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

Amaranthus hybridus (= A. patulus), an annual herbaceous plant of the family Amaranthaceae, is a noxious alien weed in Korea, which causes high economic losses in agriculture by invading farming areas and competing with crops. From September to November of 2018, A. hybridus plants showing white blister rust symptom were observed along farmlands in Wanju (36°03′40″N, 127°17′22″E) and Namwon (35°41′37″N, 127°40′07″E), Korea. Symptoms appeared as light green to yellow chlorotic spots on the upper leaf surface and on the corresponding lower surface. Blister sori were white, oval to ellipsoidal, and 3 to 4 mm in diameter. The disease incidence in the surveyed regions reached 50 to 70%. Two representative collections were deposited in the Kunsan National University Herbarium (KSNUH 292 and 320). Sporogenous hyphae were hyaline, clavate or cylindrical, mostly grouped, and (17–) 24.6 to 34.1 (–38) (avg. 29.3) µm long. Primary sporangia were hyaline, globose or subglobose, with a thick wall, and (11–) 12.6 to 15.6 (–17) (avg. 14.1) µm in diameter. Secondary sporangia were hyaline, subglobose, and (13–) 17.4 to 20.4 (–22) (avg. 18.9) µm × (10–) 13.3 to 16.5 (–19) (avg. 14.9) µm. Oogonia were irregular in shape but mostly subglobose to ellipsoidal, and (48–) 53.4 to 64.5 (–72) (avg. 58.9) µm in diameter. Oospores were globose, dark brown, reticulated, and (31–) 38.5 to 48.9 (–56) (avg. 43.7) µm in diameter. The morphological characteristics closely resembled those reported for Wilsoniana amaranthi (= Albugo amaranthi) (Choi et al. 2007; Voglmayr and Riethmüller 2006). To confirm the morphological identification, genomic DNA was extracted from white sori on a diseased leaf of KSNUH292. The internal transcribed spacer (ITS) rDNA was amplified with primers ITS1-O and LR-0 and cytochrome oxidase II (cox2) mtDNA with primers cox2-F and cox2-RC4 (Choi et al. 2015). The resulting sequences were deposited in GenBank (accession nos. MK333400 for ITS, MK335465 for cox2). A BLASTn search revealed that the Korean sample is identical with W. amaranthi; AY929824 (ex Amaranthus spinosus) and JN849471 (ex A. chlorostachys) for the ITS sequence and AY913805 (ex A. spinosus) for the cox2 sequence. To confirm pathogenicity, sporangia were kept at 4°C for 1 h to induce the formation of zoospores and inoculated onto the lower surface of leaves of 10 healthy potted A. hybridus. Five noninoculated plants served as controls. Inoculated and noninoculated plants were maintained in separate dew chambers at 20°C and 65% humidity with a 10-h photoperiod. After 2 weeks, the inoculated plants developed white rust pustules on the lower surface of leaves, whereas the control plants remained symptomless. The pathogen present on the inoculated plants was morphologically identical to the original one observed on the diseased plants, fulfilling Koch’s postulates. Based on morphological characteristics, molecular sequencing data, and pathogenicity test, the pathogen was identified as W. amaranthi. To our knowledge, this is the first report of white blister rust caused by W. amaranthi on A. hybridus in Korea. Our field surveys have shown that W. amaranthi could be developed as a biocontrol agent for inhibiting the growth and spread of this exotic weed in Korea.