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First Report of Powdery Mildew Caused by Erysiphe heraclei on Dill in Korea

Dill (Anethum graveolens L.) is a scented herb belonging to the family Apiaceae. The plant has a long and ancient history in many countries as a culinary and medicinal herb. In October 2008, plants showing typical symptoms of powdery mildew disease were found in polythene tunnels in Icheon, Korea. Symptoms first appeared as thin white colonies, which subsequently showed abundant growth on the leaves and stems. Most diseased plantings were unmarketable and shriveled without being harvested. The damage caused by powdery mildew infections on dill has reappeared every year, with confirmation of the causal agent made again in 2011. Voucher specimens were deposited in the Korea University Herbarium (KUS). Hyphae were septate, branched, and 4 to 7 μm wide. Appressoria on the mycelium were multilobed or moderately lobed. Conidiophores were unbranched, cylindrical, 80 to 140 × 8 to 10 μm, straight or slightly flexuous in foot cells, and produced conidia singly, followed by two to three cells. Conidia were oblong elliptical to oblong, 28 to 50 × 14 to 18 μm, lacked fibrosin bodies, and produced germ tubes on the subterminal position, with angular/rectangular wrinkling of the outer walls. Primary conidia were apically conical, basally subtruncate, and generally smaller than the secondary conidia. No chasmothecia were found but the above characteristics are consistent with Erysiphe heraclei DC. (1). To confirm the identity of the causal fungus, the complete internal transcribed spacer (ITS) region of rDNA from isolate KUS-F26425 was amplified with primers ITS5 and P3 as described by Takamatsu et al. (3) and directly sequenced. The resulting 630-bp sequence was deposited in GenBank (Accession No. JQ517297). Comparison with the sequences available in the GenBank database revealed that the isolate showed >99% sequence similarity with those of E. heraclei from Pleurospermum camtschaticum (GU173850) and Daucus carota (EU371725). Pathogenicity was confirmed through inoculation by gently pressing diseased leaves onto leaves of five healthy potted dill plants. Five noninoculated plants served as controls. Plants were maintained in a greenhouse at 22 ± 2°C. Inoculated plants developed signs and symptoms after 7 days, whereas the control plants remained healthy. The fungus present on the inoculated plants was morphologically identical to that originally observed on diseased plants. Powdery mildew caused by E. heraclei on dill has been known worldwide where the plant is cultivated (2). In East Asia, however, dill powdery mildew was known only from Taiwan (4). To our knowledge, this is the first report of powdery mildew infections by E. heraclei on dill in Korea. Since cultivation of dill was only recently started on a commercial scale in Korea, powdery mildew infections pose a serious threat to safe production of this herb, especially in organic farming where chemical control would be prohibited.

References: (1) U. Braun. Beih. Nova Hedw. 89:1, 1987. (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/ January 28, 2012. (3) S. Takamatsu et al. Mycol. Res. 113:117, 2009. (4) J. G. Tsay. Trans. Mycol. Soc. Repub. China 5:1, 1990.