Bergamot (Monarda didyma L.), which is native to eastern North America, is an aromatic herb in the family Lamiaceae. It is widely planted in gardens and parks for ornamental purposes and also grown indoors as a fragrant houseplant in Korea. In October 2007, several dozen bergamots planted outdoors in Bonghwa, Korea were found to be heavily infected with a powdery mildew. Symptoms first appeared as thin, white colonies, which subsequently developed into abundant growth on both sides of the leaves. Severe infections often caused leaf distortions and premature senescence. The same symptoms have also been found in bergamot plots in Osan, Suwon, Incheon, and Seoul from 2007 to 2011. Voucher specimens were deposited at Korea University, Seoul, Korea. Hyphae were septate, branched, and 4 to 8 μm wide. Appressoria on the mycelium were nipple shaped. Conidiophores arose from the lateral part of the hyphae, measured 100 to 180 × 10 to 12 μm, were simple, and produced two to four immature conidia in chains, followed by two to three cells. Conidia were hyaline, ellipsoid to barrel shaped, measured 28 to 40 × 16 to 20 μm (length/width ratio = 1.4 to 2.2), lacked distinct fibrosin bodies, and produced germ tubes on the subterminal position, with reticulate wrinkling of the outer walls. No chasmothecia were observed. The structures described above were typical of the Oidium subgenus Reticuloidium anamorph of the genus Golovinomyces, and the fungus measurements were compatible with those of Golovinomyces biocellatus (Ehrenb.) V.P. Heluta as described previously (1,4). The only other powdery mildew known on Monarda spp. is Neoerysiphe galeopsidis (1), which is clearly distinguished by its lobed hyphal appressoria and fine striations on conidial surfaces. To confirm the tentative identification based on morphological characteristics, internal transcribed spacer (ITS) rDNA sequences from two representative isolates (KUS-F23070 and F23117) were obtained using primers ITS5 and P3 as described by Takamatsu et al. (3). The resulting sequences of 523 bp were deposited in GenBank (Accession Nos. JN228358 and JN228359). A GenBank BLAST search produced an exact match for the sequences of G. biocellatus on several plants belonging to the Lamiaceae, with a 100% sequence similarity. Pathogenicity was confirmed through inoculation by gently pressing diseased leaves onto leaves of five healthy potted bergamot plants. Five noninoculated plants served as controls. Plants were maintained in a greenhouse at 25 ± 2°C. Inoculated plants developed signs and symptoms after 6 days, whereas the control plants remained healthy. The fungus present on the inoculated plants was morphologically identical to that originally observed on diseased plants. The powdery mildew infections of bergamot plants associated with G. biocellatus have been known in Europe (2) and Japan (4). The current work confirmed the occurrence of G. biocellatus infecting M. didyma in Korea.
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/, July 12, 2011, (3) S. Takamatsu et al. Mycol. Res. 113:117, 2009. (4) S. Tanda. J. Agric. Sci. Tokyo Agric. Univ. 47:274, 1997.
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