First report of downy mildew caused by Peronospora verbenae on verbena in the Czech Republic

Verbena (Verbena officinalis)is a perennial herb native to Europe, which is widely grown for traditional medicine and tea or for ornamental purposes. In August 2009, plants showing typical symptoms of downy mildew disease were collected in Brno, Czech Republic. In September 2009, plants with symptoms were found also in Olomouc. Infected leaves turned yellow or pale green, with a greyish to brownish fungal-like downy growth developing on the abaxial leaf surface (Figs. 1A, B). Detailed microscopic examination of a representative sample (KUS-F24423) was made to identify the pathogen. Conidiophores were subhyaline, tree-like, straight, 250−400 x 7−12 µm, monopodially branched in 5−7 orders, and emergent from stomata (Fig. 2A). Ultimate branchlets were substraight to slightly curved, 5−12 µm long, 2−3 µm wide, and had obtuse or subacute tips (Fig. 2B, C). Conidia were greyish brown to violet, ellipsoidal, with size in the range 25−38 x 17−20 µm, and a length/width ratio of 1.3−2.0 (Figs. 2 D, E). These data are in full agreement with those given for Peronospora verbenae (Braun et al., 2009).

The amplification and sequencing of the ITS rDNA were performed with primers DC6 and ITS4 (Cooke et al.,2000), with the sequence of the region deposited in GenBank (Accession No. GQ919014). Comparison with the sequences available in the GenBank database revealed that the ITS sequenceshares a 94 % similarity with Peronosporaflava. Since this was the first sequence submitted for P.verbenae, no comparable data were available. Downy mildew disease on verbena was first recorded under Plasmopara halstedii from New Mexico, USA (Anonymous 1960, cited in Farr & Rossman, 2009). Plasmopara halstedii, however, is reported only on Helianthus or some related Asteraceae (Leppik, 1966). In Europe, a downy mildew on Verbena spp. and identified as Peronospora verbenae was recently recorded from Germany (Braun et al.,2009). To our knowledge, this is the second report of the disease in Europe and the first in the Czech Republic. Therefore, the present record suggests the possibility that this pathogen is spreading throughout Europe. Since verbena is cultivated on a commercial scale in many countries, the downy mildew poses a serious threat to production of this medicinal herb.