Ambrosia trifida L., commonly known as giant ragweed, is native to North America and was introduced to Korea in the 1970s (3). It is now widely naturalized, and since 1999, has been designated as one of 11 ‘harmful nonindigenous plants’ by the Korean Ministry of Environment because of its adverse effects on native plants. Various strategies to eradicate this noxious weed have been tried without any success (3). In September 2009, powdery mildew infections of giant ragweed were found for the first time in Dongducheon, Korea, and specimens were isolated and deposited in the Korea University Herbarium (KUS-F24683). White mycelial and conidial growth was present mostly on adaxial leaf surfaces with sparse growth on abaxial leaf sides. Severely infected leaves were malformed. Slight purplish discoloration occurred on the leaves contiguous with colony growth. Mycelial colonies were conspicuous, amphigenous, and epiphytic with indistinct to nipple-shaped appressoria. Conidiophores were 80 to 180 μm long and produced two to five immature conidia in chains. Conidia were ellipsoid or doliiform, 28 to 38 × 16 to 24 μm, and lacked distinct fibrosin bodies. Chasmothecia were amphigenous, scattered or partly clustered, dark brown, spherical, 95 to 130 μm in diameter, and contained 6 to 16 asci. Appendages were mycelioid, numbering 10 to 24 per chasmothecium, 0.5 to 2.5 times as long as the chasmothecial diameter, 1 to 4 septate, and were brown at the base and becoming paler toward the tip. Asci were short stalked, 50 to 75 × 32 to 42 μm and contained two spores. Ascospores were ellipsoid-ovoid with a dimension of 22 to 30 × 15 to 18 μm. On the basis of these morphological characteristics, this fungus was identified as Golovinomyces ambrosiae (Schwein.) U. Braun & R.T.A. Cook (= G. cichoracearum var. latisporus (U. Braun) U. Braun) (1). To confirm the identification, the complete internal transcribed spacer (ITS) region of rDNA from KUS-F24683 was amplified with the primers ITS5 and P3 and sequenced (4). The resulting sequence of 508 bp was deposited in GenBank (Accession No. JF907589) and was identical to the ITS sequences of G. ambropsiae on A. artemisiifolia var. elatior from Japan (AB077631) and Korea (JF919680) as well as on A. trifida from the United States (AF011292). Therefore, the sequence analysis verified the pathogen to be G. ambrosiae. To our knowledge, this is the first record of powdery mildew infections on giant ragweed outside of North America (2). Although the disease incidence is still low, the disease could be a limiting factor to suppress the expansion of this noxious weed in Korea.
References: (1) U. Braun and R. T. A. Cook. Mycol. Res. 113:616, 2009. (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/, May 5, 2011. (3) S. M. Oh et al. Impacts of Invasive Alien Weeds and Control Strategies of Noxious Weeds in Korea. National Institute of Agricultural Science and Technology, Suwon, Korea, 2007. (4) S. Takamatsu et al. Mycol. Res. 111:117, 2009.