First report of powdery mildew caused by Podosphaera erigerontis-canadensis on Conyza bonariensis in Korea

Conyza bonariensis (L.) Cronq. (Asteraceae), also called wavy-leaf fleabane or Argentine fleabane, is a widespread weed of cultivated land, garden areas, pastures, wasteland, and roadsides in the tropical and subtropical regions. This weed originated from Central America or South America and was accidently introduced into Korea in the 1910s, and has been naturalized in the southern part of Korea. In November 2011, a dozen plants exhibiting symptoms of powdery mildew were found with 100% disease incidence in Busan (35°09′28″N; 129°09′17″E), southern Korea. White mycelial growth covered entire leaves, causing premature senescence. The same symptoms were also found in Daegu (35°53′51″N; 128°35′07″E) and Jeju (33°30′21″N; 126°31′59″E) in September 2016. Three specimens have been deposited in the Korea University Herbarium (KUS). Appressoria on the mycelium were nipple-shaped or nearly absent. Conidiophores were straight, 125 to 220 × 10 to 12 μm, and produced 3 to 5 immature conidia in chains with a crenate outline. Foot-cells of conidiophores were cylindric and 50 to 75 μm long. Conidia were ellipsoid-ovoid to barrel-shaped, 30 to 45 × 15 to 20 μm with a length/width ratio of 1.8 to 2.3, and had distinct fibrosin bodies. Germ tubes were produced from the lateral position of conidia. No chasmothecia were observed. These structures are typical of the Euoidium anamorph of the genus Podosphaera. The internal transcribed spacer (ITS) regions of KUS-F29294 were amplified with primers ITS1/ITS4 and sequenced. The resulting 475-bp sequence was deposited in GenBank (accession no. KY678231). A GenBank BLAST search of the Korean isolate showed >99% similarity with those of Podosphaera erigerontis-canadensis from Erigeron canadensis (syn. Conyza canadensis) from Japan (AB040313). Based on the molecular analysis, the pathogen detected on C. bonariensis was identified as P. erigerontis-canadensis (Lév.) U. Braun & T.Z. Liu, accordingly with the morphological characteristics of the pathogen described above and previously reported (Braun and Cook 2012). Pathogenicity was confirmed by pressing an affected leaf onto young leaves of three healthy plants. Three noninoculated plants were used as controls. All plants were maintained in greenhouses at 24 to 30°C. Inoculated leaves developed symptoms after 5 days, whereas the control plants remained symptomless. Association of C. bonariensis with powdery mildew caused by P. fusca (syn. Sphaerotheca fusca) has been reported in Mexico, France, Portugal, and South Africa (Farr and Rossman 2017). In Korea, Chung et al. (1977) listed Sphaerotheca sp. on C. bonariensis, but there was no proof of identification. To our knowledge, this is the first report of powdery mildew on C. bonariensis caused by P. erigerontis-canadensis in Korea, as well as worldwide. Our field observations showed that the powdery mildew infections result in reduced growth and poor seed production of this weed, suggesting the possibility to limit expansion of C. bonariensis in Korea.