First Report of Downy Mildew on Field-Grown Sweet Basil Caused by a Peronospora sp. in San Diego County, California

Sweet basil (Ocimum basilicum) is a culinary herb grown in fields and greenhouses in California. In August of 2008, samples were submitted to the San Diego County Plant Diagnostic Laboratory with a grayish brown, downy growth that covered areas of the abaxial side of the leaf. Early symptoms included leaf chlorosis followed by the appearance of scattered grayish brown conidia and conidiophores in the chlorotic areas. Conidiophore emergence progressed to a grayish brown, velvety growth with some of the chlorotic areas becoming necrotic. Similarly affected plants were submitted to the laboratory from another field 32 km (20 miles) away in the same county. Both fields were drip irrigated and located in inland valleys. Approximately 50 to 60% of the plants were symptomatic. Conidiophores, on the abaxial side of the leaves that were characteristic of Peronospora, branched dichotomously three to five times and measured 390 to 1,100 × 2.5 to 7.0 μm (675.6 × 4.9 μm average). Light brown, globose to ellipsoidal conidia measured 20 to 34 × 18 to 26 μm (26 × 22 μm average). To confirm pathogenicity, three basil plants in 10.2-cm (4-inch) pots were sprayed with a suspension of 4.5 × 10⁵ conidia/ml and incubated in a dew chamber at 20°C for 48 h in the dark. Three noninoculated plants were sprayed with water. Plants were then placed in a growth chamber at 18°C with a 16-h photoperiod. Relative humidity was maintained near 95% by placing the plants over a tray of water and covering each group with a plastic tent. Signs of grayish brown, downy growth were seen approximately 14 days after inoculation. Pathogenicity experiments were repeated once. No symptoms developed on plants sprayed with water. Sequence of the intergenic spacer regions and 5.8S gene was obtained from a sample collected in one field, through amplification with primers DC6 and ITS4 and sequencing with primers ITS6 and ITS4 (2). The sequence (Accession No. FJ436024) from the field sample was identical to GenBank Accession No. AY884605 that represented a Peronospora sp. from basil in Switzerland. On the basis of morphological and sequence information, the Peronospora sp. from San Diego County is most likely the same Peronospora sp. that was identified from sweet basil in Switzerland, Italy (1), and South Africa (4), as well as coleus in New York and Louisiana (3). The basil Peronospora sp. has been shown to be present in seed (1). Since both growers purchased their seed from the same supplier, seeds could have been a possible inoculum source. To our knowledge, this is the first report of a downy mildew on sweet basil in California. References: (1) L. Belbahri et al. Mycol. Res. 109:1276, 2005. (2) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (3) M. L. Daughtrey et al. Plant Dis. 90:1111, 2006. (4) A. McLeod et al. Plant Dis. 90:1115, 2006.