First Report of White Blister Rust Disease Caused by Wilsoniana amaranthi on Amaranthus dubius in Korea

Abstract

Amaranthus dubius (Amaranthaceae), often called “spleen amaranth,” is one of the most popular edible leafy amaranths throughout the world. The plant was accidentally introduced to Korea but currently is cultivated as a leafy vegetable. Edible young leaves and stems are cooked like spinach. In July 2019, symptoms of white blister rust were found on A. dubius on farmland in Jeongeup (35°32′09″N, 126°54′26″E), Korea, with a disease incidence of 70%. Symptoms appeared as light green to yellow chlorotic spots on the upper surface of the leaves. Blister sori formed on the corresponding lower surface were white, oval to ellipsoidal, and 3 to 4 mm in diameter. A representative sample was deposited in the Kunsan National University Herbarium (KSNUH401). Sporogenous hyphae were hyaline, clavate or cylindrical, mostly grouped, and (24 to) 32 to 42 (to 44) (av. 37.2) µm long. Primary sporangia were hyaline, globose or subglobose, and (12 to) 14.9 to 16.6 (to 18) (av. 15.26) µm in diameter, with a thick wall of 1.5 to 2.5 µm. Secondary sporangia were hyaline, subglobose, and (15 to) 16.7 to 19.4 (to 21) (av. 18.09) × (12 to) 14.0 to 16.5 (to 19) (av. 15.27) µm. Oogonia were irregular in shape but mostly subglobose to ellipsoidal and were (48 to) 56.1 to 68.1 (to 74) (av. 62.1) µm in diameter. Oospores were globose, dark brown, reticulated and (34 to) 39.6 to 50.8 (to 56) (av. 45.2) µm in diameter. The morphological characteristics were similar to those reported for Wilsoniana amaranthi (= Albugo amaranthi) (Choi et al. 2007; Voglmayr and Riethmuller 2006). To confirm this morphological identification, genomic DNA was extracted from an infected leaf of KSNUH401. Amplification and sequencing of the internal transcribed spacer (ITS) rDNA with primers ITS1-O and LR0 and cytochrome oxidase II (cox2) mtDNA with primers cox2-F and cox2-RC4 were performed (Choi et al. 2015). Two resulting sequences were deposited in GenBank (accession nos. MN526483 for ITS, MN533957 for cox2). A BLASTn search showed that this sample is identical with W. amaranthi: AY929824 (ex Amaranthus spinosus), MK333400 (ex A. hybridus), and JN849471 (ex A. chlorostachys) for the ITS and AY913805 (ex A. spinosus) and JN849486 (ex A. chlorostachys) for the cox2 sequence. For a pathogenicity test, sporangia (4 × 10⁵ sporangia/ml) were harvested from the infected leaves, kept at 4°C for 30 min to induce the formation of zoospores, and inoculated onto the lower leaf surface of three healthy A. dubius (3 weeks old). Three noninoculated plants served as controls. Inoculated and noninoculated plants were incubated in an outdoor greenhouse at 80% relative humidity and 25 to 30°C. After 2 weeks, all inoculated plants formed yellow spots on the upper surface of leaves and then typical white rust pustules on the corresponding lower surface, identical to those observed in the field, whereas the control plants remained symptomless. The W. amaranthi successfully sporulated on the inoculated plants, fulfilling Koch’s postulates as thoroughly as possible with an obligate parasite. Based on morphological characteristics, sequencing data, and pathogenicity testing, the causal organism was identified as W. amaranthi. Previously, occurrences of white blister rusts on A. dubius have been known in Puerto Rico, Venezuela, the Virgin Islands, and the West Indies (Farr and Rossman 2019). To our knowledge, this is the first report of white blister rust caused by W. amaranthi on A. dubius in Korea. Given the high incidence of this disease, this pathogen could be a potential concern for cultivation of A. dubius.