Golovinomyces macrocarpus Causing Powdery Mildew on Achillea millefolium in Korea.

Achillea millefolium L. (Asteraceae), commonly known as yarrow or common yarrow, is native to temperate regions of the Northern Hemisphere in Asia, Europe, and North America (https://www.missouribotanicalgarden.org/). The plant is not indigenous to Korea, and many commercial varieties have been introduced and are grown in gardens for ornamental purposes. Since 2014, A. millefolium has been found infested with a powdery mildew at different sites in Korea, with a disease incidence of 30-50% among the surveyed plants. Five representative voucher specimens were deposited in the Korea University herbarium (KUS-F27956, F28670, F28776, F33575, and F33963). Symptoms first appeared as circular to irregular white patches, which subsequently coalesced to develop into abundant hyphal growth on both sides of the leaves. Hyphal appressoria were nipple-shaped. Conidiophores were cylindrical, 110 to 186 × 10 to 12 µm, and produced 2 to 5 immature conidia in chains with a sinuate outline. Foot-cells of conidiophores were straight, cylindrical, and 44 to 72 μm long. Conidia were ellipsoid to doliiform-limoniform, 30 to 38 × 16 to 22 μm (l/w 1.5 to 2.0), and devoid of distinct fibrosin bodies. Germ tubes were produced at the perihilar position of the conidia. Chasmothecia were not observed until the natural senescence of the leaves in early winter. These morphological characteristics are consistent with those of Golovinomyces macrocarpus (Speer) U. Braun (Braun and Cook 2012). Sequences of the internal transcribed spacer (ITS) and large subunit (LSU) rDNA regions of three herbarium specimens (KUS-F27956, F28670, and F28776) were determined using primer pairs PM10/ITS4 and PM3/TW14, respectively (Bradshaw and Tobin 2020). A comparison of the resulting sequences using the BLASTn algorithm showed 100% identity with reference sequences of G. macrocarpus (ITS: AB077685 and LSU: AB769429) in the NCBI GenBank database. Obtained sequences were deposited in GenBank (Accession Nos: PQ682288-90 for ITS and PQ682291-3 for LSU). A maximum likelihood tree was constructed based on the ITS+LSU dataset of 20 sequences. Our sequences were clustered with sequences of G. macrocarpus and supported with a 96% BS value. A pathogenicity test was performed twice by gently dusting the conidia of the sample KUS-F33963 onto the leaves of five healthy potted plants. Five non-inoculated plants served as controls. Powdery mildew colonies developed on all inoculated plants after a week, whereas the control plants remained symptomless. The fungus on the inoculated leaves was morphologically identical to that observed on the originally diseased leaves, fulfilling Koch's postulates. Previously, Erysiphe cichoracearum (syn. G. cichoracearum) was recorded on this plant in Europe, North America, and Japan (Farr and Rossman 2024; Nomura 1997). Since G. cichoracearum has been divided into several distinct species (Qiu et al. 2020; Takamatsu et al. 2013), the current taxonomic position of Golovinomyces samples on host plants belonging to the tribe Anthemideae in the family Asteraceae is G. macrocarpus (Bradshaw et al. 2017; Braun and Cook 2012; Mieslerová et al. 2020). To our knowledge, this is the first report of powdery mildew on this plant in Korea. Since the powdery mildew infection on this plant resulted in a loss of the glossy beauty of the leaves and early senescence of the plant, appropriate control measures should be developed.