First Report of Rust Caused by Puccinia sasicola on Corylopsis coreana in South Korea

Korean winter hazel (Corylopsis coreana) is an endemic species of the South Korea (Seo et al. 2016; Kim et al. 2021), which is cultivated as an ornamental plant in this country, but also in China and Japan (Yoon et al. 2016). In July 2022, typical symptoms of a rust disease were observed on C. coreana at Jirisan National Park (35°22'07.7"N 127°34'57.7"E) in Namwon, South Korea. Spermogonia were epiphyllous, densely grouped, pale brown or orange-yellow, round, and 0.23 - 0.38 × 0.19 - 0.41 mm in size. Aecia were hypophyllous, mostly densely grouped, yellow or pale orange, resembling small wart-like galls, and 0.04 - 0.06 × 0.89 - 1.68 mm in size. Aeciospores were hyaline, mostly angularly globose, ellipsoid or oblong-ellipsoid, and 17.8 - 25.2 × 15 - 26.5 μm (average 19.2 × 19.1 μm; n=50) in size. Aeciospore walls were echinulate-verrucose, and 1.1 - 2.2 µm (average 1.7 µm; n=50) in thickness. In December 2022, dark brown telia were observed on the lower surface of Sasa borealis leaves near C. coreana. Telia were mostly scattered but often compacted, brown to dark brown, round, and 1.5 - 1.95 × 1.24 - 1.55 mm in size. Teliospores were either one- or two-celled with a long tapering apex, and light brown to brown in color. One-celled teliospores were globose, and 95.1 - 186.5 × 20.5 - 36.4 μm (average 136.4 × 27.7 μm; n=50) in size. Two-celled teliospores were ellipsoid-cylindrical, and 111.4 - 180.3 × 13.5 - 32.6 μm (average 149 × 21.1 μm; n=50) in size. Side walls of teliospores were golden and 2.2 - 5.5 µm thick (average 3.5 µm; n=50), and pedicels were hyaline, measuring 150 - 300 μm long. Uredinial stage was not observed. Disease symptomology and pathogen morphology were mostly consistent with that of Puccinia sasicola reported in Japan (Hino. 1955). For phylogenetic analysis, genomic DNA was extracted from the aeciospores collected from C. coreana and the teliospores collected from S. borealis. The internal transcribed spacer (ITS) rDNA and the large subunit (LSU) rDNA regions were amplified using ITS5u/ITS4rust (Pfunder and Schürch 2001) and LRust1R/LRust3 (Beenken et al. 2012) primers, respectively. Both sequences were identical for the spores collected from the two different hosts. The sequences were deposited in GenBank (PP171665, PP174211 ITS, PP171709, PP174356 LSU). A GenBank BLAST search revealed 89.53% and 96.78% similarity with Puccinia kusanoi (KX610657) and Puccinia sp. (MT7298241) for ITS and LSU sequences, respectively. In maximum-likelihood phylogenetic analysis of ITS and LSU sequences, the isolates from C. coreana and S. borealis formed a separate clade from other Puccinia species. To test Koch's postulates, leaf disks with telia from S. borealis were directly attached to the adaxial surface of six healthy C. coreana leaves with tape. As controls, healthy S. borealis leaf disks were attached to the adaxial surface of six C. coreana leaves. After four weeks, four inoculated leaves developed small yellow wart-like galls on the abaxial surface, while the control leaves remained symptom-free. The isolates obtained from the inoculated leaves had identical sequences to the original isolate. There are no publicly available sequences for P. sasicola, nor did we find any sequences that match our Puccinia samples. Nevertheless, based on morphological characteristics and life cycle, our isolates closely matched with the previous description of P. sasicola by Hino (1955). To our knowledge, this is the first report of P. sasicola causing leaf rust in C. coreana in South Korea.