Xanthium orientale L. (syn. Xanthium canadense Mill., Asteraceae), known as cocklebur, is an annual weed native to North America, which is now a neophyte distributed throughout the world. This plant was accidentally introduced to Korea in the late 1970s ( So et al. 2008) and is considered a problematic exotic weed in orchards, for which many herbicides are ineffective (Kim et al. 2020). In September 2018, powdery mildew was observed on X. orientale in Jeju, Korea. The disease incidence ranged from 40 to 60%. Voucher specimens were deposited in the Korea University Herbarium (Accession No. KUS-F30795) and Kunsan National University Herbarium (KSNUH1988). Symptoms appeared as round to irregular white patches with abundant hyphal growth on the leaf surface. Hyphal appressoria were nipple-shaped, and 3 to 6 μm diam. Conidiophores (n = 30) were 145 to 206 × 9 to 11.6 µm and produced 2 to 5 immature conidia in chains with a sinuate outline. Foot-cells of the conidiophores were straight, cylindrical, and 43 to 100.9 μm long. Conidia (n = 30) were ellipsoid-ovoid, doliiform to somewhat limoniform, 25.2 to 31.8 × 13.6 to 16.8 μm (l/w 1.6 to 2.1), and devoid of distinct fibrosin bodies. The morphological characteristics corresponded to those of Golovinomyces ambrosiae (Schwein.) U. Braun & R.T.A. Cook (Braun and Cook 2012, under Golovinomyces spadiceus (Beck. & M.A. Curtis) U. Braun; Qiu et al. 2020). To confirm the identity of the causal fungus, the internal transcribed spacer (ITS), large subunit (LSU) (Bradshaw and Tobin 2020), the intergenic spacer (IGS) of rDNA, and the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene (Bradshaw et al. 2022) were amplified for a herbarium specimen (KUS-F30795). A BLASTn search of these sequences revealed 100% identity with reference sequences of G. ambrosiae on diverse Asteraceae plants (AB077644 for ITS, AB077643 for LSU, ON361171 for IGS, and ON075648 for GAPDH). However, there was a single nucleotide difference on both the IGS and GAPDH sequences when compared to the closely related species Golovinomyces latisporus. The sequences were deposited in GenBank (Accession No. OQ165157 (ITS), OQ165164 (LSU), OR050524 (IGS), and OR086076 (GAPDH)). Phylogenetic analyses of ITS, LSU, IGS, and GAPDH sequences revealed the Korean sample formed a well-supported group with other G. ambrosiae sequences, confirming its identity. A pathogenicity test was performed through inoculation by gently pressing diseased leaves onto the leaves of five healthy plants. Five non-inoculated plants served as controls. All plants were maintained in a greenhouse at 25±2°C. Powdery mildew colonies developed on the inoculated plants after ten days, whereas the control plants remained symptomless. The fungus present on the inoculated leaves was morphologically identical to that observed on the initially diseased leaves, fulfilling Koch's postulates. Powdery mildew on X. orientale has previously been reported as Golovinomyces cichoracearum (≡ Erysiphe cichoracearum) sensu lato in the USA, G. ambrosiae (= G. spadiceus) throughout all continents, and Podosphaera fusca sensu lato (now P. xanthii) in Korea (Braun and Cook 2012; Farr and Rossman 2023). To date, powdery mildew in Korea has been reported only on Xanthium strumarium as G. cichoracearum s. lat. and Podosphaera xanthii (KSPP 2022). To our knowledge, this is the first report of powdery mildew caused by G. ambrosiae on X. orientale in Korea.

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