First report of rust caused by Coleosporium asterum on Aster spathulifolius in Korea

Aster spathulifolius Maxim. is a perennial herb in the family Asteraceae, which is narrowly distributed and endemic in coastal regions of Korea and Japan (Nguyen et al. 2013). This plant has long been grown in gardens for ornamental purposes in Korea and has recently been studied as medicinal herb (Hwang and Choung 2016; Won et al. 2013). During autumn 2015 and spring 2016, plants were observed in gardens displaying typical rust lesions. In a survey of a private garden in Gangneung (37°41′34″N; 128°51′38″E), Korea, the disease incidence was 100%. Similar symptoms were observed in pot-grown A. spathulifolius in Pocheon (38°02′58″N; 127°18′13″E) and Donghae (37°36′05″N; 129°04′24″E). Examination of a diseased plant revealed that orange-yellow rust pustules were formed on the lower leaf surface with corresponding small yellowish to chlorotic lesions on the upper surface. Infections often resulted in leaf yellowing and premature senescence, reducing aesthetic value of the plants. Four representative voucher specimens were deposited in Korea University (KUS-F24712, F29051, F29053, and F29054). The morphology of the fungus was examined by light microscopy and scanning electron microscopy. Uredinia were mostly hypophyllous, orange-yellow, erumpent, and 70 to 146 μm in diameter. Urediniospores were subglobose to ellipsoid, but somewhat irregular and variable in shape, yellow-orange, verrucose, 22 to 32 × 16 to 22 μm, including 2 μm in wall thickness. No telial stage was found. Based on the morphological characteristics, the rust fungus was identified as Coleosporium asterum (Dietel) Syd. & P. Syd., as described by Hiratsuka et al. (1992). To confirm the identification, genomic DNA was extracted from urediniospores of the dried herbarium specimen (KUS-F29053) and the D1/D2 region of 28S rDNA was amplified and sequenced using the primers LROR and LR6. The obtained sequence was deposited in GenBank (accession no. KX420703). An NCBI BLAST search showed 100% identity to the sequence of C. asterum on A. pilosus (JF273968) and >99% similarity to sequences of C. asterum on other asteraceous hosts (HQ317530, GU058009, AF522164, AF522165). Pathogenicity was confirmed by rubbing diseased fresh leaves containing urediniospores onto three pot-grown plants. Three noninoculated plants were included as controls. All plants were placed in a dew chamber for 24 h at 20°C, and then in a growth chamber with a 12-h photoperiod. Inoculated leaves developed typical rust symptoms with the uredinia appearing after 16 days, no symptoms developed on any control plants. C. asterum has been reported on more than 60 species of Aster from North America and East Asia, but not on A. spathulifolius (Farr and Rossman 2017). Therefore, this is the first record of C. asterum on A. spathulifolius globally. This rust fungus has been reported to produce spermogonial and aecial stages on Pinus densiflora (Hiratsuka et al. 1992); however, the occurrence of other spore stages has not been recorded in Korea. This information would be helpful for Aster producers, breeders, and gardeners.