In the past, rice bakanae was considered an endemic disease that did not cause significant losses in Korea; however, the disease has recently become a serious threat due to climate change, changes in farming practices, and the emergence of fungicide-resistant strains. Since the bakanae outbreak in 2006, its incidence has gradually decreased due to the application of effective control measures such as hot water immersion methods and seed disinfectants. However, in 2013, a marked increase in bakanae incidence was observed, causing problems for rice farmers. Therefore, in this review, we present the potential risks from climate change based on an epidemiological understanding of the pathogen, host plant, and environment, which are the key elements influencing the incidence of bakanae. In addition, disease management options to reduce the disease pressure of bakanae below the economic threshold level are investigated, with a specific focus on resistant varieties, as well as chemical, biological, cultural, and physical control methods. Lastly, as more effective countermeasures to bakanae, we propose an integrated disease management option that combines different control methods, including advanced imaging technologies such as remote sensing. In this review, we revisit and examine bakanae, a traditional seed-borne fungal disease that has not gained considerable attention in the agricultural history of Korea. Based on the understanding of the present significance and anticipated risks of the disease, the findings of this study are expected to provide useful information for the establishment of an effective response strategy to bakanae in the era of climate change.

Lettuce (Lactuca sativa L.) is a vegetable belonging to the family Asteraceae, and one of the major leafy vegetables in Korea. Lettuce has long been consumed as a fresh vegetable. In 2020, 96,774 tons were produced in about 3,800 ha according to Statistics Korea, and various types of lettuce have been bred for year-round production in line with consumer needs (Jang 2007). In August 2020, symptoms of a disease on young leaves of green lettuce (cv. Cheongpungyeoreumchima) were observed in the commercial greenhouse, Jeongeup (35°37'17.5"N, 126°53'40.4"E), Korea. About 60% of plants in 500 m2 of the cultivation area were infected. The tips of infected leaves appeared water-soaked, with brown to black lesions and were covered with dark masses of sporangiophores. Two fungal isolates NC20860 and NC20861 were isolated from monosporous sporangiola in infected leaves. Two isolates produced fast-growing colonies on potato dextrose agar medium (PDA) with abundant white mycelium at the top and bright yellow pigmentation at the underside in only 2 days. Morphological characteristics of the two fungal isolates were investigated from 30 days after inoculation on PDA. Sporangiophores bearing sporangia were erect, non-septate, unbranched, hyaline, and straight. Sporangia were hyaline to brown, globose to subglobose, 28.4 to 33.6 µm diameter. Sporangiospores from sporangia were brown to dark brown, ellipsoid to ovoid, distinctly longitudinally striate, with hyaline appendages at each pole, 6.3 to 7.5 µm wide and 13.0 to 16.0 µm long. Sporangiophores bearing sporangiola were erect, unbranched, hyaline and sporangiola were fusiform, ellipsoid to broadly ellipsoid, 7.6 to 9.8 µm wide, 13.3 to 17.8 µm long. Based on the morphological and cultural characteristics, this fungus was identified as Choanephora cucurbitarum (Berk.&Ravenel) Thaxt (Kirk 1984). To identify the species of the two isolates, DNA was extracted by using the Maxwell® RSC PureFood GMO and Authentication Kit (Promega). The internal transcribed spacer (ITS) region of ribosomal DNA was amplified using nTaq-Tenuto polymerase chain reaction (PCR) kit and therein specified protocol (Enzynomics). The primers ITS1F and ITS4 were used for PCR and sequencing (Gardes 1991). The ITS sequences of the isolates NC20860 and NC20861 from Korea (NCBI GenBank accession no.MZ960299, MZ960300) and the sequences of C. cucurbitarum strains CBS 150.51 (MH856791.1), and KUS-F29113 (KU316934.1) were compared. The sequences of the isolates were identical to those of the reference strains of C. cucurbitarum CBS 150.51 (reference/isolates base pairs, 566/566) and KUS-F29113 (572/572) at the 100% of sequence similarity. Pathogenicity test was conducted using seedlings of 8-week-old green lettuce (cv. Cheongsimchima) by spraying spore suspension (2⨉104 spores/ml). The plants were covered with plastic bags and incubated at 25 ℃ and 70 to 90% RH with a 12-h photoperiod for 20 days. The symptoms were observed only on the inoculated plants within 20 days after inoculation. The inner leaves of the inoculated lettuces had water-soaked lesions. No symptoms were observed in controls. C. cucurbitarum was reisolated from inoculated lettuce, fulfilling Koch's postulates. This fungus has been reported as the causal pathogen of fruit and blossom rot in cucurbits and other plants (Akwaji 2014). However, to our knowledge, occurrence of this pathogen on lettuce has not been reported as yet. This disease is crucial for producers because infected leaves directly results in yield loss of lettuce. This is the first report of Choanephora rot on lettuce caused by C. cucurbitarum in Korea and worldwide.