Two Erysiphe species, E. diervillae Miyabe ex U. Braun and E. weigelae Z.X. Chen & S.B. Luo, were listed in Braun’s monograph in 1987 as powdery mildew pathogens infecting Weigela spp. [1]. E. diervillae was first introduced by Salmon in 1900, based on the herbarium material collected by Miyabe from Diervilla japonica DC. (currently classified as Weigela hortensis (Siebold. & Zucc.) K. Koch.) in Japan [2]. Jaczewski later reclassified it as a forma of Erysiphe communis (Wallr.) Schltdl. (referred to as E. communis f. diervillae) [3]. Later in 1983, Braun raised it to the species level as E. diervillae, in accordance with its earliest classification [3]. This fungus has long been considered endemic to Japan. Another powdery mildew species, E. weigelae, was introduced in 1984, having been found on W. japonica var. sinica (Rehder) L.H. Bailey, in Fujian, China [4]. In 1990, Heluta reduced it to a variety of E. diervillae, as E. diervillae var. weigelae (Z.X. Chen & S.B. Luo) V.P. Heluta [5]. Based on the current concept, the three varieties of E. diervillae – var. diervillae, var. weigelae, and var. chasanensis Heluta – were classified as powdery mildew pathogens of Weigela spp. [6]. According to Braun and Cook [6], var. weigelae differs from var. diervillae by having smaller chasmothecia (85–130(–140) µm vs. 110–160 µm) and ascospores (15–20 × 8–12 µm vs. 15–36 × 9–17 µm). E. diervillae var. chasanensis, which is known only from W. praecox L.H. Bailey (currently classified as W. florida (Bunge) A.DC.) in Russia, differs from the other two varieties by having irregularly branched ascomatal appendages [6].

In Korea, Erysiphe powdery mildew on W. florida and W. subsessilis L.H. Bailey was initially recorded as E. weigelae in 2003, based on its similarities to the original description of teleomorphic traits [4], and the differences in anamorphic features from those of E. diervillae as described by Nomura [7]. Additionally, the authors reported the presence of secondary hyphae in this fungus for the first time [8].

During our extensive field forays to study phytopathogenic fungi in Korea, 42 samples of powdery mildew on Weigela spp. were collected from various regions across Korea since 1999 and have been preserved in the Korea University herbarium (KUS). Four recently collected samples on W. subsessilis were examined as part of our study: KUS-F31246 (Oct 9, 2019; Gapyeong), F32902 (Jun 3, 2022; Wanju), F33739 (Jun 8, 2023; Jinan), and F34400 (Oct 31, 2024; Namyangju).

Detailed morphological characteristics of the fungus were examined using fresh samples under 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). Mycelial mats with conidiophores and conidia were amphigenous and caulicolous, persistent, in white patches, and occasionally causing reddish discoloration (Fig. 1A–C). Hyphae were superficial, straight to wavy, branched, and 4–8(–10) µm wide. Hyphal appressoria were multi-lobed or moderately lobed, occurring singly or in pairs, and 3–8 µm wide (Fig. 1D–E). Conidiophores were predominantly single or in groups of up to three on a hyphal cell, emerging from the upper part of the mother cells, 56–84 × 9–12 µm, straight at foot-cells, becoming broader or narrower upwards, with slight constrictions at the branching point, and producing conidia singly, followed by 2 straight shorter cells (Fig. 1F–I). Conidia were variable in shape, mostly doliiform to subcylindrical, 28–42 × 12–17 µm with a length/width ratio of 1.8–2.8, devoid of conspicuous fibrosin bodies, and occasionally guttulate (Fig. 1J); primary conidia were asymmetrical at both ends, exhibiting an apically rounded and basally subtruncate morphology (Fig. 1K and L); germ tubes were produced on the perihilar position of the conidia and variable in shape (Fig. 1M–O). Chasmothecia were gregarious and 110–136 µm in diameter; peridium cells were irregularly polygonal and 8–20 µm in diameter (Fig. 1P and Q); chasmothecial appendages were located in the lower half, mycelioid, simple, straight when young, becoming geniculate and wavy as they matured, interlaced with each other and the surrounding hyphae, with a variable length reaching up to 4 times the chasmothecial diameter, 3–6 µm wide, usually aseptate, occasionally with a septum at the base, and with thin, smooth, and hyaline walls (Fig. 1P and Q). Asci were 8–16 per chasmothecium, ellipsoid-obovoid, saccate, 50–68 × 30–40 µm, shortstalked or subsessile, 8-spored, with a 1–1.5 µm thick wall (Fig. 1R). Ascospores were ellipsoid, 15–18 × 9–10 µm with a length/width ratio of 1.5–2.1, and hyaline to faintly yellow (Fig. 1S). These characteristics were consistent with the description of Erysiphe diervillae, and based on the size of the chasmothecia and ascospores, our isolates were identified as Erysiphe diervillae var. weigelae [6].