The romaine lettuce (Lactuca sativa L.) is one of the most frequently consumed vegetables in Korea. In January 2023, the romaine lettuce cultured within an indoor hydroponic farm in South Korea displayed severe disease, with an incidence of approximately 13.7% of 300 plants. The diseased plants showed symptoms of stunted growth, lower leaf yellowing, and brown or black-colored soft and mushy root rot, in which the outer layer of root was sloughed off, leaving a thread-like appearance. These symptoms were similar to those of Pythium root rot previously reported to occur on lettuce (McGehee et al., 2018; Stanghellini and Kronland, 1986). Samples of romaine lettuce with rot symptoms were collected from the hydroponic farm. The infected roots were rinsed three times with sterilized distilled water (SDW), dried on sterilized filter paper, and sliced into segments (about 0.5 cm in length), which were placed into Petri dishes (9 cm in diameter) containing V8 juice agar (V8A: 8% V8 juice and 1.5% agar powder) and cultured at 25°C for 2 days. The emerging hyphae were transferred to new Petri dishes containing V8A. After four rounds of sub-culturing, a total of 11 strains were isolated and all of them exhibited the same morphology. Strain KNU2301TP was purified via isolation of a single zoospore and stored at -80°C. The mycelia were non-pigmented. The hyphae obtained from a three-day-old culture grown in V8A were aseptate and the diameter of major hyphae was up to 7 µm. The filamentous sporangia were not to slightly inflated and formed dendroid-like branches. Vesicles containing zoospores were formed on the filamentous sporangia. The encysted zoospores were spherical with a diameter of 8.1 to10 µm (average 8.9 ± 0.6 µm; n = 50). These morphological characteristics of KNU2301TP were similar to those of a previously reported oomycete, Pythium dissotocum (Van der Plaats-Niterink 1981). The genomic DNA was extracted from mycelia cultured on V8A by a previously described extraction method (Chi et al. 2009). Sequences of internal transcribed spacer (ITS) region and cytochrome c oxidase subunit II (cox2) gene were amplified with paired primers ITS1/ITS4 and cox2_F/R, respectively (Callaghan et al., 2022; Hudspeth et al. 2000; White et al. 1990). The resulting sequences were deposited in GenBank (GenBank Accession Nos.; ITS: OQ683867; cox2: OQ700848). The sequences of ITS and cox2 gene of strain KNU2301TP were 99.61% and 100% identical to the sequences of ITS (MG719858) and cox2 gene (MG719859) of P. dissotocum strain YNP-3, respectively. Based on the result of the morphological characterization and sequence analysis, the strain KNU2301TP was identified as P. dissotocum. For Koch's postulates, 15 lettuce seedlings (eighteen-day-old) were inoculated by immersing the roots in a spore suspension (1 × 105 zoospores/ mL) and incubated at 25°C under 16/8 h light/dark cycles for 8 days. Fifteen plants of same age were treated with SDW in the same manner as a control The symptoms resembling those originally found at the farm were developed on the inoculated plants, but not on controls. The strain reisolated from the inoculated plants by same method mentioned above was confirmed as strain KNU2301TP by analysis of morphology and ITS and cox2 sequences. To our knowledge, this is the first report of root rot on hydroponically grown lettuce caused by P. dissotocum in Korea. Root rot on lettuce caused by P. dissotocum has been previously reported in USA, Canada, and Finland (McGehee et al. 2018; Stanghellini and Rasmussen 1994). Since lettuce is an important and popular leafy vegetable around the world, further work would focus on developing efficient strategies to manage this Pythium root rot disease.
Keywords: Pythium dissotocum; lettuce; root rot.