First Report of Downy Mildew Caused by a Peronospora Species on Sweet Basil (Ocimum basilicum) in Massachusetts

In September of 2008, downy mildew was discovered to be causing a serious foliar blight of sweet basil at several farms and greenhouses in Massachusetts. Infected leaves had chlorotic vein-bounded patches and diffuse chlorosis, and a characteristic gray, fuzzy growth was on the abaxial surface. Microscopic observations revealed branched sporangiophores that measured 187.5 to 325 μm (average 285 μm) long. Sporangia measured 22.5 to 30 × 20 to 22.5 μm (average 26.7 × 20.9 μm). No oospores were found. Sporangium measurements are comparable to unnamed Peronospora species reported previously on basil from Italy, Switzerland, and South Africa (1,2). Sequence analyses were conducted on five isolates of 'Nufar' basil by extracting DNA from a sporangial suspension washed from leaves and infected leaf tissues using the Qiagen DNeasy plant tissue kit (Qiagen, Valencia, CA). PCR amplification of the ITS1, 5.8S, and ITS2 region was performed using primers ITS6 and ITS4 (3). The sequences of the five isolates were identical. BLAST analyses of the sequences revealed a 99% similarity to the unnamed Peronospora species on sweet basil in Europe and South Africa (1,2). To our knowledge, this is the first report of a Peronospora species on sweet basil in Massachusetts. References: (1) L. Belbahri et al. Mycol. Res. 109:1276, 2005. (2) A. McLeod et al. Plant Dis. 90:1115, 2006. (3) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, San Diego, 1990.