First report of powdery mildew caused by an Erysiphe sp. On Actinostemma lobatum in Korea

Actinostemma lobatum Maxim. (Cucurbitaceae) is a climbing vine widely distributed in subtropical and temperate regions of Asia. It has been used for medicinal purposes, such as diuretic and antidote in Korea (Kim et al. 2008). Saponins are proposed to be the main bioactive components (Li et al. 2012). In August 2007, powdery mildew of the plants was first observed in natural ecosystems in Yangpyeong (37°32′16″N; 127°19′02″E), Korea. Signs first appeared as white colonies, which subsequently developed into abundant growth on both sides of the leaves. Symptoms included distortion and early defoliation of the leaves. Numerous chasmothecia were also formed on the leaves and stems in autumn. Powdery mildew on this plant has been found additionally in several localities in Korea. Five voucher specimens including 2016 collection have been deposited in the Korea University Herbarium (KUS). The hyphal appressoria were multilobed or moderately lobed. Conidiophores were straight, 70 to 120 × 7 to 9 µm, and composed of 3 to 4 cells. Foot-cells of conidiophores were straight or slightly flexuous at the base, and 25 to 35 µm long. Primary conidia were apically rounded and basally subtruncate. Conidia were oblong-elliptical, 31 to 42 × 15 to 21 µm with a length/width ratio of 1.6 to 2.5, devoid of distinct fibrosin bodies, and showed angular/rectangular wrinkling of outer walls. Germ tubes were produced in the perihilar position of the conidia. Chasmothecia were amphigenous, cauligenous, embedded in the mycelial mat, 80 to 130 μm in diameter, and contained 4 to 7 asci. Appendages were mycelioid, numerous, septate, and brown at the base. Asci were ellipsoid to obovoid, short-stalked, and 50 to 65 × 30 to 40 μm. Ascospores numbered 4 to 6 were ellipsoidal, 13 to 23 × 8 to 13 μm. The internal transcribed spacer (ITS) regions of KUS-F25657 selected for molecular identification were amplified with primers ITS1/ITS4, and sequenced directly. The resulting 674 bp sequence was deposited in GenBank (accession no. KY290257). A GenBank BLAST search of the Korean isolate showed >99% similarity with Erysiphe sp. on crassulaceous plants (e.g., EU185636 to EU185641). Therefore, the Korean isolate was determined to be an Erysiphe sp. Pathogenicity was confirmed through inoculation tests by gently pressing a diseased leaf onto three young leaves of three asymptomatic, potted plants. Three noninoculated plants were used as controls. Plants were maintained in a greenhouse at 24 to 30°C. Inoculated leaves developed symptoms 7 days after inoculation, whereas the control plants remained symptomless. Powdery mildews of A. lobatum have been recorded as E. actinostemmatis, E. pisi var. pisi, E. polygoni, and Golovinomyces cichoracearum from Japan and China (Amano 1986; Farr and Rossman 2016). However, the identity of the species of Erysiphe is still unclear and has to be proven and confirmed. To our knowledge, this is the first report of powdery mildew on A. lobatum in Korea. Common occurrence of the powdery mildew, especially in shady areas, can be a potential threat to the commercial cultivation of this medicinal vine in future.