First Report of Powdery Mildew Caused by Podosphaera xanthii on Verbena bonariensis in Korea.

Abstract

Verbena bonariensis L. (Verbenaceae), known as purple-top vervain or Argentinian vervain, is a perennial, herbaceous plant, native to tropical South America. It is widely grown as an ornamental worldwide. During summer and autumn of 2022, V. bonariensis of an unknown cultivar with purple-violet flowers was found infected by powdery mildew fungus with 100% disease incidence in a public garden in Jeonju, Korea. White, superficial mycelia developed on infected plants and subsequently covered whole surfaces of leaves and stems, resulting in leaf discoloration and early defoliation. High disease severity caused poor growth of the plants, resulting in premature senescence and reduced flowering. A representative voucher specimen was submitted in the Korea University herbarium (KUS-F33160). Morphological characterization and measurements of the fungus were carried out using a fresh sample. Appressoria on the mycelium were poorly developed, nipple-shaped or nearly absent. Conidiophores were 90 to 246 × 10 to 12 μm and produced 2 to 7 immature conidia in chains with a crenate outline. Foot-cells of conidiophores were straight, cylindrical, relatively short, 42 to 64 μm long, and constricted at the branching point from the hypha. Conidia were hyaline, ellipsoid to ovate, measured 28 to 40 × 18 to 22 μm (length/width ratio of 1.4 to 2.0), and contained conspicuous fibrosin bodies. Germ tubes were produced from the lateral position of conidia. No chasmothecia were observed throughout the growingseason. These diagnostic structures were typical to the anamorph of the genus Podosphaera. The morphological characteristics and measurements were consistent with those of P. xanthii (Castagne) U. Braun & Shishkoff (Braun & Cook 2012). To confirm morphology-based identification, the nucleotide sequences of the internal transcribed specer region (ITS1 and ITS2) and large subunit gene (LSU) of the rDNA were determined in this study outlined by Bradshaw and Tobin (2020). The resulting sequences were submitted to GenBank (OQ061318 for ITS, OQ061319 for LSU) and were 100% identical with sequences of Podosphaera xanthii (MT242593, LC371331 etc.) for both ITS and LSU gene from the BLAST'n search results. Thus, based on morphology and results of molecular analysis, the isolate on V. bonariensis in Korea was identified as P. xanthii. The pathogenicity test was carried out by touching a diseased leaf onto healthy leaves of five pot-grown plants. Five non-inoculated plants were used as controls. After 7 days, typical powdery mildew colonies started to appear on the inoculated leaves. All control plants remained symptomless. The fungus isolated from the inoculated leaves was morphologically identical to that observed on the originally diseased leaves, which supports Koch's postulates. Hitherto, powdery mildew of Verbena spp. associated with Podosphaera sp. (including Sphaerotheca sp.) has been globally reported (Farr & Rossman 2022). Podosphaera xanthii on V. bonariensis was recorded from China and Japan (Hong et al. 2021, Farr & Rossman 2022), while in Korea, this fungus was recorded on Verbena brasiliensis (Cho et al. 2014), but not on V. bonariensis. To our knowledge, this is the first report of powdery mildew disease caused by P. xanthii on V. bonariensis in Korea. Our field observations suggest that this powdery mildew occurs on V. bonariensis planted in the shade. This finding could be useful for the breeding programme of Verbena spp. and for planting strategies in gardens.