2021 Volume 62 Issue 6 Pages 390-394
The phylogeny and taxonomy of Podosphaera filipendulae (including P. filipendulensis, syn. nov.) have been examined. Asian, European and North American collections were examined and the nucleotides sequences of their partial rDNA region were determined. In particular, the relationship between P. filipendulae and P. spiraeae was analysed. The results confirmed P. filipendulae and P. spiraeae as two separate, morphologically similar species. The phylogenetic analysis revealed a similar phylogeny to that of the host genera. Although ITS sequences retrieved from Asian, European and North American specimens of P. filipendulae on various Filipendula spp. are identical to sequences from P. macularis on hop, there is consistently one base substitution at the 5'-end of 28S rRNA gene between the species. This result provides evidence that the hop powdery mildew and P. filipendulae are biologically and morphologically clearly distinguished, and should be maintained as two separate species.
The genus Filipendula comprises about 15 species of perennial herbs distributed in the temperate zone of the Northern Hemisphere (Schanzer, 1994). In the past, Filipendula was treated as a part of the genus Spiraea until its reintroduction as separate genus by Maximowicz (1879). Spiraea is a larger genus of about 80 to 100 deciduous shrub species distributed in the temperate Northern Hemisphere, with a few species in subtropical regions. The highest diversity of Spiraea occurs in Asia, most notably in China (Drábková, Pospíšková, & Businský, 2017). Numerous species are used as popular ornamental shrubs worldwide. Filipendula and Spiraea pertain to the family Rosaceae, but they are not closely allied. Filipendula is a part of the subfamily Rosoideae, but with an isolated position in phylogenetic multilocus analyses, i.e., it does not belong to any tribe (Potter et al., 2007). Spiraea is the type genus of tribe Spiraeeae, which belongs to the subfamily Amygdaloideae (previously Spiraeoideae, Potter et al., 2007; Drábková et al., 2017). Powdery mildews (Erysiphaceae) are common detrimental pathogens on numerous rosaceous host plants. Salmon (1900) assigned powdery mildew on Filipendula vulgaris (as Spiraea filipendula) to Sphaerotheca humuli, followed by Homma (1937), who included Japanese powdery mildews on Filipendula spp. and Spiraea spp. under Sph. humuli. Sawada (1951) described Sph. spiraeae on Spi. thunbergii from Japan. Based on Chinese type material on F. ulmaria, Zhao (1981) introduced the new species Sph. filipendulae. The two species on Filipendula and Spiraea spp. are barely distinguishable morphologically with regard to their asexual or sexual morphs. Therefore, Braun (1987) merged the two species. Later, Braun and Takamatsu (2000) reallocated Sph. spiraeae to Podosphaera. The first comprehensive phylogenetic analysis of powdery mildews on rosaceous hosts suggested that Podosphaera on Filipendula and on Spiraea, although barely distinguishable morphologically, belong to two different clades (Takamatsu, Niinomi, Harada, & Havrylenko, 2010). Based on these results, Braun and Liu (in Liu, 2010) proposed the combination P. filipendulae. A new powdery mildew species, P. filipendulensis, has recently been described from India on F. vestita (Yadav, Verma, & Singh, 2021). The authors compared results of sequence analyses with sequences from P. ferruginea, P. macularis and P. spiraeae, but unfortunately without inclusion of sequences retrieved from P. filipendulae. To clarify the phylogeny, taxonomy, distribution and host range of P. filipendulae, a new phylogenetic examination, with a large sampling effort including collections and sequences from Asia, Europe and North America, has been performed.
The nucleotide sequences of the 5'-end of the 28S rRNA gene (including domains D1 and D2) and internal transcribed spacer (ITS) regions including the 5.8S rRNA gene were determined in this study. Extractions were accomplished by the Chelex method (Walsh, Metzger, & Higuchi, 1991; Hirata & Takamatsu, 1996). In the United States the primers PM10 (Bradshaw & Tobin, 2020) and ITS4 (White, Bruns, Lee, & Taylor, 1990) were used. One sample from Canada (DAOM 984908) was amplified with P7 (Mori, Sato, & Takamatsu, 2000) and PMITS2 (Cunnington, Takamatsu, Lawrie, & Pascoe, 2003). New sequences from this study were deposited in GenBank (Supplementary Table S1). These sequences were aligned with other sequences of the Erysiphaceae using MUSCLE (Edgar, 2004) implemented in MEGA 7 (Kumar, Stecher, & Tamura, 2016). Alignments were further manually refined using the MEGA 7 program and were deposited in TreeBASE (http://www.treebase.org/) under the accession number S28364. Phylogenetic trees were constructed using maximum parsimony (MP) and maximum likelihood (ML) methods as described in Maeda et al. (2021).
The present phylogenetic analyses (Fig. 1; Supplementary Fig. S1), confirmed that P. filipendulae on Filipendula spp. and P. spiraeae on Aruncus spp. and Spiraea spp. pertain to separate highly supported clades. The analyses confirm that they are separate, morphologically similar species, i.e., the asexual and sexual morphs of two taxa are barely distinguishable. These results are in line with the phylogenetic differentiation of the host genera; Filipendula belongs to the family Rosaceae subfamily Rosoideae, and Aruncus and Spiraea belong to subfamily Amygdaloideae [previously Spiraeoideae] (Potter et al., 2007; Drábková et al., 2017). Sequences of P. spiraeae from Argentina, China, Japan and the USA cluster together and form a well-supported clade distant from sequences obtained from P. filipendulae. A single sequence (AF154325, ex VPRI20247, Korea, asexual morph on Spiraea salicifolia; see Cunnington et al., 2003) was identified as“Sph. spiraeae”. This was undoubtedly misidentified, as the specimen only contained the asexual morph, and it clustered with a Japanese sequence of P. minor. Podosphaera minor pertains to Podosphaera sect. Podosphaera (chasmothecial appendages stiff, setiform, apically dichotomously branched), whereas P. spiraeae belongs to sect. Sphaerotheca (chasmothecial appendages mycelioid, not dichotomously branched). The taxonomic affiliation of powdery mildew on Aruncus spp. was previously not clear. Based solely on morphological similarities, Braun (1987) and Nomura (1997) assigned Sphaerotheca collections on Aruncus to Sph. ferruginea. However, phylogenetic studies of Takamatsu et al. (2010) suggested a close relationship to P. spiraeae, which aligns with the phylogenetic affinity of the genus Aruncus (Arunucs is in the tribe Spiraeeae). The present analyses, including new sequences obtained from Podosphaera collections on Aruncus spp., confirmed the inclusion of this powdery mildew in P. spiraeae. Podosphaera ferruginea on Sanguisorba spp. is distantly allied and clusters close to sequences from P. filipendulae (Fig. 1).
All sequences obtained from Asian, European and North American specimens on various Filipendula spp. are identical and cluster distantly from P. spiraeae. An ex-type rDNA ITS sequence of the recently described P. filipendulensis (Yadav et al., 2021), from India on F. vestita, is 100% identical with all other P. filipendulae sequences, including all sequences from China, the country from which this species was described. Filipendula vestita, the type host of P. filipendulensis, was cited as a host of P. filipendulae in Braun and Cook (2012), which refers to a record from Pakistan. The asexual morph of P. filipendulensis was not described by Yadav et al. (2021) and is, therefore, not comparable with the anamorph of P. filipendulae. All postulated morphological differences between the latter species and P. filipendulensis listed in Yadav et al. (2021: 138, tab. 1) are overlapping, i.e., the corresponding characteristic of P. filipendulensis fall within the variation of these traits in P. filipendulae. The most characteristic features of P. filipendulae, viz., rather long chasmothecial appendages, straight to curved, mycelioid, but not distinctly geniculate-sinuous, at least to some extent arising from the upper half of the fruiting body, and pigmented, with thickened walls towards the base, are also evident in P. filipendulensis. However, the main issue with the introduction of the later species name lies in that sequences of P. filipendulae were not compared with the ex-holotype sequence of P. filipendulensis. On the basis of 100% identical sequences between P. filipendulensis and P. filipendulae, overlapping morphological traits of the sexual morphs, and previous records of the latter species on F. vestita, it is evident that the two taxa are conspecific.
Collections of P. filipendulae on various Filipendula spp. from Asia, Europe and North America agree morphologically and are characterised by having identical rDNA ITS sequence data, suggesting that a single species is involved with a wider host range and geographical distribution. Podosphaera filipendulae sequences are nearly identical to those obtained from hop (P. macularis) providing evidence of a close affinity of the two species and the involvement of two phylogenetically young species. There are various other examples of closely allied species that are indistinguishable solely based on ITS data, such as the Erysiphe aquilegiae compex (Takamatsu, Ito, Shiroya, Kiss, & Heluta, 2015; Bradshaw et al., 2020) and the E. alphitoides clade (Takamatsu et al., 2015). The common hop powdery mildew, P. macularis, is biologically distinct from P. aphanis and P. filipendulae, as previously established by Salmon (1907), who showed in inoculation experiments, using ascospores and conidia of the hop powdery mildew, that cross infections of Potentilla reptans and F. ulmaria were not possible. Furthermore, the two species are morphologically clearly distinguished (chasmothecial appendages usually in the lower half of the fruiting body, thin-walled, pluriseptate, yellowish to brown, asci (2-)4-8-spored in P. macularis, vs. appendages often arising from the upper half of the fruiting body, moderately thick-walled, above all in the lower half, aseptate to sparingly septate, dark brown when mature, asci 6-8-spored in P. filipendulae; Braun & Cook, 2012). A detailed study of the genetic population diversity of the hop powdery mildew based on North American and European samples has recently been published by Gent et al. (2020). Furthermore, this study revealed one base substitution at the #523 nucleotide from the 5'-end of the 28S rRNA gene between the two species, viz., thymine in P. filipendulae vs cytocine in P. macularis. Owing to this result and the clear biological and morphological differences between the hop powdery mildew and the fungus on Filipendula spp., it is justified to maintain P. macularis and P. filipendulae as two different species.
In conclusion, the new phylogenetic analyses support two different species: P. filipendulae on Filipendula spp., which is widespread in the northern hemisphere, and P. spiraeae on Aruncus and Spiraea spp., an Asian species introduced in Europe, North America and South America:
≡ Sphaerotheca filipendulae Z.Y. Zhao, Acta Microbiol. Sin. 21: 439, 1981, nom. cons. (Braun 2013: 1331). Holotype: China, Xinjiang Uygur Autonomous Region, Ili Kazakh Autonomous Prefecture, Habahe County, on Filipendula ulmaria, 28 Aug. 1975, Z.Y. Zhao (HMAS 39000).
= Torula botryoides Corda, in Sturm, Deutschl. Fl., III. Abt. Die Pilze Deutschlands, 8. Heft: 77 + T. 35, Nürnberg 1829. Lectotype (designated here, MycoBank, MBT10001615): Corda (1829: Plate 35), original illustration. Epitype (designated here, MycoBank, MBT10001616): Germany, Sachsen, Landkreis Görlitz, Herrnhut, OT Ruppersdorf, on Filipendula ulmaria 21 May 2011, H. Boyle (GLM-F-96874), ex epitype sequence = MZ242069.
≡ Oidium botryoides (Corda) Ces., in Rabenh., Klotzschii Herb. Viv. Mycol., Cent. XVII, no. 1671, Dresden 1852.
= Podosphaera filipendulensis Sa. Yadav, Sanj. K. Verma & Ragv. Singh, Phytotaxa 491: 137, 2021. Holotype: India, Uttarakhand, Chamoli, Valley of Flowers National Park, on Filipendula vestita, Sep. 2017, S. Yadav (AMH 9934).
Description: Braun and Cook (2012: 140).
Illustrations (selection): Nomura (1997: 37, fig. 37), Shin (2000: 249, fig. 86), Liu (2010: 206, Fig. 100). Braun and Cook (2012: 140, fig. 85).
Host range and distribution: On Filipendula angustiloba (Asia, China), F. auriculata (Asia, Japan), F. intermedia (China, Russia [Far East]), F. camtschatica (Japan, Russia [Far East]), F. multijuga (Japan), F. palmata [including F. nuda = F. palmata var. glabra] (Asia, China, Japan, Russia [Far East]), F. purpurea (Asia, Japan), F. rubra (North America, USA [Indiana, New York, Vermont], Canada [Ontario]), F. ulmaria [including subsp. ulmaria, subsp. denudata and subsp. pentapetala] (Asia, Kazakstan; Europe, Austria, Bulgaria, Czech Republic, Denmark, Estonia, Faroe Islands, Finland, France, Germany, Hungary, Italy, Latvia, Lichtenstein, Lithuania, Netherlands, Norway, Poland, Russia, Serbia, Slovakia, Spain, Sweden, Switzerland, UK, Ukraine; North America, Canada [Ontario], USA [Vermont]), F. vestita (India, Pakistan), F. vulgaris (Caucasus, Armenia; Europe, Denmark, Estonia, Germany, Hungary, Lithuania, Portugal, Romania, Slovakia, Spain, Sweden, Switzerland), Rosaceae [Rosoideae].
Notes: Type material of Torula botryoides is not preserved (see Pilát 1938: 145). Therefore, the original illustration of this species is designated as lectotype, and is supplemented by an epitype with an ex-epitype sequence. The type of the recently described Podosphaera filipendulensis has not been re-examined.
≡ Sphaerotheca spiraeae Sawada, Bull. Gov. Forest Exp. Sta. Meguro 50: 104, 1951. Lectotype (designated by Braun 1987): Japan, Iwate Pref., Morioka, on Spiraea thunbergii, 27 Jun. 1897, Takahashi (TNS-F-214412). Syntype: IUM. Epitype (designated here, MycoBank, MBT10001617): Japan, Toyama Pref., Kami-ichi, on Spiraea thunbergii, 11 Jun. 1995, Y. Sato (TNS-F-87186 [ex TSU-MUMH s66]); ex epitype sequence - AB026153.
Description: Braun and Cook (2012: 161).
Illustrations (selection): Nomura (1997: 41, fig. 41, 42), Liu (2010: 216, Fig. 106). Braun and Cook (2012: 162, fig. 120).
Host range and distribution (Amano, 1986; Braun & Cook, 2012; Lee, Kim, Hong, & Lee, 2009; Takamatsu et al., 2010; Talgø et al., 2011; Moparthi, Grove, & Bradshaw, 2018; Chater & Woods, 2019): On Aruncus sylvester [= A. asiaticus, A. dioicus var. kamtschaticus, A. sylvester var. vulgaris] (Asia, China, Japan, Korea, Russia [Far East]), Spiraea betulifolia (Asia, Russia [Far East]; Europe, Norway), Spiraea × bumalda (Europe, Slovakia), Spi. cantoniensis (Asia, Japan; South America, Argentina), Spi. japonica (Asia, China, Japan; Europe, UK [Wales]; North America, USA, Washington), Spi. media (Asia, Kazakhstan, Russia [Far East, Siberia]), Spi. palmata [= Spi. digitata] (Asia, Russia [Far East, Siberia]), Spi. prunifolia (Asia, Japan; ?North America, USA), Spi. sericea (Asia, Russia [Far East]), Spi. salicifolia (Asia, Korea, Russia [Far East, Siberia]), Spi. thunbergii (Asia, Japan).
Notes: Podosphaera spiraeae is an Asian species, introduced in Europe, North and South America on cultivated Spiraea spp. Talgø et al. (2011) reported this species from Norway on Spi. betulifolia with chasmothecia. Other reports, above all those based on anamorphic collections, have to be classified as uncertain, i.e., they are in need of confirmation by sequencing. The record of “Podosphaera cf. clandestina” on Spiraea cantoniensis from Argentina (Delhey, Braun, & Kiehr, 2003), based on an asexual morph, most likely belongs to P. spiraeae, which is a phylogenetically proven host of this species in Argentina (Takamatsu et al., 2010). All collections of P. spiraeae on Aruncus refer to A. sylvester (Asian goat's-beard), which has often been synonymous with A. dioicus as subspecific taxon, but we follow the taxonomy in Flora of China, which considers this taxon a species of its own (http://www.efloras.org/florataxon.aspx?flora_id=2&taxon_id=200010639). There are additional doubtful reports of “Sphaerotheca” on Aruncus and Spiraea, such as Sph. macularis on Aruncus sp. (Germany; Amano, 1986), P. leucotricha, asexual morph on Spi. × bumalda in Argentina, Patagonia (Havrylenko & Takamatsu, 2005), Sph. fuliginea on A. sylvester in California (Gardner, Yarwood, & Raabe, 1970), and various unproven records on Spiraea spp. from Europe (Czech Republic, Denmark, Russia; Amano, 1986; Braun, 1995).
The authors declare no conflicts of interest. All the experiments undertaken in this study comply with the current laws of the countries where they were performed.
This work was financially supported in part by the National Natural Science Foundation of China (31970019).
