First report of powdery mildew caused by Podosphaera elsholtziae on Ajuga reptans in Korea

Ajuga reptans L. (Lamiaceae), known as bugle or bugleherb, is a perennial herbaceous plant native to Europe. Though invasive in parts of North America, it is an ornamental commonly grown as a ground cover in temperate climates. In September 2015, hundreds of A. reptans individuals of an unknown cultivar grown in a public garden (34°47′40″ N; 126°22′25″ E) in Mokpo, Korea, were found damaged by a powdery mildew with approximately 50% disease incidence. Symptoms first appeared as white colonies, which subsequently covered both sides of leaves, detracting from the aesthetic value of affected plants. Chasmothecia were abundantly formed by November. Voucher specimens (KUS-F29062, F29224) were deposited in the Korea University Herbarium (KUS). The hyphae were septate, branched, and 3 to 8 μm in width. Conidiophores (n= 20) were straight, 130 to 200 × 10 to 12 μm, and produced 2 to 5 immature conidia in chains with a crenate outline. Foot-cells of conidiophores were straight, cylindric, and 35 to 95 μm long. Conidia (n= 20) were hyaline, ellipsoid-ovoid to barrel-shaped, 24 to 33 × 21 to 27 μm with a length/width ratio of 1.1 to 1.5, and contained distinct fibrosin bodies. Dark brown chasmothecia were scattered to gregarious, spherical, and 75 to 95 µm in diameter. Peridium cells were polygonal to irregular, 15 to 32 μm diam. Appendages were few, mycelioid, 1- to 5-septate, 5 to 9 μm wide, brown at the base and becoming paler upwards. Each chasmothecium contained a single ascus. Asci with a terminal oculus of 10 to 15 µm wide were sessile, broadly ellipsoid-ovoid to subglobose, 8-spored, and 65 to 92 × 47 to 58 μm. Ascospores (n= 20) were colorless, ellipsoidal, and 15.0 to 22.5 × 12.5 to 16.5 µm. These characteristics were consistent with those of Podosphaera elsholtziae(Z.Y. Zhao) T.Z. Liu & U. Braun (Braun and Cook 2012). The internal transcribed spacer (ITS) region of isolate KUS-F29062 was amplified using universal primers ITS1/ITS4 and sequenced (White et al. 1990). The obtained ITS sequence was deposited in GenBank (accession no. KX668270). A GenBank BLAST search of the Korean isolate showed 100% identity with P. elsholtziaeon A. reptans from Japan (AB026142). Pathogenicity was confirmed by pressing a diseased leaf onto young leaves of three healthy, potted A.reptans. Three noninoculated plants served as controls. Plants were maintained in a greenhouse at 24 to 30°C. The first lesions developed on leaves of inoculated plants after 7 days, whereas the control plants remained symptomless. There have been two previous records of powdery mildew infection caused by Podosphaera spp. on A.reptansfrom Italy and Japan; P. fuscafrom Italy (cf.Braun 1995) andP. elsholtziae from Japan (Ito and Takamatsu 2010). To our knowledge, this is the first report of powdery mildew caused by P. elsholtziae on A.reptansin Korea. Due to shade tolerance and growth habit in dense mats, A. reptansis preferred to be planted as ground cover under trees. Therefore, powdery mildew seems to be inevitable. Breeding of resistant varieties is urgently necessary.