Sorghum (Sorghum bicolor (L.) Moench) is one of the top five cereal crops in the world, but the cultivation area in Korea is estimated to be about 3,000 ha (MIFFAF, 2012). In August 2014, anthracnose symptoms on sorghum leaves were observed in two fields in Yecheon (36.62°, 128.41°) and Youngwol (37.20°, 128.49°), South Korea. Symptoms on leaves were brownish red irregular lesions with yellow and tan borders. Some darkened conidiomata and setae were observed on the lesions of infected leaves. Approximately 20% of sorghum plants (cv. Hwanggeumchal) were affected in each field with an area of about 0.1 ha. Fragments of diseased infected leaves were surface sterilized with 1% NaOCl for 30sec. The pieces were placed on water agar and incubated at 25°C for 7days. Two isolates were obtained through single sporing and cultured on synthetic nutrient poor agar at 25°C for 14days. Conidia (n=30) of YN1458 isolate were falcate and measured 22.0 to 32.7 × 4.2 to 6.4 µm. Brown to black setae (n=20) had 1-3 septa, with tapering acute apices and 53.7 to 95.2 × 4.7 to 7.8 µm in size. Appressoria (n=30) were dark brown, usually irregular and 10.5 to 16.9 × 8.6 to 13.6 µm in size. Colonies on PDA produced salmon spore masses in the center of the colony, and whitish grey to dark color in reverse. The morphological characteristics of two isolates were similar. Based on morphology, two isolates were tentatively identified as Colletotrichum graminicola species complex (Cannon et al. 2012; Crouch and Tomaso-Peterson 2012). To clarify taxonomic placement, DNA extracted from mycelia of the two isolates was PCR amplified and sequenced targeting internal transcribed spacer (ITS) regions of rDNA, actin (ACT), chitin synthase 1(CHS-1), and beta-tubulin (TUB) genes (Weir et al. 2012). The sequences of the above four loci of YN1458 and YN1728 were deposited in GenBank with accession numbers KT351801, KT351802 (ITS); KY769869, KY69870 (ACT); KY769871, KY769872 (CHS-1); and KY769873, KY769874 (TUB), respectively. The sequencing results of two isolates showed 99.6% (ITS), 99.6% (ACT of YN1458), 100% (ACT of YN1728), 100% (CHS-1), 100% (TUB of YN1458) and 99.8% (TUB of YN1728) similarity with C. sublineola CBS 131301 (JQ005771, JQ005834, JQ005792, and JQ005855) by BLASTn. Based on the morphological characteristics and multigene sequence analysis, the two isolates were identified as C. sublineola. Pathogenicity of two isolates was confirmed by spraying conidial suspensions (106 conidia/mL) on leaves of 3-week-old sorghum seedlings (cv. Hwanggeumchal) using a pot assay (5 plants per isolate). The same number of seedlings were sprayed with sterile distilled water and served as controls. All plants were maintained in a greenhouse at 25/32°C with natural light. After one week, symptoms similar to those in the field were observed on the leaves inoculated with the pathogen, but not on the control leaves. Colletotrichum sublineola was consistently re-isolated from the inoculated leaves showing anthracnose symptoms and the pathogen identity was confirmed by observing morphological characteristics. So far, C. graminicola was known as the only causal agent pathogen of sorghum anthracnose in South Korea (KSPP, 2009). To our knowledge, this is the first report of C. sublineola causing anthracnose on sorghum in South Korea. Although sorghum is a small-scale crop in South Korea, it is necessary to study the biological and pathogenic characteristics of C. sublineola for effective control of sorghum anthracnose.
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