First report of powdery mildew on Deutzia spp. in the UK

Deutzia is a genus of about 60 species of shrubs in the Hydrangeaceae, native to eastern Asia and Central America, and widely planted as an ornamental garden plant. During April 2006, in the Royal Horticultural Society Gardens, Deutzia crenata and D. scabra ‘Candidissima’ were recorded as suffering from powdery mildew. Leaves exhibited yellowing and chlorotic circular patches, later merging to form irregular patterns, followed by total leaf cover and senescence (Fig. 1).

White superficial colonies, with abundant sporulation, developed amphigenously, on leaves (mostly adaxial surface) and non-lignified parts of the stem. Hyphae were 5.4-6.4 µm wide, septate, branched and smooth. Appressoria were lobed, singular or opposite in pairs (Fig. 2). Conidiophores were produced from external mycelium, unbranched, mostly erect, cylindrical, 61.1-98.5 x 6.8-8.6 µm, consisting of a foot cell, 28.6-38.3 x 7.5-8.4 µm, with a basal septum near the branching point, followed by two or three straight cells (Fig. 3). The measurements of the foot cell were shorter than mentioned by Bolay et al. (2005) at 40-70 x 7-10 µm, but fit well with the illustrations given by the same authors using the bar scale. Conidia developed singly and were dolliform, ellipsoid-cylindrical, 26.2-35.7 x 12.4-15.9 µm (Fig. 4), aseptate, mostly having oil drops and no fibrosin bodies seen.

Chasmothecia were absent but based on the morphological characteristics of the anamorph the organism conforms to Oidium subgenera Pseudoidium (Cook et al., 1997). Previous records of powdery mildew on Deutzia (Bolay et al., 2005) indicate it is most likely Erysiphe deutziae (syn. Microsphaera deutziae) (Bunkia, 1974).

The ITS region was amplified, using PMITS1 and PMITS2, as described by Cunnington et al. (2003), except that no Tween 20 was used and the annealing temperature was 60°C. The amplified fragment was gel purified and directly sequenced (GenBank DQ861917 and DQ861918). With no sequences of E. deutziae available in public databases, results of the sequence search came back as unique.

E. deutziae has been recently introduced from Asia to mainland Europe (Bolay et al., 2005). This is the first report of powdery mildew on Deutzia spp. in the UK. Kew Gardens Herbarium hold deposited infected plant material samples (D. crenata, K(M)140025; D. scabra ‘Candidissima’, K(M)140024).