First report of Podosphaera macrospora on Heuchera in the United Kingdom

Heuchera are important ornamental herbaceous plants providing year-round coloured foliage. During January 2015 Heuchera cultivars ‘Caramel’, ‘Galaxy’, ‘Marmalade’ and ‘Obsidian’ at RHS Garden Wisley, Surrey, were observed with symptoms associated with powdery mildew. Infected plants generally exhibited symptoms on less than half the leaves of the plant. In extreme cases symptoms were observed on all leaves. Empirical evidence suggested that introduction of a symptomatic Heuchera ‘Marmalade’ plant to a glasshouse with clean tissue culture plants in May 2015 resulted in all plants exhibiting symptoms within 12 weeks.

Braun & Kummer in Ale-Agha et al. (2008) found Podosphaera alpina f. alpina and P. alpina f. macrospora to be distinct morphologically and raised f. macrospora to species level as P. macrospora. P. alpina f. macrospora was described initially from North America on Saxifraga (Braun, 1985) and was reported subsequently from North America on Heuchera, Tellima, Tiarella and Tolmiea (Braun, 1987) and in Europe on Tellima, Tiarella and Tolmeia (Braun & Cook, 2012). Braun & Kummer in Ale-Agha et al. (2008) reported that P. alpina occurs on two species of Saxifraga in Europe. Bolay (2005) recorded P. alpina on Heuchera sanguinea in Switzerland but did not distinguish between P. alpina f. alpina and f. macrospora although this material was subsequently determined by Braun & Kummer in Ale-Agha et al. (2008) to be P. macrospora. To our knowledge there are no records of powdery mildew on Heuchera in the UK.

Abundant amphigenous greyish white, later yellow-brown, mycelia were observed on leaf surfaces (Figs. 1, 2) with branched, hyaline hyphae (n=8) 8-13 µm wide (mean 9 µm). Appressoria were indistinct. Conidiophores (n=178) were 191-289 x 10-13 µm (mean 229 x 11 µm) with straight foot cells (n=47) of 71-144 x 10-13 µm (mean 98 x 11 µm) (Fig. 3). Conidia (n=87) were catenescent, and broadly obovoid-ellipsoid with rounded apices and almost truncated bases, 33-45 x 15-22 µm (mean 39 x 19 µm), length/width ratio 1.7-2.7 (mean 2.1). Chasmothecia (n=20) were initially cream-yellow and later dark brown measuring 84-122 µm (mean 106 µm) with irregularly shaped peridial cells 20-56 µm and a thick wall 3-5 µm (Fig. 3). Appendages (n=35) were unbranched, mycelioid, measuring 156-742 µm (mean 387 µm) and brown though paler towards the apices. The single ascus (n=25) per chasmothecium was round to obovoid-saccate, 90 x 73 µm and 6-8-spored with walls (n=25) 1.3-2.8 µm thick (mean 1.9 µm). Ascospores (n=46) were broadly ellipsoid-ovoid, 16-39 x 13-23 µm (mean 28 x 18 µm), and colourless.

No sequences of the ITS region of P. macrospora have been deposited previously in GenBank. The ITS region of three samples of P. macrospora from Heuchera was analysed and deposited in GenBank (Accession Nos. KP966080-KP966082). These were augmented by sequencing of five further samples of P. macrospora on host Tellima grandiflora from Kew Fungarium (Kew Accession Nos. 171016, 171193, 171927, 187918 & 188271) (KX032527-KX032536). Sequences matched via the GenBank BLAST.

Ascospore lengths >20 µm, asci wall thickness <3 µm, progression of mycelial colour from white to yellowish-brown and ITS DNA identify the causal fungus as P. macrospora. This is the first record of powdery mildew on Heuchera in the UK. Heuchera are widely grown in nurseries throughout Europe and trade may have facilitated movement of P. macrospora and thus may threaten other hosts. The annual value of UK production of herbaceous perennials, including Heuchera, is estimated at £97m (Denny, 2014) and prompt recognition and treatment of disease helps to sustain this market.