First Report of Postharvest Corm Rot on Taro ( <i>Colocasia esculenta</i> ) Caused by <i>Agroathelia rolfsii</i> (syn. <i>Sclerotium rolfsii</i> ) in Guangxi, China

Taro (Colocasia esculenta) is an important vegetable and grain economic crop in China. In 2023, China was the top exporting trade country of taro and its output was the second largest country after Nigeria. The cultivation of taro mainly distributed in the southern China, including the Pearl River Basin, the Yangtze River Basin, the Huaihe River Basin and Taiwan Province. In December 2024, 28 taro corms infected rot disease on surface with white mycelium and sclerotia was collected from Nanning vegetable market of Guangxi province. After sealed storage at room temperature for one week, taro corm showed water-soaked softening rotten, the incidence rate reached 100%. To identify the disease causal agent, sclerotia produced on taro corm were picked and transferred on potato dextrose agar (PDA) medium. Numerous white small globoid white sclerotia were formed on PDA after 5 days of culture. The sclerotia were white at first, and gradually turned dark brown (2 to 4 mm in diameter, n=50). Aerial mycelia usually formed many narrow hyphal strands 4 to 8 μm wide. Three uniform strains (YS1, YS2, YS3) were obtained from the diseased taro corm. Pathogenicity of the representative strain YS2 was performed by placing a mycelial agar discs (6 mm in diameter) on 5 taro corms, and while inoculation with sterile PDA was used as a negative control. The inoculated and non-inoculated taro corms were sprayed sterile water, then incubated with plastic film for maintained high moisture. All the corms were placed inside of a growth chamber at 28 ± 2°C with a 14-h photoperiod and 80% relative humidity. After one week, all inoculated corms showed white mycelium with a large number of white fine granular sclerotial, while the control corms were asymptomatic. After two weeks, the sclerotium particles on the corms became coarse and turned brown. Water-soaked soft rot appeared in the internal tissue of corm, the symptoms were the same as those occurring naturally, and the fungus was re-isolated. To further confirm the species-level identification, partial sequences of the mitochondrial small subunit (SSU), ribosomal DNA internal transcribed spacer (ITS) and nuclear ribosomal large subunit (LSU) genes were amplified and sequenced using the primers pairs of ITS-Fu-F/ITS-Fu-R, ITS1/ITS4 (White et al. 1990) and SRLSU1//SRLSU2 (Kumar et al., 2016), respectively. The resulting sequences of SSU, ITS and LSU were uploaded in GenBank (Accession no. PV798860 to PV798862, PV939281 to PV939283, PV798865 to PV798867) and shared 100%, 100% and 99.89%-100% sequence identity with Agroathelia rolfsii of strains W1, SR001 and SrB1, respectively. Phylogenetic analysis conducted with neighbor-joining (NJ) method using MEGA6.0 revealed that the strains shared a common clade with the reference sequence of A. rolfsii in GenBank Data Library. Based on the symptoms, morphological and molecular characteristics, the fungi were identified as A. rolfsii (syn Sclerotium rolfsii) (Paparu et al. 2020; Lao et al., 2024). Although the pathogen of Sclerotium has been previously reported causing petiole and female taro rot during the growth stages at field in China (Xie, 2002). To our knowledge, this is the first report of taro corm rot caused by A. rolfsii during the postharvest period in China. The discovery showed that the disease has a great influence on the safe preservation of taro during postharvest period, and the disease prevention and control of taro should be paid more attention in the future.