First Report of Powdery Mildew Caused by Golovinomyces adenophorae on Adenophora triphylla var. japonica in Korea.

ABSTRACT

Adenophora triphylla var. japonica (Campanulaceae), known as Japanese lady bell, is native to East Asia. It has been used as a medicinal plant but is widely cultivated in Korea as an indigenous vegetable (Park et al. 2011). In the summer of 2020, about 100 plants in an experimental plot at the National Institute of Forest Science, Seoul, Korea, showed powdery mildew symptoms with a 100% disease incidence. Signs first appeared as white colonies, subsequently expanding over the leaves, stems, and inflorescences. Infected young shoots were elongated and became slender. Chasmothecia were found in late October. Voucher specimens were deposited in the Korea University Herbarium (KUS-F). Conidiophores arising from the lateral part of the hyphae were upright, 100 to 220 × 10 to 12 µm, and produced 2 to 5 immature conidia in chains with sinuate edge lines. Basal parts of foot-cells in conidiophores were curved. Conidia were barrel-shaped to ellipsoid, 26 to 40 × 14 to 20 µm, and produced germ tubes on the perihilar position of the conidia. Chasmothecia with short mycelioid appendages were gregarious, 144 to176 µm in diam., and contained 8 to 22 asci. Asci were clavate-saccate with short stalks, 60 to 82 × 28 to 42 µm, and contained two spores. Ascospores were broadly ellipsoid, cytoplasm-dense without vacuoles, colorless, and 22 to 28 × 12 to 18 µm. The structures and measurements were consistent with those of Golovinomyces adenophorae (R.Y. Zheng & G.Q. Chen) Heluta (Braun & Cook, 2012). To confirm the morphology-based identification, two herbarium specimens (KUS-F29252 and F31898) were sequenced for the internal transcribed spacer (ITS) and large subunit (LSU) regions with PM10/ITS4 and PM3/TW14 primers, respectively (Bradshaw and Tobin, 2020). A Blastn search revealed high similarities in the ITS and LSU sequences, with 99.81% (538/539 bp) and 99.86% (697/698 bp) to G. adenophorae sequences (AB077633 and AB077632), respectively. All resulting sequences were deposited in GenBank under accession numbers OR841069-70 for ITS and OR841071 for LSU. A pathogenicity test was performed through inoculation by gently dusting the conidia from a detached symptomatic leaf onto the leaves of five healthy plants. Five non-inoculated plants served as controls. Following inoculation, plants were covered with plastic film and maintained in a greenhouse (24 to 32°C) until symptoms developed. Powdery mildew colonies developed on the inoculated plants after twelve days, whereas the control plants remained symptomless. The inoculated pathogen was confirmed morphologically and molecularly by the sequence comparison aforementioned, fulfilling Koch's postulates. Based on morphological characteristics and the sequencing data, the powdery mildew was identified as G. adenophorae. The association of G. adenophorae and Adenophora spp. has been known in China, Japan, Kazakhstan, and the Far East of Russia (Farr and Rossman, 2023). This is the first report of powdery mildew caused by G. adenophorae on A. triphylla var. japonica in Korea. Since the commercial cultivation of this plant aims to harvest young shoots as one of the most popular vegetables in Korea, appropriate control measures for the powdery mildew should be considered.