The genus Zelkova Spach, belonging to the family Ulmaceae, consists of six species distributed across Europe and East Asia. Although Zelkova species are widespread, only Zelkova serrata (Thunb.) Makino has been reported as a host for powdery mildew fungus Erysiphe zelkovae (Henn.) U. Braun [as ʻzelkowae’], which is also encountered only in East Asian countries, such as Japan, China, and Korea [1]. Furthermore, E. zelkovae is the only known powdery mildew pathogen found on Zelkova globally. Traditionally, the powdery mildew pathogen of Z. serrata was identified as a variety of Uncinula kusanoi Syd. & P. Syd. [currently Erysiphe kusanoi (Syd. & P. Syd.) U. Braun & S. Takam.] based on the similarities in their morphological traits [2,3]. Japanese scientists followed this concept and recorded Zelkova powdery mildew as U. kusanoi var. zelkovae (P. Henn) U. Braun [4]. In Korea, E. zelkovae was first recorded as Uncinula zelkovae P. Henn. [5]. Shin [5] recognized U. zelkovae as a distinct species differing from U. kusanoi var. zelkovae on the basis of the number of ascospores in an ascus, while also emphasizing the close affinity of the anamorph morphology of the two powdery mildew species.

Erysiphe kusanoi is an obligate parasite of Celtis L. species [1]. Previously, Celtis spp. were classified within the elm (Ulmaceae) family, but were later transferred to the Cannabaceae (hemp) based on the molecular phylogenetic analysis [6]. Both the Ulmaceae and Cannabaceae families belong to the order Rosales; therefore, the molecular and morphological similarities of E. zelkovae and E. kusanoi can be understood within the framework of the phylogenetic relationship between host families. Considering the above-mentioned issues between these two powdery mildew species, detailed mycological characterization is required to distinguish them. Furthermore, a single sequence from a Japanese specimen (MUMH 403) for the internal transcribed spacer (ITS) and large subunit ribosomal DNA (LSU) regions provides insufficient molecular evidence to recognize E. zelkovae as a distinct species. Therefore, this study aimed to characterize the holomorph morphology of E. zelkovae in detail, to elucidate its molecular phylogenetic position using ITS and LSU sequence data derived from Korean specimens, and to compare its phylogenetic relationships with other closely related Erysiphe powdery mildews.

A total of 28 powdery mildew specimens associated with Z. serrata were collected from multiple localities in Korea and are currently preserved at the Korea University Herbarium (KUS). Accession numbers for the specimens are listed as follows: KUS-F15571 (Oct 27, 1998; Seoul), F15636 (Nov 03, 1998; Seoul), F17168 (Oct 22, 1998; Namyangju), F17795 (Oct 09, 2000; Dongducheon), F22395 (Oct 28, 2006; Jinju), F23620 (Aug 29, 2008; Gongju), F24645 (Sep 20, 2009; Mungyeong), F25070 (Jul 18, 2010; Jecheon), F25084 (Jul 19, 2010; Youngwol), F25252 (Aug 08, 2010; Dongducheon), F25511 (Oct 18, 2010; Pocheon), F27062 (Oct 06, 2012; Seoul), F27952 (Jul 25, 2014; Hongcheon), F28198 (Sep 22, 2014; Jinju), F28249 (Sep 25, 2014; Busan), F28349 (Oct 09, 2014; Miryang), F28389 (Oct 14, 2014; Hongcheon), F28549 (Nov 27, 2014; Ulsan), F28912 (Oct 12, 2015; Yangpyeong), F29630 (Oct 27, 2016; Yangpyeong), F30353 (Nov 09, 2017; Daegu), F30809 (Sep 11, 2018; Hongcheon), F31296 (Oct 15, 2019; Daegu), F33414 (Oct 23, 2022; Imsil), F33898 (Oct 23, 2023; Uijeongbu), F33905 (Oct 23, 2023; Seoul), F34384 (Oct 21, 2024; Namyangju), F34444 (Nov 15, 2024; Paju), and F34595 (Aug 21, 2025; Chuncheon).

The morphological features of the powdery mildew pathogen were examined using an Olympus BX50 light microscope (Olympus, Tokyo, Japan), and images were captured using a Zeiss AX10 microscope equipped with an AxioCam MRc5 camera (Carl Zeiss, Oberkochen, Germany). Microscopic observations and measurements were primarily conducted on fresh specimens. The lactic acid technique was used to examine the dried herbarium materials [7]. For each diagnostic structure, quantitative assessment was based on 30 measurements.

Mycelial mats with conidiophores and conidia were amphigenous and abundantly epigenous, forming circular to irregular white colonies (Fig. 1A). In the later stages of the disease, chasmothecia were formed abundantly on the lower side of the leaves (Fig. 1B, C). The appressoria on the mycelia were moderately lobed to multi-lobed (Fig. 1D, E). Conidiophores emerging from the side of the hyphae were 55–87 × 7–9 μm, and producing conidia singly, followed by 1–2(–3) cells (Fig. 1F–I). Foot cells of conidiophores were curved or L-shaped from the basal part, cylindrical, and 18–32 μm long. Conidia were hyaline, cylindrically ovoid to oblong, variable in shape and size, 26–36 × 13–16 μm (length/width ratio = 2.0–2.8), occupied by vacuoles and oil drops, and lacking distinct fibrosin bodies (Fig. 1J). Germ tubes were produced at the perihilar position of the conidia (Fig. 1K, L). Chasmothecia were amphigenous, abundantly hypogenous, scattered to gregarious, dark brown, depressed globose, and 91–125 μm in diameter, and containing 4–7 asci per chasmothecium (Fig. 1M, N). Chasmothecial appendages were positioned in the lower half of the chasmothecium, numerous, 12–24 in number, not branched, substraight to mildly curved, apex circinate to uncinate, 4–6 μm wide but up to 9 μm wide at the uncinate part, 1–1.5 times as long as the chasmothecial diameter, aseptate, and hyaline throughout (Fig. 1O). Asci were obovoid, saccate, subsessile, 42–56 × 32–42 µm, and moderately thick-walled, thinner at the upper part (Fig. 1P, Q). Ascospores were 3–4 in an ascus, oval to cylindrically ovoid, 24–30 × 11–14 µm (length/width ratio = 1.8–2.2), and containing oil drops (Fig. 1R, S).

The authors declare that there are no conflicts of interest.