First Report of Basil Downy Mildew Caused by Peronospora belbahrii in the Czech Republic

Sweet basil (Ocimum basilicum L.) is an annual aromatic and medicinal plant in the Lamiaceae that is originally native to India but is grown in warm regions all over the world. It is a popular culinary herb used fresh and dried, and is used in traditional folk medicine. In the Czech Republic, sweet basil is grown commercially in South Moravia or by home gardeners as a potted plant. In 2012, severe downy mildew was observed in a field of basil plants (cv. Dark Green) at the Crop Research Institute (CRI) in Olomouc, Czech Republic. Infected leaves each exhibited large, interveinal, chlorotic lesions, and violet-gray, fuzzy growth on the lower leaf surface. Within a few days, lesions turned necrotic and severely infected leaves dropped prematurely. Microscopic observations revealed hyaline conidiophores typical of Peronospora Corda, emerging from stomata. Conidiophores (n = 100) were usually 239.9 to 296.5 × 8.7 to 10.6 μm, straight, and were branched 4 or 5 times submonopodially at the upper ends. Ultimate branchlets (n = 100) were slightly curved and obtuse, with the longer branchlets usually 17.8 to 22.7 μm and the shorter branchlets 10.0 to 12.9 μm, and each bearing a single conidium. Conidia (n = 100) were olive-brown, mostly ellipsoidal to subglobose, and typically 29.0 to 31.0 × 23.2 to 25.4 μm, with a length/width ratio of 1.2 to 1.3. Oospores were not observed. Based on these morphological characteristics, the pathogen was identified as Peronospora belbahrii Thines (5). The specimen was deposited in a local herbarium at the CRI in Olomouc, as voucher PB-1. Genomic DNA was extracted from conidia, and the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) amplified with primers DC-6 (1) and LR-0 (4). A sequence was deposited in the NCBI database (GenBank Accession No. KJ960193). A BLAST search of the NCBI database revealed 99% identity to the deposited ITS sequences of P. belbahrii from basil and other host species (EU863410, FJ394334-7, GQ390794, GQ390795, HM462241, HM462242, HM486901, HQ702191, HQ730979, KC756923, KF419289, and KF419290). P. belbahrii was first described by Thines et al. (5) as a pathogen of sweet basil and coleus (Solenostemon scutellarioides), but can also infect Agastache spp. (2). There are many reports indicating the pathogen is spreading throughout the world (5). In Europe, chronologically, basil downy mildew has been reported from Italy, France, Switzerland, Germany, Hungary, and Cyprus (2,3,5). To our knowledge, this is the first report of natural occurrence of downy mildew on sweet basil in the Czech Republic. References: (1) D. E. L. Cooke et al. Fung. Genet. Biol. 30:17, 2000. (2) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, USDA ARS. Retrieved from http://nt.ars-grin.gov/fungaldatabases/ , 16 June 2014. (3) A. Garibaldi et al. Plant Dis. 89:683, 2005. (4) O. Spring et al. Eur. J. Plant Pathol. 114:309, 2006. (5) M. Thines et al. Mycol. Res. 113:532, 2009.