First Report of Powdery Mildew Caused by Podosphaera xanthii on the Invasive Weed, Bidens pilosa, in Korea

Abstract

Bidens pilosa L., known as Spanish needle or cobbler's peg, is a cosmopolitan weed, originating from South America and common in all tropical and subtropical areas of the world. In Korea, this plant was first reported in 1992 (3), and has become naturalized widely by replacing indigenous plants and disrupting native ecosystems. In the autumn of 2012, hundreds of Spanish needles growing around an industrial complex in Milyang in southern Korea were observed to be severely infected with a powdery mildew. Symptoms first appeared as circular to irregular white patches, which subsequently showed abundant hyphal growth on both sides of the leaves, and on young stems and inflorescences. Red-purplish discoloration and crinkling of the leaves were also observed. Appressoria on the mycelium were nipple-shaped or nearly absent. Conidiophores measured 100 to 190 × 11 to 13 μm, and produced 2 to 5 immature conidia in chains with a crenate outline, at the end of 2 to 3 straight cells of the conidiophore. Conidia were hyaline, ellipsoid to barrel-shaped, measured 28 to 42 × 16.5 to 21 μm (length/width ratio of 1.4 to 2.0), and had distinct fibrosin bodies. Dark brown chasmothecia were found partly embedded in the mycelial felt on leaves and stems, were spherical, gregarious to scattered, and 80 to 100 μm in diameter. Each contained a single ascus and had a terminal oculus 17 to 23 μm wide. Asci were sessile, 70 to 90 × 52 to 65 μm, and contained eight ascospores, each 22 to 28 × 16 to 20 μm. The structures and measurements were consistent with those of Podosphaera xanthii (Castagne) U. Braun & Shishkoff (1). Specimens were deposited in the Korea University Herbarium (KUS). To confirm identity of the causal fungus, the complete internal transcribed spacer (ITS) region of rDNA of isolate KUS-F27201 was amplified with primers ITS5 and P3 (4), and sequenced directly. The resulting 476-bp sequence was deposited in GenBank (Accession No. KC484700). A GenBank BLAST search of this sequence revealed >99% similarity with those of many P. xanthii isolates on plants in the Asteraceae. Pathogenicity was confirmed through inoculation by gently pressing diseased leaf onto leaves of five healthy, potted, young Spanish needle plants. Five non-inoculated plants served as a control treatment. Plants were maintained in a greenhouse at 25 ± 2°C. Inoculated plants developed symptoms after 8 days, whereas the control plants remained symptomless. The fungus present on the inoculated plants was identical morphologically to that originally observed on diseased plants, fulfilling Koch's postulates. Powdery mildews of B. pilosa caused by Podosphaera species have been reported in North America, Africa, and Asia (India and China), but not from Korea (2). To our knowledge, this is the first report of powdery mildew caused by P. xanthii on Spanish needles in Korea. The field observations suggest that the powdery mildew could limit expansion of this invasive weed in Korea.