First Report of <i>Colletotrichum chlorophyti</i> Causing Anthracnose on Peanut in the Republic of Korea

The annual consumption per person and the grain self-sufficiency ratio of peanut (Arachis hypogaea L.) in the Republic of Korea are 0.85 kg and 24.3%, respectively (MAFRA 2023). In autumn of 2023, small-to-medium, irregularly shaped tan to dark brown lesions were observed on peanut leaves, and similar symptoms recured over two years in an 880 m² disease surveillance field at the National Institute of Crop and Food Science (35°50'31.1"N 127°02'41.3"E) with the incidence of less than 5% in complex infection conditions. Ten leaf cuttings (5 mm x 5 mm) in 2023 and 40 in 2024 were surface-sterilized with 70% EtOH for 1 min, followed by 1% NaClO for 1 min, and rinsed 3 times with sterile water. Leaf pieces were placed on Potato Dextrose Agar (PDA) and incubated at 25 °C in the dark for 5 days. Of the 50 isolates recovered, 15 showed the same characteristics and 5 representative isolates (1 in 2023 and 4 in 2024) were designated as isolates F23028 and F24021~F24024, respectively and deposited in the Korean Agricultural Culture Collection (https://genebank.rda.go.kr) under acc. no. 410724 and 411054~411057. Fungal colonies were initially white and turned pale gray to gray showing felt-like surface with pink, brighter margin. Chlamydospores were dark brown and bulbous in chains, spherical or sub-spherical, 8.3 to 14.6 μm in diameter (mean = 10.9 μm ± 1.9, n = 50). Conidia were colorless and crescent-shaped having one acute end and the other blunt end, 16.9 to 21.2 μm x 3.9 to 5.3 μm (mean = 21.2 μm ± 1.0 x 4.5 μm ± 0.8, n = 50). The morphology was consistent with descriptions of Colletotrichum chlorophyti (Damm et al. 2009). Five loci, ITS (ITS5/ITS4), actin (ACT-512F/ACT-783R), beta-tubulin (T1/Bt-2b), chitin synthase (CHS79F/CHS354R), and glyceraldehyde-3-phosphate dehydrogenase (GDF1/GDR1) were sequenced (Damm et al. 2009). The locus of representative isolate F24023 (GenBank acc. no. PX090881, PX098305, PX098317, PX098309, and PX098313) showed high nucleotide identity over 99.0% with thoses of C. chlorophyti strain IMI 103806 (GU227894, GU227992, GU228188, GU228384, and GU228286), respectively. MLSA revealed the five isolates were clustered with C. chlorophyti strain IMI 103806, thus identified as C. chlorophyti. To determine the pathogenicity, a conidial suspension (1 x 106 conidia/mL) was sprayed on the leaves of five 3-week-old peanut seedlings and distilled water as a negative control in one seedling. Sprayed plants were placed in a dew chamber at 24℃ for 2 days and grown in a growth chamber at 28℃ and 80% relative humidity with a 14-h/10-h light/dark cycle. Three weeks later, tan to dark brown lesions appeared only on leaves sprayed with conidia and no symptom in the negative control. The pathogenicity test was repeated three times, and each time the pathogen was re-isolated and identified by morphology and ITS sequences, thus fulfilling Koch's postulates. To date, two species of the genus Colletotrichum, C. gloeosporioides and C. sojae were reported as anthracnose on peanut in the Republic of Korea (Kim et al. 1998; Kim et al. 2024) and C. chlorophyti was recently reported as the pathogen of peanut anthracnose in China (Zhao et al. 2021). To the best of our knowledge, this is the first report of C. chlorophyti causing anthracnose on peanut in the Republic of Korea. Anthracnose is known to occur severely in high-temperature, humid conditions, identifying the new causal agent and monitoring the disease could help reduce damage during hot and humid late summers in the Republic of Korea.