First report of Phragmidium rubi‐oldhami causing rust disease on Rubus pungens in Korea

Abstract

Rubus pungens (‘Korean creeping raspberry’; Rosaceae) is a deciduous, broad-leaved tree native to East Asia. Its edible fruit is consumed fresh or used as a medicinal ingredient for diabetes and asthma (Moon, 1991). In May 2021, symptoms of a rust disease were observed on Rubus pungens in the Korea National Arboretum, Pocheon, Korea, with a disease incidence of 50% amongst the 30 plants surveyed. Bright yellow-orange rust pustules of the uredinial stages were found on both sides of the infected leaves. The pustules were often erumpent and 1 to 3 mm in diameter (Figure 1). The infected leaves were examined under a light microscope. Uredinia (Figure 2; n = 20) were erumpent, round, yellow to pale yellow, mostly scattered, rarely grouped and 200–536 µm in diameter (Figure 2). Urediniospores (= 50) were hyaline, globose to sub-globose and (16.0–)18.4–21.2(–23.6) × (12.6–)15.3–18.5(–19.7) µm (mean 19.8 × 16.9 µm), with an echinulate wall of 1.0 µm thick (Figures 3, 4). Paraphyses (n = 30) were hyaline, oblong or curved, and 56.2–11.1 µm, with a thin wall of 0.5–1.0 µm, occasionally slightly thickened at apex, 1.2–2.5 µm (Figure 5). The telial stage was not found. Based on morphological characteristics, the rust fungus was identified as Phragmidium rubi-oldhami Togashi & Y. Maki (Hiratsuka, 1934). To confirm the identification of the rust, genomic DNA was extracted from a representative specimen (KSNUH1322). Two ribosomal DNA regions were amplified, the internal transcribed spacer (ITS) with primers ITS5u and ITS4rust (Pfunder & Schürch, 2001) and the large subunit (LSU) with primers LRust1R and LRust3 (Beenken et al., 2012). The resulting sequences were deposited in GenBank (Accession Nos. ON180674 for ITS and ON170371 for LSU). A BLASTn search revealed that the Korean isolate had 99.43% identity in the LSU region to the only P. rubi-oldhami sequence in Genbank, which was obtained from Rubus pungens in China (MG669116). Close identities were found with other Phragmidium species when analysing the ITS sequences, however, there is no ITS reference sequence in GenBank for P. rubi-oldhami. In a maximum likelihood phylogenetic analysis using ITS (Figure 6) and LSU (Figure 7) sequences, the Korean isolate grouped with the reference sequence of P. rubi-oldhami in the LSU tree and formed a distinct branch among other Phragmidium species in the ITS tree. Phragmidium rubi-oldhami was originally described as a rust on Rubus oldhami in Japan (Togashi & Maki, 1934) and has since been recorded on R. amabilis, R. pungens and R, thibetanus in China and Japan (Farr & Rossman, 2022; Liu et al., 2018; Zhuang & Wei, 2009). To our knowledge, this is the first report of P. rubi-oldhami and of the rust disease on R. pungens in Korea. Given that urediniospores can easily disperse long distances by wind, this can pose a potential risk of rust disease on other Rubus species in Korea. This work was supported by the Korea National Arboretum grant (KNA3-1-10, 21–4) from the Korea Forest Service, Korea.