First Report of Potato Wilt Caused by <i>Plectosphaerella cucumerina</i> in Korea

Potatoes (Solanum tuberosum L.) are one of the most important food crops (Spooner et al. 2010). In Korea, potatoes are mainly grown in the Gangwon state region, which contains many highlands. As temperatures in spring and summer have recently increased, new diseases have been documented in potato fields in Korea. In July 2024, we observed wilt symptoms in a potato field initially in Pyeongchang-gun, Gangwon-state (37°37'15.3" N, 128°28'19.9" E). The same symptoms began to appear in other areas in Gangwon-state. The incidence rate in the fields investigated was approximately 1%. Chlorosis and wilt symptoms were observed from the lower to the upper parts of the stem, and wilt symptoms were observed gradually on the leaves. When we cut the xylem part of the stem, we could see that the water vessels were turned brown . The symptoms are similar to those of Verticillium wilt (Johnson and Dung 2010). Five diseased plants were collected and transported to the laboratory. Symptomatic tissue samples were surface-sterilized with 1% NaOCl for 1 min and 70% EtOH for 2 min. The samples were cultured at 25 °C on water agar medium, and when mycelia emerged from the tissue, single conidium was subcultured on potato dextrose agar (PDA) medium to isolate the fungal strain. Colonies cultured for 7 d on PDA were 41–43 mm in diameter and had a pinkish-cream color. The conidia were colorless, hyaline, 0-1 septate, ellipsoidal or cylindrical, and occasionally curved. Conidia were (7.86-)8.89(-9.85) × (3.18-)4.16(-5.26) μm in diameter (n = 100). The morphological characteristics of the colonies and conidia were similar to those of Plectosphaerella cucumerina (Arzanlou et al. 2013). The genomic DNA of the strains HARI24D137 and HARI24D157 were extracted. The internal transcribed spacer (ITS) of rDNA was amplified using the ITS1F/ITS4 primers (Gardes and Bruns 1993), and the translation elongation factor 1-α (TEF) region was amplified using the EF-688F/EF-1251R primers (Alves et al. 2008). The ITS sequence showed 99.09% identity with P. cucumerina MH858371.1, and the TEF sequence showed 98.95% identity with P. cucumerina KY421330.1. The concatenated DNA sequence formed a monophyletic group in the neighbor-joining phylogenetic tree. Strain HARI24D137 was deposited in the Korean Agricultural Culture Collection (KACC), and the DNA sequences were deposited in GenBank (ITS: PV366895, TEF: PV440315). For the pathogenicity test, 20 potato seedlings were prepared approximately two weeks after germination. Ten seedlings were planted in plastic pots after soaking in a conidial suspension (5×105 spores/ml) for 30 min; control group seedlings were soaked in distilled water for 30 min. After four weeks in the greenhouse (25-30°C), symptoms including wilting stems and yellowing leaves developed, which were consistent with symptoms observed in the field. The test was performed twice with similar results. To complete Koch's postulates, the fungal strain was re-isolated from the inoculated plants and the identity was confirmed based on morphology and DNA sequence. Incidence of wilt caused by P. cucumerina has been increasing in potato fields in Korea since 2024. Because P. cucumerina is a soil-borne fungus (Zhao et al. 2025), chemical control is currently very difficult, and studies on soil management and other cultural controls are needed.