Anthracnose Caused by Colletotrichum spaethianum on Polygonatum odoratum var. pluriflorum in Korea.

Polygonatum odoratum var. pluriflorum (Miq.) Ohwi (Asparagaceae), commonly known as Lesser Solomon's seal, is a perennial herbaceous plant widely distributed in the temperate regions of the Northern Hemisphere. It is known for tea and contains various pharmacologically active compounds (Chang et al. 2007). In September 2021, previously unseen anthracnose was encountered on Polygonatum odoratum var. pluriflorum in Chuncheon (37°45'07''N; 127°47'45''E) and Wanju (35°38'47''N; 127°31'16''E), Korea, with a disease incidence of 20 to 30% of the inspected plants. The disease symptoms begin with leaf yellowing (chlorosis) along the margins, progressing toward the central part of the leaf. As the disease progressed, brown necrotic lesions developed, covered with small, dark dots, indicating the presence of conidiomata of Colletotrichum. The disease often led to leaf blight, resulting in concentric lesions of a periodic and concentric arrangement of conidiomata, forming circular patterns within the affected tissue. It expanded rapidly during the rainy season producing water-soaked lesions but nearly slowed down during the dry season, leaving the plant with a ragged appearance. Monoconidial isolates were obtained from five fresh samples collected in Wanju, of which a representative isolate was deposited in the Korean Agricultural Culture Collection (KACC410442), and the dried specimen was housed at the herbarium of Jeonbuk National University (JBNU0135). Morphological characteristics of the fungus were examined with fresh and naturally infected leaves. Setae (n = 30) were dark brown to nearly blackish, 45 to 172 um long, 4.1 to 6.1 µm wide at the base, and becoming narrower upwards, apex pointed, 2 to 4-septate. Conidia were 1-celled, sometimes uniseptate before germination, slightly curved, with an acute or rounded apex and a more or less truncate base, and 11.4 to 23.2 × 3.1 to 4.2 µm. Appressoria were single or in groups, dark brown, lobate, irregularly shaped, and 6.0 to 11.9 × 7.0 to 14.8 µm. Two-week-old colonies grown on PDA at 25 ℃ reached 30-35 mm in diam., initially white, turning gray with age, with cottony aerial mycelia. These morphological characteristics were in good agreement with those of C. spaethianum (Damm et al. 2009; Liu et al. 2020). To confirm the morphology-based determination, genomic DNA was extracted from KACC410442, and multi-loci sequences of the internal transcribed spacer region (ITS) rDNA, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), actin (actA), chitin synthase (CHS1), histone (HIS3) and tubulin (TUB2) genes were determined, as outlined by Cannon et al. (2012) and Damm et al. (2009). The resulting sequences obtained in this study were registered to GenBank (PP739190 for ITS, PP741985 for GAPDH, PP741986 for actA, PP741987 for CHS1, PP741988 for HIS3, and PP741989 for TUB2) and compared with other sequences on GenBank using the BLASTn search tool. The results showed 100% identity to other C. spaethianum sequences (e.g., MT611068 for ITS, MN400659 for actA, OP965007 for CHS1, OR060963 for GAPDH, PP480643 for HIS3, and OQ613727 for TUB2). In a maximum parsimony tree, the Korean isolate formed a well-supported clade (100% bootstrap value) with reference sequences of C. spaethianum. Pathogenicity was tested by spraying a conidial suspension (1×104 conidia/ml) from a two-week-old culture onto ten leaves of three potted plants, without wounding. Ten water-sprayed leaves were served as controls. The plants were maintained in a dew chamber and later moved to a greenhouse at 25 to 30 °C. After two weeks, all inoculated plants developed anthracnose symptoms, while the control plants remained symptomless. The fungus isolated from the inoculated plants was morphologically identical to that initially observed, fulfilling Koch's postulates. C. spaethianum has been known on various plant species, but mostly on the Asparagaceae, including Anemarrhena (Okorley et al. 2019), Hosta (Damm et al. 2009; Cheon and Jeon 2016), and Polygonatum (Liu et al. 2020; Ma et al. 2021). To our knowledge, this is the first report of anthracnose caused by C. spaethianum on P. odoratum var. pluriflorum in Korea, highlighting the continuous need for disease management and control in the cultivation of this plant.