Spanish needles (Bidens bipinnata L.) is an annual herb that belongs to a genus of flowering plants in family Asteraceae native to United States, and tropical regions around world. The plant produces important flavonoid compounds quercitin and hyperoside that function as anti-allergens, anti-inflammatories, anti-microbials, and anti-cancer agents. Between July and October 2011 and 2012, white superficial mycelia were observed initially on leaf and stem portions, but later progressed to the flower head. Surveys showed that the disease was widespread in Gwangju and most areas of South Korea. Abundant, necrotic, dark brown spots showing chasmothecia were frequently observed in October and were abundant on the adaxial surface of leaves. Chasmothecia were blackish brown to yellow without typical appendages. They ranged from 51.2 to 71.1 (mean 66.8) μm in diameter. Conidia were formed singly and the primary conidia were ellipsoid, rounded at the apex, truncated base, and ranged from 25.4 to 33.2 (mean 27.3) μm long × 10.2 to 12.2 (mean 11.3) μm wide. Conidiophores were erect, 60.1 to 101.3 (mean 98.3) μm long × 6.2 to 9.2 (mean 7.3) μm wide. From extracted genomic DNA, the internal transcribed spacer (ITS) region inclusive of 5.8S and 28S rDNA was amplified with ITS1F (5′-TCCGTAGGTGAACCTGCGG-3′) and LR5F (5′-GCTATCCTGAGGGAAAC-3′), and LROR (5′-ACCCGCTGAACTTAAGC-3′) and LR5F primer sets, respectively. rDNA ITS (GenBank Accession No. JX512555) and 28S (JX512556) homologies of the fungus (EML-BBPW1) represented 99.6% (532/534) and 100% (661/661) identity values with Podosphaera xanthii (syn. P. fusca) AB040349 and P. xanthii (syn. P. fusca) AB462798, respectively. The rDNA sequence analysis revealed that the causal fungus matched P. xanthii (syn. P. fusca), forming a xanthii/fusca group (3,4). A pathogenicity test was performed on three plants in a greenhouse. The treated leaves were sealed in vinyl pack in humid condition for 2 days. Seven days after inoculation, similar symptoms were observed on the inoculated Spanish needles plant leaves. No symptoms were observed on control plants treated with distilled water. Koch's postulates were fulfilled by re-observing the fungal pathogen on the inoculated leaves. Podosphaera (syn. Sphaerotheca) xanthii (or fusca) has been known as an ubiquitous species with a broad host range. So far, five records regarding P. xanthii (=P. fusca) have been found in plants of genus Bidens. P. xanthii has been reported to occur on B. cernua in Belarus and Switzerland. In addition, the powdery mildew species was reported to occur on B. frondosa and B. tripartita in Korea, Russia, and Switzerland (2). To our knowledge, this is the first report of powdery mildew caused by P. xanthii on Spanish needles (B. bipinnata) in Korea.
References: (1) U. Braun et al. Schlechtendalia 10:91, 2003. (2) D. F. Farr and A. Y. Rossman. Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/, 2012. (3) H. B. Lee. J. Microbiol. 51:1075, 2012. (4) S. Takamatsu, et al. Persoonia 24:38, 2010.