New report of a powdery mildew on Wisteria in the UK

Since 1999, the advisory service at the Royal Horticultural Society has received samples of Wisteria spp. (family Papilionaceae) showing dark necrotic spots and marks with yellowish background on both surfaces of the leaves (Fig.1). Microscopic examination of the spots consistently showed typical mycelium of a powdery mildew. Erysiphe trifolii has been recorded on W. floribunda in Scotland (1979) and New Zealand (1986) (Farr et al., 2006). Also, Hirata (1966) listed Oidium spp. on Wisteria spp. in Australia and USA and an Erysiphe communis (= E. section Erysiphe sensu) in Yugoslavia. Usually sporulation was too sparse in English material for species identification. However in 2006, more abundant sporulation mainly on the upper leaf surfaces allowed a morphological and DNA description of powdery mildew infecting two specimens of W. sinensis located in different locations in Surrey.

The conidia were cylindrical, lacking fibrosin bodies, length 27 - 37 µm (mean 33.1 µm), width 10 – 15.0 µm (mean 13.3 µm). The wrinkling pattern on the conidia was angular/rectangular. Conidiophores were erect consisting of a foot-cell measuring 11 - 23 µm (mean 18.1 µm) x 5 - 7 μm (mean 6.0 μm) which was predominantly flexuous sometimes straight and followed by 1-2 distal cells (Fig. 2). Conidia were formed singly. Appressoria on the mycelium were lobed. No chasmothecia were present but the above characteristics are fully consistent with Oidium subgenus Pseudoidium, the anamorph of Erysiphe.

The ITS region of the two isolates was amplified using the primers PMITS1 and PMITS2, as described by Denton and Henricot (2006) and cloned into pGEM®-T Easy vector as described in the manufacturer’s instructions (Promega). The cloned fragments were sequenced using universal primers M13 reverse and T7. One isolate (GenBank Acc. No EF210375) had 5 ambiguous bp which could not be resolved by sequencing of different clones or by direct sequencing. This was probably due the variation in the sequences of the ITS copies. The other isolate had identical ITS sequences (GenBank Acc. No EF183498) with isolates of E. alphitoides, Oidium mangiferae, an anamorph of E. alphitoides, and E. hypophylla, possibly inseparable from E. alphitoides, as well as E. wallrothii, originally described in Japan as Microsphaera alni, an older name for E. alphitoides inter alia (Braun, 1987). A description of the foot cells is not available for E. wallrothii, but the distinctive foot cells of our Wisteria isolates were reflected in the other species including E. trifolii (Braun, 1987). However, the DNA analysis rules out E. trifolii. It also rules out Hirata’s E. communis since our isolates group with E. sect. Microsphaera. Thus the evidence now points strongly to E. alphitoides as the causal agent of Wisteria powdery mildew.