First Report of Downy Mildew Caused by Peronospora belbahrii on Basil (Ocimum spp.) in Ontario

Basil (Ocimum spp.) is one of the most commercially significant fresh culinary herb crops worldwide. In Ontario, basil is grown both in the field and in the greenhouse. In the summer of 2011, basil plants grown in a research field at the Simcoe Research Station in Norfolk County, Ontario, Canada (44°15'N, 77°35'W), were infected with downy mildew. Infected leaves exhibited interveinal chlorotic lesions on the upper surface and clear to black sporulation on the abaxial leaf surfaces. Leaf senescence and defoliation occurred at high disease severity, which reduced marketable yield. Basil downy mildew symptoms were severe on leaves of cultivars Genovese and Sweet Basil, with 40 to 100% disease incidence. Based on morphological characteristics, the basil downy mildew causal agent was identified as Peronospora belbahrii Thines (4). Infected leaves were collected and microscopic observations of the sporulating lesions were carried out and the structures measured. Sporangiophores (n = 20) were hyaline with relatively long, straight trunks and were monopodially branched, with a length of 150 to 360 μm (average 285 μm). Sporangiophores ended with two slightly curved branchlets, the longer one measuring 15 to 27 μm (average 19 μm) and the shorter one 5 to 15 μm (average 9 μm). Sporangia (n = 50) were round, or slightly ovoid, olive to brown in color, and measured 29 × 25 μm (25 to 35 × 20 to 30 μm). Genomic DNA was extracted from 10 isolates and the nuclear ribosomal internal transcribed spacer (ITS) region was amplified with ITS4 and ITS5 primers and sequenced. The sequences of the 10 isolates were nearly identical. A BLAST search of the NCBI database with the ITS sequences (GenBank Accession No. KC756923) revealed a 98 to 100% similarity to the sequences of P. belbahrii (HQ730979, FJ436024, and HQ702191) isolated from sweet basil in Florida (3), California (1), and Hungary (2), respectively. To confirm pathogenicity, 5-week-old 'Genovese' seedlings were sprayed with a suspension of 1 × 10⁵ sporangia/ml. Plants were kept in a growth chamber maintained at 23/18°C, 60 to 85% relative humidity, and 12/12 h light/dark. Non-inoculated plants served as controls. Basil downy mildew symptoms developed after 8 days on the inoculated plants and the pathogen was identified in association with symptoms consistent with downy mildew. The non-inoculated controls remained healthy. In North America, the occurrence of basil downy mildew has been reported since 2007 (3) and the disease has spread into several U.S. states. To our knowledge, this is the first report of downy mildew on sweet basil in Canada. References: (1) C. L. Blomquist et al. Plant Dis. 93:968, 2009. (2) G. Nagy and A. Horvath. Plant Dis. 95:1034, 2011. (3) P. D. Roberts et al. Plant Dis. 98:199, 2009. (4) M. Thines et al. Mycol. Res. 113:532, 2009.