In October 2016, hundreds of Saxifraga stolonifera plants, commonly known as cree** saxifrage, were found to be damaged by a powdery mildew with a 100% disease incidence in a public garden in Wonju, Korea (37°22′07″N; 127°57′43″E). White colonies were present on both sides of the leaves and stems, detracting from their beauty as landscape plantings (Fig. 1a). Infected leaves later showed partial distortion and diffuse discoloration. Samples from these plants were used for morphological characterization and molecular analyses, and they were preserved in the Korea University Herbarium (KUS; accession no. KUS-F29581).

Fig. 1
figure 1

Powdery mildew on Saxifraga stolonifera caused by Erysiphe sp. (a) Symptoms on leaves. (b) Hyphal appressoria. (c, d) Conidiophores. (e) Primary conidium. (f) Conidia. (g) Conidium in germination

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Previously, five samples of powdery mildews found on S. stolonifera were collected during our routine field forays and deposited in KUS tentatively determined as Pseudoidium sp., viz., KUS-F23438 (Seoul, 22 Jun 2008), F23716 (Seoul, 28 Sep 2008), F23950 (Yangpyeong, 3 Nov 2008), F25801 (Gyeongju, 17 Jun 2011), and F27811 (Busan, 28 Mar 2014). To confirm that these samples were very close or identical to KUS-F29581 in their morphological characteristics, the lactic acid technique (Shin 2000) was used.

Morphological characterization and specific measurements of the fungal structures were performed using an Olympus BX51 microscope (Olympus, Tokyo, Japan). Photographs were captured using an AxioCam MRc5 camera mounted on a Zeiss AX10 microscope (Carl Zeiss, Oberkochen, Germany). Hyphae were septate, branched, and 3 to 7 μm wide. Appressoria on the hyphae were well developed, lobed, and mostly positioned in pairs (Fig. 1b). Conidiophores were cylindrical, 70 to 125 × 8 to 10 μm, and composed of 3 to 4 cells (Fig. 1c, d). Foot-cells of conidiophores were straight, cylindrical, and 25 to 40 μm long. Primary conidia were apically rounded, basally subtruncate, and generally smaller than the secondary conidia (Fig. 1e). Conidia produced singly were variable in shape, oval to cylindrical oval or oblong-elliptical, 30 to 52 × 16 to 22 μm with a length/width ratio ranging from 1.6 to 2.3, devoid of distinct fibrosin bodies, and showed angular/rectangular wrinkling of outer walls (Fig. 1f). Germ tubes were produced on the perihilar position of conidia (Fig. 1g). No chasmothecia were observed until the leaves senesced and died. These structures are typical of the powdery mildew Pseudoidium anamorph of the genus Erysiphe. Therefore, we tentatively determined the isolates as Erysiphe sp.

To confirm the initial identification, the internal transcribed spacer (ITS) regions and 28S rDNA of KUS-F27811 and KUS-F29581 were amplified with the primers ITS5/P3 and PM3/TW14, respectively. Two ITS and 28S sequences were identical to each other, and the obtained sequences were deposited in GenBank (Accession Nos. MF115140, MF115141, MF446962, MF446963). Phylogenetic trees based on the neighbor-joining (NJ) method were generated using MEGA 7 (Kumar et al. 2016) with 30 ITS sequences and 19 28S rDNA sequences. The robustness of the NJ trees was evaluated with 1000 bootstrap (BS) values. The NJ trees showed that the Korean isolates from S. stolonifera were placed in the E. aquilegiae clade, as defined by Takamatsu et al. (2015) on the basis of ITS +28S rDNA data, with 99% and 100% BS value, respectively (Figs. 2 and 3). In addition, a Pseudoidium sp. infecting S. stolonifera from Japan (LC010064) formed a single cluster with E. aquilegiae, E. catalpae, E. macleayae, E. pileae, E. sedi, Pseudoidium hortensiae, P. neolycopersici, etc. Therefore, the phylogenetic position of the powdery mildew isolates on S. stolonifera was confirmed by the present phylogenetic analyses.

Fig. 2
figure 2

Phylogenetic relationship between Erysiphe sp. on Saxifraga stolonifera and some reference isolates retrieved from NCBI, inferred by the neighbor-joining method using the ITS regions. Bootstrap values (≥70%) based on 1000 replications are indicated above the branches. The scale bar represents 0.05 nucleotide substitutions per site. The isolates used in this work are indicated in bold

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Fig. 3
figure 3

Phylogenetic relationship between Erysiphe sp. on Saxifraga stolonifera and some reference isolates retrieved from NCBI, inferred by the neighbor-joining method using the 28S rDNA. Bootstrap values (≥70%) based on 1000 replications are indicated above the branches. The scale bar represents 0.01 nucleotide substitutions per site. The isolates used in this work are indicated in bold

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Pathogenicity was confirmed via inoculation tests by gently pressing a diseased leaf onto young leaves of five asymptomatic potted plants. Five non-inoculated plants were used as controls. The plants were kept in greenhouses at 24–30 °C. The inoculated leaves started to develop signs of powdery mildew after 6 days, whereas the control plants remained symptomless. The fungus present on the inoculated leaves was morphologically identical to that observed on the original infected leaves. The pathogenicity tests were performed twice with similar results, fulfilling Koch’s postulates.

The genus Saxifraga includes as many as 440 species (Anonymous 2017). Like many other species of Saxifraga, S. stolonifera has been used for its medicinal value (Chen et al. 2008; Liu et al. 2016) and for horticultural purposes (Harrison 2006; Suh et al. 2007; Ju and Yoon 2010). Two reports of powdery mildew on S. stolonifera were found. One was from Japan, in which it was simply listed as a host plant of Erysiphe communis without description or information (Amano 1986). A monographic study of Japanese powdery mildews compiled by Nomura (1997) did not mention Amano’s record. The other was from Korea (Park et al. 2010) regarding a Pseudoidium sp. as a host fungus of the hyperparasitic Ampelomyces quisqualis. However, there were no description and illustration demonstrating the taxonomic characteristics of the record.

On the other hand, Braun (1980) proposed that Erysiphe krumbholzii accommodated Pseudoidium powdery mildews found on Saxifragaceae, housing Erysiphe communis f. chrysosplenii and E. communis f. saxifragae (Braun and Cook 2012). This species was recorded on S. stolonifera from Asia including Japan and Russian Far East. However, the identity of E. krumbholzii has been not supported by molecular data. Therefore, further studies with morphological and molecular characterization are necessary to prove the taxonomic status of the species.

Takamatsu et al. (2015) performed molecular sequence analyses of Pseudoidium sp. on S. stolonifera and showed that the fungus pertains to the E. aquilegiae clade which comprises numerous morphologically defined powdery mildew species with insufficient resolution just based on ITS data. A morphological description or a detailed discussion of the isolate were not provided in the paper. Therefore, the status of the Japanese isolate remained as a Pseudoidium sp. In this study, we also prefer to determine the Korean collections on S. stolonifera as Erysiphe sp. based on morphology and phylogenetic placement. A more precise identification of the causal agent on species level requires phylogenetic analyses of the E. aquilegiae clade with better resolutions including additional markers.

Because of its shade tolerance and growth habit in dense mats, S. stolonifera is preferentially planted as a groundcover near walls or along the edges of a border. Therefore, powdery mildew seems to be inevitable. Our field observations suggest that the powdery mildew is a serious threat to the widespread ornamental plantings of cree** saxifrage in Korea, especially in cases where the plants are growing in the shade. Therefore, continuous attention should be paid to powdery mildews in gardens containing this plant. This information might help saxifrage growers and breeders to control the disease.