Rust Disease of Aster pilosus Caused by Coleosporium asterum in Korea

Abstract

Aster pilosus Willd. [syn. Symphyotrichum pilosum (Willd.) G.L.Nesom var. pilosum ], commonly known as frost aster or white heathaster, is a perennial plant belonging to the Asteraceae. This plant isnative to North America and was introduced into Korea during theKorean War in the 1950s (Park, 1995). It is now widely naturalizedand officially designated as one of 11 ‘harmful non-indigenousplants’ by the Korean Ministry of Environment. Since 2004, fromJune to November, severe occurrence of rust infections has beenconsistently observed on plants growing wild in Korea. Examinationof a typical plant revealed yellow-orange rust pustules wereproduced on the stem and lower leaf surface with correspondingsmall yellowish to chlorotic lesions on the upper surface (Figs 1A &.1C). No symptoms were observed on flowers. Both the cauline andradical leaves were equally susceptible to the pathogen. Severeinfections often resulted in leaf distortion, withering and prematuresenescence (Fig. 1B). More than 20 voucher specimens have beendeposited in the herbarium of Korea University (KUS).Uredinia were mostly hypophyllous, yellow-orange, erumpent,and 90–150 µm in diameter (Fig. 1D). Urediniospores were sub-globose to ellipsoid, but somewhat irregular and variable in shape,yellow-orange,ver rucose, 22.5–32.5 × 17.5–20 µm i,ncluding 2 µmin wall thickness (Figs. 1E–1H). No telial stage was found. Thisspecies is supposed to overwinter in the uredinial state in Korea.Based on the morphological characteristics, the rust pathogen wasidentified as Coleosporium asterum (Dietel) Syd. & P. Syd., asdescribed by Hiratsuka et al. (1992).Genomic DNA was extracted from urediniospores of the driedherbarium specimens (KUS-F23395, 24985, 25002). The D1/D2region of 28S rDNA was amplified using the primers LROR andLR6, and directly sequenced. The resulting sequences of the threespecimens were identical to each other, and have been deposited inGenBank with accession numbers (JF273968–JF273970). A NCBIBLAST search showed that the present sequences had 99%similarity to those of Coleosporium asterum on Aster sp. andSolidago sp. In the phylogenetic tree (Fig. 2), inferred usingneighbour-joining method in MEGA4, the Korean isolates and C.asterum formed a monophyletic clade with moderate bootstrapvalue. C. asterum on A. pilosus has been reported from Canada and theUnited States (Farr and Rossman, 2011). This is the first recordoutside of North America, as well as in Korea, for a rust infection onA. pilosus caused by C. asterum . This rust fungus has been reportedto produce spermogonial and aecial stages on Pinus densiflora(Hiratsuka et al., 1992). However, its occurrence has not beenconfirmed in Korea. Our observations suggest that the rustinfections markedly suppress the growth of this noxious weed andecologically retard the spread and expansion to the naturalenvironment. This rust fungus may thus have the potential to besuccessfully exploited as a biocontrol agent for the weed.