First record of downy mildew, caused by Peronospora belbahrii, on Solenostemon scutellarioides in the UK

Solenostemon scutellarioides (Lamiaceae), commonly known as coleus, is a popular ornamental houseplant and summer bedding plant in the UK. In 2014, several S. scutellarioides cv. 'Chocolate Mint' plants were received by the Royal Horticultural Society (RHS) advisory service with brown necrotic blotches visible on the leaves that became dry and crisp over a period of two to five days (Fig. 1). 

Symptom-bearing leaves were clarified with an acetone/industrial methylated spirits solution (1:1 v/v) followed by staining with lacto-fuchsin and methyl blue (Koroch et al., 2013). Mycelium and conidiophores, typical of a downy mildew, were observed on the abaxial surface arising directly out through stomata. Conidia were abundantly attached to the conidiophores when infected leaves were placed under humid conditions. The conidiophores were hyaline, with a size range 331-680 µm x 10-17 µm (mean 480 x 13 µm) with (sub)monopodial branching (Fig. 2). The conidiophores branched 3-5 times before terminating in slender branchlets, set obtusely, of uneven length, longest 11–25 µm (mean 15 µm), shortest 5-10 µm (mean 8 µm) (Fig. 2). Conidia were light brown, ellipsoid and non-papilate ranging 20-25 µm x 17-20 µm (mean 24 x 18 µm) (Fig. 2); measuring slightly smaller than those described by Thines et al. (2009) but fitting with the description by Daughtrey et al. (2006). Oospores were not observed. When infected plants were kept in dry, warm conditions no necrosis was observed although mycelium and conidiophores were present on the lower leaf surface (Fig. 3). 

The ITS region was analysed using the semi-nested PCR as described by Cooke et al. (2000), using primers DC6/ITS4 followed by ITS6/ITS4 and deposited in GenBank (Accession No. KP164987). The sequence had 99-100% identity with Peronospora belbahrii from Solenostemon sp. (Accession Nos. FJ394333, FJ394334, FJ394337). The molecular and morphological results in this study fit with the description of P. belbahrii (Thines et al., 2009), previously recorded as causing downy mildew of S. scuttellarioides. To our knowledge this is the first record of P. belbahrii infection on Solenostemon spp. in the UK. Peronospora belbahrii is recorded infecting Solenostemon spp. in the USA and Germany (Farr & Rossman, 2014). In the UK, P. belbahrii has been confirmed on basil and Agastache sp. (Webb et al., 2012), both of which are propagated by seed, thereby limiting the risk of downy mildew spread and infection. Solenostemon spp. are predominantly propagated via cuttings and the detection of P. belbahrii surviving and producing conidiophores on asymptomatic tissue potentially increases the risk of rapid spread through the plant trade undetected.