First Report of Leaf Spot Caused by Septoria lactucae on Lactuca serriola in Korea

Abstract

Lactuca serriola L. (syn. L. scariola L.), called prickly lettuce, has become naturalized globally in temperate regions. Because the plant was accidentally introduced to Korea in the late 1970s (Yim and Jeon 1980), they are now distributed throughout the country, threatening native plant communities. Thus, prickly lettuce has been designated as one of the 12 harmful nonindigenous plants by the Korean Ministry of Environment (Kim et al. 2013). In June 2019, leaf spot symptoms on this weed were found at a public garden in Gunsan (35°57′59″N, 126°43′28″E), Korea. Leaf lesions were 2 to 10 mm in diameter, mostly angular, vein-limited, dark brown, and exhibited numerous black pycnidia with a hyaline cirrhus. A representative specimen was deposited in the Kunsan National University Herbarium (KSNUH379). Pycnidia were amphigenous, globose, dark brown to blackish, and measured 70 to 90 μm in diameter. Ostioles were 15 to 22 μm in diameter, with a hyaline, slender cirrhus. Conidia were hyaline, filiform, slightly curved to straight, aguttulate, uniseptate, subtruncate at the base, tapering to a rounded apex, and measured 20 to 32 × 1.5 to 2.0 μm. Single-conidial isolations on potato dextrose agar formed dark grayish colonies and grew slowly (10 mm 5 weeks after inoculation at 25°C). On the basis of morphological and cultural characteristics, the fungus was identified as Septoria lactucae Pass. (Verkley et al. 2013). An isolate was deposited in the Korean Agricultural Culture Collection (KACC48731). To confirm the morphological identification, genomic DNA was extracted from a diseased leaf of KSNUH379. Three DNA regions were sequenced: internal transcribed spacer (ITS rDNA) with primers ITS1-F and ITS4, large subunit (LSU) rDNA with LSU1Fd and LR5 (Verkley et al. 2013), and actin gene region (ACT) with ACT512F and ACT783R (Carbone and Kohn 1999). The resulting sequences were deposited in GenBank (accession nos. MN267567 for ITS, MN267564 for LSU, and MN284910 for ACT). A BLASTn search revealed that the Korean sample is identical with the sequences of S. lactucae parasitic to Lactuca sativa (KF251439 and KF251440 for ITS, KF251943 and KF251944 for LSU, and KF253743 and KF253744 for ACT). Pathogenicity was confirmed by inoculating the leaves of three 4-week-old lettuce plants with a conidial suspension (∼1 × 10⁶ conidia/ml). Three noninoculated plants served as controls. For 72 h after inoculation, the plants were incubated in a growth chamber at 100% relative humidity and darkness at 25°C and then kept in an outdoor greenhouse at 80% relative humidity and 25 to 30°C. After 3 weeks, typical leaf spots, identical to those observed in the field, developed on the inoculated leaves. No symptoms were observed on the control plants. The fungus was successfully reisolated from the symptomatic plants, fulfilling Koch’s postulates. Based on the morphological characteristics, sequencing data, and pathogenicity test, the pathogen was identified as S. lactucae. Previously, the association of L. serriola with Septoria leaf spot has been known in Australia, the Canary Islands, Ethiopia, and the United States (Farr and Rossman 2019). To our knowledge, this is the first report of S. lactucae on prickly lettuce in Asia. Because of its potential as a biocontrol agent, further studies are needed.