First report of powdery mildew caused by Golovinomyces biocellatus on apple mint in Korea

Apple mint (Mentha suaveolens Ehrh., family Lamiaceae) is an herbaceous perennial plant that is most commonly grown as a culinary herb or for ground cover. In September 2014, dozens of apple mint plants in a plastic greenhouse in Namwon (35°26′02.3″ N; 127°32′58.4″ E), Korea, were found to be infected with a powdery mildew. Symptoms first appeared as circular to irregular white patches, which subsequently coalesced to develop into abundant hyphal growth on both sides of the leaves. As the disease developed, affected leaves became senescent and necrotic. A voucher specimen (KUS-F28392) was deposited in the Korea University Herbarium (KUS). Appressoria on the mycelium were nipple-shaped. Conidiophores were 70 to 170 × 9 to 11 μm and produced 2 to 5 immature conidia in chains with a sinuate outline. Foot-cells of the conidiophores were straight, cylindrical, slightly constricted at the base, and 36 to 60 μm long. Conidia were hyaline, ellipsoid to barrel-shaped, 26 to 40 × 16 to 22 μm (length/width ratio = 1.4 to 2.2), devoid of distinct fibrosin bodies, and showed reticulate wrinkling of the outer walls. Primary conidia were obconically rounded at the apex and subtruncate at the base. Germ tubes were produced at the perihilar position of conidia. No chasmothecia were observed until December. The structures were typical of the Euoidium anamorph of the genus Golovinomyces. The morphological characteristics were in agreement with those of G. biocellatus (Ehrenb.) V.P. Heluta (Braun and Cook 2012). To confirm the identification, molecular analysis of the sequence of the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) of isolate KUS-F28392 was conducted. The complete ITS rDNA sequence was amplified using primers ITS1/ITS4, and sequenced. The resulting sequence was deposited in GenBank (Accession No. KT335972). A GenBank BLAST search of the Korean isolate sequence showed 100% identity with the ITS sequences of many G. biocellatus isolates on plants in the Lamiaceae (e.g., Accession Nos. HM156494, HM053470, AB307675, EU035602, etc.). Pathogenicity was confirmed by gently pressing a symptomatic leaf onto leaves of five healthy, potted apple mints. Five noninoculated plants served as controls. Plants were maintained in a greenhouse at room temperature. Inoculated plants developed symptoms after 8 days, whereas the control plants remained symptomless. The fungus present on inoculated plants was identical morphologically to that observed on the original diseased plants. Many species of Mentha have been recorded as host plants of G. biocellatus (Farr and Rossman 2015). Among them, G. biocellatus on Mentha suaveolens was recorded in Switzerland and Australia (Bolay 2005; Liberato and Cunnington 2007). To our knowledge, this is the first report of powdery mildew caused by G. biocellatus on M. suaveolens in Korea. Occurrence of powdery mildew on apple mint would be a potential threat to safe production of this herb.