First Report of Leaf Spot Caused by Septoria erigerontis on Conyza sumatrensis in Korea.

Abstract

Conyza sumatrensis (Retz.) E. Walker (syn. Erigeron sumatrensis) or commonly known as fleabane, is an annual herbaceous plant native to South America. It/span>was accidentally introduced to Korea in the 1930s and became invasive in natural and managed ecosystems of the country (Kim et al. 2008). Leaf spots on this plant were first observed in March 2013 in Jeju (33°29'45"N; 126°26'26"E), and then in February 2017 in Seoguipo (33°14'30"N; 126°32'58"E), Korea. The severity of the disease was estimated to be greater than 50%. Initial symptoms were small, distinct, reddish-brown, then turned into brown spots with dark purplish-brown margins. Conidiomata were pycnidial, epigenous, occasionally hypogenous, scattered, dark brown to rusty brown, globose, embedded in host tissue or partly erumpent, 70 to 165 µm in diameter, with ostioles measuring 10 to 26 µm in diameter. Conidia were straight to flexuous, 24 to 65 × 1.5 to 2.0 µm, hyaline, and 1 to 5 septate. Morphological circumscriptions were consistent with Septoria erigerontis Peck (Verkley et al. 2013). To obtain a monoconidial isolate, conidia were collected from lesions, placed in an Eppendorf tube containing sterile water and streaked onto the surface of 2% water agar (WA) plates supplemented with 100 mg/L of streptomycin sulfate, and after five days transferred onto potato dextrose agar (PDA). The two-week-old colonies incubated at 25°C on PDA had a slightly ruffled, but mostly colorless margin; colonies were 5 to 7 mm in diameter, slightly elevated in the center, surface black, covered with a diffuse to a dense mat of grey aerial mycelium. Voucher specimens were housed in the Korea University Herbarium (KUS-F27274 and F29725), and two cultures were deposited in the Korea Agricultural Culture Collection (Accession Nos: KACC47219 and KACC48297). Nucleotide sequences of the internal transcribed spacer (ITS), translation elongation factor 1-α (EF), actin (ACT), 28S rDNA (LSU), and RNA polymerase II second largest subunit (RPB2) genes obtained from KACC47219 were determined (Verkley et al. 2013) and deposited in GenBank (Accession Nos: OM909018, OM908934, OM974318, OM974319, OM974320). Results of BLASTn search for ITS and LSU were 99-100% identical with reference sequences of Septoria erigerontis (MH865036, MH876473), S. lactucae (MK617321), and S. phlogis (MH876550) in GenBank. Whereas it showed 97% similarity for ACT, 93-94% for EF and RPB2 genes with sequences of S. erigerontis (JQ325031, KF253363, KF252411). Pathogenicity was tested by spraying 20 leaves of four-month-old three potted plants with a conidial suspension (1×104 propagules/mL) harvested from a four-week-old culture (span style="font-family:'Times New Roman'">KACC47219). Ten leaves were used as controls. The plants were placed in a dew chamber at 26°C for 24 h, then moved to a greenhouse. Inoculated leaves developed typical symptoms after seven days, whilst no symptoms were observed on control ones. S. erigerontis was re-isolated from symptomatic lesion and its identity was confirmed by microscopic studies, fulfilling Koch's postulates. Although S. erigerontis has been recorded to be associated with leaf spots of Erigeron spp. and Conyza spp. (Farr and Rossman 2022), there is no previous record of this fungus on C. sumatrensis. To our knowledge, this is the first report of leaf spot caused by S. erigerontis on C. sumatrensis worldwide as well as in Korea. We presume that this fungus may have potential as a biocontrol agent on fleabanes, particularly glyphosate-resistant Conyza species (Sansom et al. 2013).