First Report of Powdery Mildew Caused by Golovinomyces ambrosiae on Brachyscome multifida in Korea

Abstract

Brachyscome multifida DC. (Asteraceae), known as cut-leaved daisy, is an annual plant native to Australia. Although the species and its cultivars have been popular in horticulture globally, a previously unknown violet variety has recently grown in popularity in Korea. In June 2017, several indoor-grown potted plants were found to be displaying symptoms of powdery mildew in Wanju, Korea (35°50′05″N; 127°01′55″E). Symptoms first appeared as white colonies, which subsequently covered both sides of leaves and young stems. The leaves eventually became distorted and blighted, which severely detracted from the aesthetic and horticultural value of the plant. Three voucher specimens were deposited in the herbarium of Korea University (accession nos. KUS-F29815, KUS-F29866, and KUS-F29878). Appressoria were nipple shaped. Conidiophores were cylindrical, 90 to 200 × 9 to 11 μm, and produced 2 to 5 immature conidia in chains with sinuate outline. Foot cells were cylindrical, straight, and 50 to 75 μm long. Conidia were ellipsoid to barrel shaped, 29 to 37 × 20 to 25 μm, with length/width ratio of 1.4 to 2.0, and devoid of fibrosin bodies. Germ tubes were produced on the perihilar position of the conidia. Primary conidia were apically rounded and basally subtruncate. No chasmothecia were found until the plants died in winter. The morphological characteristics were typical of the conidial stage of the genus Golovinomyces. The internal transcribed spacer (ITS) region of the specimen KUS-F29815 was amplified using primers PMITS1/PMITS2 and the amplicon sequenced (Cunnington et al. 2003). The ITS sequence was deposited in GenBank (accession no. MG601056). BLAST results revealed 99% similarity with G. ambrosiae on Helianthus spp. (e.g., AB769419 and KM657962) and G. spadiceus on Coreopsis tinctoria (AB077640). Because G. spadiceus has much narrower conidia (<20 μm wide) than G. ambrosiae, the fungus was identified as G. ambrosiae (Schwein.) U. Braun & R.T.A. Cook (Braun and Cook 2012). Pathogenicity tests were conducted by touching an infected leaf onto five healthy leaves of a pot-grown plant, and three noninoculated plants were used as controls. The plants were kept in a greenhouse at 24 to 30°C. Typical powdery mildew symptoms started to appear on the inoculated leaves after 4 to 5 days, and the controls remained free of symptoms. Inoculations were repeated twice with similar results. Occurrence of powdery mildew on Brachyscome spp. seems to be uncommon. Amano (1986) listed three cases of powdery mildews including Spaherotheca fuliginea on B. iberidifolia and Oidium sp. on B. ciliaris and B. iberidifolia. Powdery mildew on B. multifida was recently reported as G. cichoracearum in Switzerland (Bolay 2005). To our knowledge, this is the first report of G. ambrosiae on B. multifida in Korea. Our in situ observation of the disease and experience of artificial inoculation showed that the powdery mildew can cause considerable impact on the health of B. multifida both indoors and in the greenhouse.