Curled dock (Rumex crispus L.) is a perennial flowering plant in family Polygonaceae, native to Europe and western Asia. Curled dock is a widespread naturalized species throughout the temperate world that has become a serious invasive species as a weed in many areas. In contrast, the plant has been widely used as a folk medicine for treatment of indigestion and dermatoses in Asia countries. The plant roots are known to have an antifungal effect against barley powdery mildew pathogens. In late October 2010 to 2011, plants showing typical symptoms of powdery mildew disease were observed in a river bank area located in Gwangju, South Korea. Symptoms included generally white, superficial mycelia and abundant necrotic black spots showing superficial chasmothecia. Mycelia were ectophytic with lobed appressoria. Conidiophores were cylindrical, straight, or slightly flexuous in foot cells and bore single conidia. The foot cell of the fungus had a greater range of size than Erysiphe polygoni. Conidia and conidiophores were 25.4 to 45.4 (36.5) μm long × 10.5 to 18.6 (15.0) μm wide and 34.7 to 126.0 (91.4) μm long × 8 to 10 (8.7) μm wide, respectively. The teleomorph included spherical to subspherical ascocarps that were (blackish) brown to yellow and formed hyphoid appendages. Appendages were slightly flexuous and 62.0 to 128.1 (71.6) μm wide. Mature chasmothecia were 75.1 to 140.9 (105) μm. The ascocarps contained multiple asci that were saccate, ellipsoidal and papillate in apices, bore 3 to 5 ascospores, and were 59.4 to 66.1 (60.9) μm long × 32.6 to 43.9 (38.3) μm wide. Ascospores were subhyaline, oval to ellipsoid, and 17.9 to 24.8 (21.1) μm long × 10.9 to 15.2 (13.3) μm wide. From extracted genomic DNA, the internal transcribed spacer (ITS) region inclusive of 5.8S and 28S rDNA were amplified with ITS1F (5′-TCCGTAGGTGAACCTGCGG-3′) and LR5F (5′-GCTATCCTGAGGGAAAC-3′), and LROR (5′-ACCCGCTGAACTTAAGC-3′) and LR5F primer sets, respectively. rDNA ITS (JX499184) and 28S (JX888470) homologies of the fungal strain (EML-RCPW1) via NCBI BLASTn search represented 99.7% (618/620) and 100% (667/667) identity values with E. heraclei AB104510 and AB103366, respectively. The identification of the fungus as E. heraclei was based on morphological data combined with the results of sequence analysis. Although there were no 28S sequence data from E. polygoni in GenBank, the phylogenetic tree based on ITS sequence data showed that our strain was differentiated from E. polygoni, forming a separate clade consisting of E. heraclei. So far, 26 records with respect to powdery mildews on curled dock represent those caused by only E. polygoni worldwide (1). E. heraclei has been reported to occur on various herbaceous plants including Angelica spp., Daucus spp., and Torilis japonica, and a woody plant such as Quercus myrsinaefolia in China, Japan, and Korea. To our knowledge, this is the first report of leaf powdery mildew caused by E. heraclei on curled dock in Korea or elsewhere in the world, although the fungus causes powdery mildew on various species of families Polygonaceae and Apiaceae with wide host range (2,3,4).
References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases, Syst. Mycol. Microbiol. Lab., ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/, June 22, 2012. (2) D. A. Glawe et al. Online. Plant Health Progress. PHP-2005-0114-01-HN, 2005. (3) M. J. Park et al. New Dis. Rep. 21:14, 2010. (4) G. Rodríguez-Alvarado et al. Plant Dis. 94:483, 2010.