First Report of Powdery Mildew Caused by Neoerysiphe galeopsidis on Lamium purpureum in Korea

Abstract

Lamium purpureum L. (Lamiaceae), known as red dead-nettle, is native to Europe and Asia Minor. It has been widely naturalized in East Asia including Korea, where it is considered an invasive species. This plant is also known as one of primary overwintering sources for cucumber mosaic virus with a broad host range including many horticultural crops (Tomlinson et al. 1970). In May 2018, several hundred plants of red dead-nettle were found heavily infected with a powdery mildew in Gongju (36°19′59″N; 127°05′31″E), Korea. Symptoms first appeared as thin white colonies, which subsequently developed into abundant hyphal growth on both sides of the leaves. Voucher specimens were deposited in the Korea University herbarium (KUS-F30500). Hyphal appressoria were moderately lobed to multilobed. Conidiophores were 90 to 240 × 10 to 12 μm and produced two to six immature conidia in chains with a sinuate outline, followed by one to three cells. Foot cells of conidiophores were straight or slightly swollen at the base, cylindrical, and 30 to 60 μm long. Conidia were hyaline, cylindric oval to ellipsoid, measured 24 to 38 × 18 to 22 μm (length/width ratio = 1.3 to 2.1), and devoid of distinct fibrosin bodies. Primary conidia were rounded at the apex and subtruncate at the base. Germ tubes were produced on the perihilar position of conidia. No chasmothecia were observed during field survey until the plants were killed by frost damage in November. The structures described above were typical of powdery mildew Striatoidium anamorph of the genus Neoerysiphe. The measurements and morphological characteristics were compatible with those of N. galeopsidis (DC.) U. Braun (Braun and Cook 2012). To confirm the identification, the internal transcribed spacer (ITS) regions of KUS-F30500 were amplified by polymerase chain reaction using primers ITS5/P3 as described by Takamatsu et al. (2009) and sequenced directly. The resulting 579-bp sequence was deposited in GenBank (accession no. MK156132). A GenBank BLAST search using the Korean isolate showed 99% (575/579) similarity with those of N. galeopsidis isolates on Lamium purpureum (AB498942 from Ukraine and KY660982 from the United Kingdom). Pathogenicity was confirmed through inoculation tests by gently pressing a diseased leaf onto young leaves of three asymptomatic potted plants. Three noninoculated plants were used as controls. Plants were maintained in a glasshouse without temperature control, between about 20 and 40°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. The powdery mildew infection of L. purpureum associated with N. galeopsidis has been known in most European countries (Farr and Rossman 2018) and Japan (Takamatsu et al. 2008). Although N. galeopsidis has been recorded on Stachys affinis and Lamium amplexicaule in Korea, this is the first report of powdery mildew caused by N. galeopsidis on L. purpureum. The field observations suggest that the powdery mildew may limit the spread of this invasive weed in Korea.