New Disease Reports
New Disease Reports (2013) 28, 15. [http://dx.doi.org/10.5197/j.2044-0588.2013.028.015]
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First report of powdery mildew on Sorbaria

G.J. Denton 1*, J.O. Denton 1 and R.T.A. Cook 2

*geoffdenton@rhs.org.uk

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Received: 27 Jan 2013; Published: 19 Nov 2013

During 2006, leaves of Sorbaria sorbifolia (Rosaceae) with chlorotic patches on the adaxial surface were observed (Fig. 1) at RHS Garden Wisley, UK. Epiphyllous superficial mycelium typical of powdery mildew was initially observed with a few conidia but lacking conidiophores or chasmothecia. However, in recent years more conidia and conidiophores were seen allowing morphological and molecular analysis. Neither Sorbaria sorbifolia (syn. P. spiraea sorbifolia, Schizonotus sorbifolius) nor the genera Sorbaria and Schizonotus have previous records of powdery mildew infection attributed to them (Braun & Cook, 2012). Spiraea spp. have several Podosphaera spp. recorded including P. spiraeae belonging to Podosphaeria sect. Sphaerotheca and a probable Pseudoidium recorded as Oidium spiraeae (Braun & Cook, 2012).

Mycelia were sparse, often lacking conidiophores. Hyphae were branched, septate, nodose, 3–5 µm wide and hyaline. Appressoria were highly lobed, solitary or in opposite pairs, 6–9 µm diameter (Fig. 2). Conidiophores, 38–91 x 5–10 µm (mean 74 x 7 µm) with foot cells curved or flexuous, 19–55 x 5–9.5 µm (mean 39.5 x 6.8 µm) (Fig. 3). Conidia were formed singly, cylindrical or ellipsoid-cylindrical, 29–46 x 11–17 µm (mean 38 x 13.5 µm), length:breadth ratio 2.1-3.6. Conidial germination was terminal and ending in a moderately lobed appressorium; germ tube length 0–3.5 times the width of the conidium. Collapsed conidia had longitudinal, angular/rectangular wrinkling pattern (Fig. 4). Chasmothecia were extremely scarce and those found were immature, lacking both asci and full appendage development (Fig. 5). However, initials of dichotomous branching were apparent on some of the appendages.

Morphological features are consistent with the genus Pseudoidium (anamorph of Erysiphe). The initials of dichotomous branching on chasmothecial appendages suggest Erysiphe sect. Microsphaera. The ITS region was analysed as by Cunnington et al. (2004) and deposited in GenBank (Accession No. KC489094). The sequence matched 100% with Erysiphe alphitoides even though the morphology (especially the relatively long, narrow cylindrical conidia) does not fully match the description of this species according to Takamatsu et al. (2007) and Braun & Cook (2012). Until this discrepancy is resolved, the pathogen is best labelled E. alphitoides sensu lato. To our knowledge this is the first UK or worldwide record of powdery mildew on Sorbaria.

Figure1+
Figure 1: Sorbaria sorbifolia leaves with the chlorotic patches caused by powdery mildew.
Figure 1: Sorbaria sorbifolia leaves with the chlorotic patches caused by powdery mildew.
Figure2+
Figure 2: Lobed appressoria of Erysiphe sp. on Sorbaria. Bar = 10 µm
Figure 2: Lobed appressoria of Erysiphe sp. on Sorbaria. Bar = 10 µm
Figure3+
Figure 3: Distinctive flexuous conidiophores of Erysiphe sp. on Sorbaria showing foot cell followed by one (14–44 x 6.5–9 µm) or two (11–27 x 6.5–10 µm) shorter cells. Bar = 10 µm
Figure 3: Distinctive flexuous conidiophores of Erysiphe sp. on Sorbaria showing foot cell followed by one (14–44 x 6.5–9 µm) or two (11–27 x 6.5–10 µm) shorter cells. Bar = 10 µm
Figure4+
Figure 4: Fresh conidia, germinated conidia and wrinkling pattern observed on conidia of Erysiphe sp. on Sorbaria. Bar = 10 µm
Figure 4: Fresh conidia, germinated conidia and wrinkling pattern observed on conidia of Erysiphe sp. on Sorbaria. Bar = 10 µm
Figure5+
Figure 5: Immature chasmothecia of Erysiphe sp. observed on Sorbaria. Bar = 20 µm
Figure 5: Immature chasmothecia of Erysiphe sp. observed on Sorbaria. Bar = 20 µm

References

  1. Braun U, Cook RTA. 2012. Taxonomic manual of the Erysiphales (powdery mildews). CBS Biodiversity Series 11 1-707.
  2. Cunnington JH, Lawrie AC, Pascoe IG, 2004. Unexpected ribosomal DNA internal transcribed spacer sequence variation within Erysiphe aquilegiae sensu lato. Fungal Diversity 16, 1-10.
  3. Takamatsu S, Braun U, Limkaisang S, Kom-Un S, Sato Y, Cunnington JH, 2007. Phylogeny and taxonomy of the oak powdery mildew Erysiphe alphitoides sensu lato. Mycological Research 111, 809-826. [http://dx.doi.org/10.1016/j.mycres.2007.05.013]
To cite this report: Denton GJ, Denton JO, Cook RTA, 2013. First report of powdery mildew on Sorbaria. New Disease Reports 28, 15. [http://dx.doi.org/10.5197/j.2044-0588.2013.028.015]

©2013 The Authors