30 s, rinsed with 2.5% sodium hypochlorite for 3 min and washed three times with sterile water. The segments were placed on Potato Dextrose Agar (PDA) adjusted to a pH of 6.2. Isolates were purified by hyphal tipping. Ten isolates exhibited moderate growth rates and produced colonies that were white to fluffy black after 4 to 5 days of growth on PDA at 28℃. Microscopic observation showed that there were a large number of ellipsoidal microsclerotia between the hyphae. The microsclerotia were subfusiform, black and approximately 50 to 300 µm long. The morphological features matched the description of Macrophomina phaseolina (Abd et al. 2019), the causal agent of charcoal rot. Three representative strains of the ten that were isolated from the three warehouses were selected for identification. PCR was performed using the primer sets ITS1/ITS4 to amplify the internal transcribed spacer (ITS) region of the rDNA (White et al. 1990). The PCR products with positive amplicons were sent to Sangon Biotech (Guangzhou, China) for sequencing. The analysis showed that the sequences of the three strains were consistent. Sequences were deposited in GenBank with the accession number PP887785.1, PQ727081.1 and PQ727082.1. Select one of the sequences for analysis (CRI-MP2: PP887785.1), It had 596 bp and was 100% homologous with the reference sequence of the M. phaseolina ITS (GenBank accession numbers OR240859.1, OR240847.1, OR240855.1, OR240846.1, and OR240843.1). Further verify whether it is the M. phaseolina. The genomic DNA of the three strains was amplified by the specific primer of M. phaseolina (Babu et al. 2003), and the specific fragments that were 350 bp long were amplified in all three strains. The pathogenicity was determined using one isolate (Chengmai CRI-MP2). The pathogenicity of the isolate was confirmed by inoculating three healthy storage roots of the sweet potato cultivar `Pushu 32` with a mycelial plug (0.5 cm × 0.5 cm) of M. phaseolina. An additional three `Pushu 32` storage roots were each inoculated with a sterile plug of PDA agar (0.5 cm × 0.5 cm) to serve as controls. All the storage roots were incubated at 25°C. All the inoculated storage roots exhibited the same symptoms observed in the field at 5 days after inoculation, whereas the controls remained asymptomatic. The fungi that were reisolated from the symptomatic roots were morphologically and molecularly identical to M. phaseolina, while no pathogens were isolated from the mock inoculated controls. This experiment was repeated three times with similar results and thus, fulfilled Koch’s postulates. M. phaseolina is a generalist soilborne fungus present all over the world. Under high temperatures and low soil moisture, the fungus has a broad host range and can survive for a long time (Nathalie et al. 2021). The high temperature in southern China is suitable for the cultivation of sweet potato, but it interferes with the control of diseases, which can be challenging. To our knowledge, this is the first report of charcoal rot of sweet potato caused by M. phaseolina in southern China. It provides new information and clues for the next step of disease prevention, which is to control the spread of this pathogen.