First Report of Storage Tuber Rot in Sweetpotato (Ipomoea batatas) Caused by Plenodomus destruens in Korea

Sweetpotato, Ipomoea batatas L., contains substantial levels of protein, carbohydrates, vitamins, and minerals and is popular in tropical and subtropical countries in Asia (China, Korea, and Japan), the Pacific Islands, Latin America, and Africa. Sweetpotato foot and storage tuber rot caused by the fungus Plenodomus destruens Harter are significant threats to the production of sweetpotato worldwide (Loebenstein and Thottappilly 2009). This fungus can cause disease on seedlings, mature plants, and storage roots. Decay or rot begins at the attached end, preliminarily as slight sunken lesions that become larger with time. Storage tuber rot caused by P. destruens was first observed in China in 2014 (Gai et al. 2016). Outbreaks of storage tuber rot disease were frequently observed on sweetpotato storage roots in Korea from July to August in 2015 to 2018, and it was important to confirm the cause of the disease. We collected samples from storehouses and surface sterilized with 1% NaOCl for 5 min, washed three times with sterilized distilled water, cut into small pieces, placed onto potato dextrose agar (PDA), and isolated. The selected fungi were stored at the Sweetpotato Research Laboratory, Bioenergy Crop Research Institute, Rural Development Administration, Muan, Korea (isolates SPL15025, SPL18007, and SPL18008). The representative isolate SPL15025 was cultured for 14 days on PDA and kept at 25°C with a 12-h light/dark photoperiod for sporulation. Different sized conidiomata observed and single-celled oblong to ellipsoid conidia with rounded ends were produced, measuring 6.4 to 10.6 × 1.9 to 3.9 μm (average 8.2 × 2.8 μm). The internal transcribed spacer (ITS) and translation elongation factor 1-α (EF-1α) genes were sequenced and analyzed to confirm species identity. The rDNA-ITS region was amplified with primers ITS5 and ITS4 (White et al. 1990), and EF-1α was amplified with primers EF 1 and EF 2 (O’Donnell 2000). These sequences were deposited in GenBank (accession nos. MH465671 for ITS and MH560611 for EF-1α). BLASTn search analysis showed 99 to 100% sequence similarity with P. destruens references (accession nos. JX421687/ITS and KP990656/EF-1α). Based on the conidial characteristics and DNA sequence analysis, the fungus was identified as P. destruens (Gai et al. 2016). Pathogenicity assays were conducted on two local sweetpotato varieties (Annobeni and Yonghwangmi) using a 5-mm P. destruens plug, with three replications, and a blank PDA plug served as a control. After 3 weeks of incubation in plastic containers at 25°C with high humidity, typical symptoms were detected. No symptoms were observed on the control samples, and P. destruens was reisolated from the artificially inoculated storage roots. To the best of our knowledge, this is the first report of storage tuber rot of sweetpotato caused by P. destruens in Korea.