Powdery mildew on Actinostemma lobatum Maxim., Franch. & Sav. (syn. Actinostemma tenerum Naudin) had been described as Erysiphe communis (Wallr.) Schltdl. in a broad sense [1]. In 1976, this fungus was treated as E. communis f. cucumidis Marchenko [2], presumably due to its unique parasitism on cucurbitaceous hosts among the wide host range of E. communis. In 1983, Braun [3] raised this fungus to the species level as E. actinostemmatis U. Braun.

According to Braun and Cook [4], the holotype of E. actinostemmatis designated by Braun was collected on October 23, 1910, in Japan and preserved in the National Museum of Nature and Science, Japan (accession no. TNS-F-229705). As the holotype accommodates only the sexual state of the fungus, the original description of E. actinostemmatis was devoid of anamorph characteristics, solely providing the conidial dimension as “approximately 30–38 × 13–17.5 µm”. Furthermore, no sequence data are available for E. actinostemmatis to date. Therefore, the basic information required to characterize E. actinostemmatis as a distinct species in the genus Erysiphe is insufficient for taxonomic purposes.

During our field forays in Korea, eight samples of A. lobatum infected by powdery mildew were collected and preserved at the Korea University Herbarium (KUS). They were labeled as KUS-F22806 (29 Aug 2007, Yangpyeong), F22927 (22 Sep 2007, Yangpyeong), F25657 (7 Oct 2010, Jinju), F27614 (16 Sep 2013, Yangpyeong), F29460 (11 Sep 2016, Imsil), F33083 (20 Jul 2022, Imsil), F33124 (24 Aug 2022, Imsil), F33299 (29 Sep 2022, Imsil), and F33569 (17 Nov 2022, Imsil).

Detailed morphological characteristics of the powdery mildew were examined using an Olympus BX50 microscope (Olympus, Tokyo, Japan). Photomicrographs were captured using a Zeiss AX10 microscope equipped with an AxioCam MRc5 camera (Carl Zeiss, Oberkochen, Germany). All microscopic characterizations and measurements were performed using fresh samples.

Powdery mildew symptoms were observed on the leaves, stems, and fruit (Fig. 1A–F). Chasmothecia were formed in September and October (Fig. 1C–F). The morphological characteristics of powdery mildew are as follows: Hyphae were straight or wavy, hyaline or almost hyaline, septate, branched, and 4–7 µm wide; hyphal appressoria were well-developed, moderately lobed to multi-lobed, and solitary or in opposite pairs (Fig. 1G–I). Conidiophores were 76–120 × 7–9 µm, producing solitary conidia, followed by 2–3 cells; foot-cells of conidiophores were straight to slightly curved, relatively short, and 26–36 µm long (Fig. 1J–K). Conidia were hyaline, oblong-elliptical to ellipsoid, and 30–46 × 15–22 µm, with a length/width ratio of 1.6–2.8 (Fig. 1L). They lacked fibrosin bodies and produced germ tubes in the perihilar position (Fig. 1M–O). Chasmothecia were amphigenous, cauligenous, gregarious to scattered, partly embedded in the mycelial mat, dark brown, spherical, and 90–124 µm in diameter, and contained 4–7 asci per chasmothecium (Fig. 1P–Q). Chasmothecial appendages were mycelioid, about 10–20 in number, septate, not branched, reaching up to four times the chasmothecial diameter, and brown throughout or paler upwards (Fig. 1P). Asci were obovoid, saccate, short-stalked, and 52–64 × 30–38 µm, and contained 6–8 spores per ascus (Fig. 1R). Ascospores were hyaline to subhyaline, oblong-ovoid, and 14–18 × 9–10 µm (Fig. 1S–T). The fungus was initially identified as E. actinostemmatis based on its morphological characteristics, including sexual morph and host specification [4].

To confirm the morphology-based identification and reveal the molecular phylogeny of the fungus, nucleotide sequences of the 5ʹ-end of the large subunit (LSU) gene and the internal transcribed spacer (ITS) region, including 5.8S rDNA gene, of the rDNA were determined from three collections, as outlined by Choi et al. [5]. The forward and reverse sequences were assembled in MEGA11, and consensus sequences were deposited to GenBank under the following accession numbers: PV366477, PV366478, and PV366479 for ITS, and PV366483, PV366484, and PV370038 for LSU. To determine the genetic affinity of our isolates, a comparison was performed against reference sequences using a BLAST search in the NCBI database. Results revealed 100% identity to Pseudoidium hortensiae (MG654731), E. begoniae (MZ958709–MZ95870912), and Erysiphe sp. (EU185636–EU185641) in the ITS, and 99.8% similarity to Pseudoidium pedaliacearum (LC342963, LC342966, LC342967), E. caricae-papayae (LC228614), and