First Report of <i>Agroathelia rolfsii</i> Caused Southern Blight on Taro in Guangdong Province, China

Taro (Colocasia esculenta) is one of the world’s most widespread food crops, cultivated in tropical to temperate regions (Li et al. 2025). China is the second-largest producer and largest exporter of C. esculenta, with a cultivation history of more than 2000 years (Wang et al. 2020). In July 2022, an outbreak of southern blight disease was observed on taro plants in some agricultural fields in Wujiang District and Lechang County, Shaoguan City, Guangdong Province, China (24°35′N, 113°20′E). Infected plants initially exhibited browned petioles, yellowed leaves, and eventually wilted and died. White mycelium and numerous brown spherical sclerotia appeared on the surface of petioles near the soil line. Disease incidence reached 10% in severely infested fields. Using a five-point sampling method, ten plants exhibiting typical symptoms were collected from the field (approximately 0.2 hectares). Diseased stem segments (1.0-1.5 cm) were excised, rinsed with sterile water, and surface-sterilized by sequential immersion in 75% ethanol for 30 s and 2% sodium hypochlorite solution for 90 s. The segments were then rinsed three times with sterile water, air-dried, and plated onto PDA (6g/l potato infusion powder, 20.0g/l glucose, 20.0g/l agar), all culture experiments included 3 replicates, with negative controls using sterile PDA under identical incubation conditions. Plates were incubated at 26°C in the dark for 3 days to isolate the causal pathogen. Ten fungal isolates with similar colony morphology were obtained and purified. These isolates formed radial colonies, and abundant sclerotia developed after 4-7 days of incubation at 26°C. Sclerotia diameter ranged from 0.9 to 1.8 mm (n = 80), with their color transitioning from white to pale yellow and finally dark brown. These characteristics matched those described for Agroathelia rolfsii (Li et al. 2020; Redhead and Mullineux 2023). A representative isolate, WUJIANG01, was selected for further molecular identification. The internal transcribed spacer (ITS) region, 18S small subunit rRNA (SSU), and 28S large subunit rDNA (LSU) were amplified using the primers ITS1/ITS4 (Utama et al. 2022), NS1/NS4 (Li et al. 2018), and LR0R/LR5 (Rehner and Samuels 1994), respectively. The ITS, SSU, and LSU amplicons were sequenced and deposited in GenBank with the accession numbers OP349638, OP355438, and OP355432, respectively. Sequence analysis revealed that the ITS, SSU, and LSU sequences of the WUJIANG01 isolate exhibited ≥97% identity with the ITS (PQ417144, 613/622 and OP279917, 621/639), SSU (PV495703, 1073/1086 and PP549544, 1073/1086), and LSU (PV495696, 930/932 and PP549543, 929/934) sequences of Agroathelia rolfsii. Moreover, a phylogenetic tree of the ITS sequences of WUJIANG01 was constructed using the Neighbor-Joining method in MEGA7 software and revealed that the isolate WUJIANG01 clustered with Agroathelia rolfsii isolates. For pathogenicity tests, five sclerotia from a fifteen-day-old colony and a fungal mycelial mat were inoculated at the stem bases of ten healthy taro seedlings (30-day-old). Five healthy seedlings inoculated with sterile PDA plugs served as controls. All plants were placed in a greenhouse at 25°C and 90% relative humidity with ambient light. Five to seven days after inoculation, all plants inoculated with WUJIANG01 showed typical symptoms of stem rot, whereas control plants remained symptomless. The pathogen was re-isolated from infected tissues, thus fulfilling Koch’s postulates. To our knowledge, this is the first report of Agroathelia rolfsii in Guangdong Province, China. This finding is important to raise awareness of the spread of Agroathelia rolfsii and design management strategies for taro.