Downy mildew of sweet basil (Ocimum basilicum) caused by Peronospora sp. in Argentina

In the green belt surrounding Buenos Aires and La Plata cities, Argentina, sweet basil (Ocimum basilicum) is grown commercially under greenhouse mainly for fresh consumption. In February 2008, plants showing typical symptoms of downy mildew were found in greenhouses in La Plata. The disease was widespread in the cropped area with 100% prevalence. Infection resulted in chlorotic leaves with a greyish to dark brown fungal-like growth on the lower surfaces (Figs. 1 & 2). A sample was deposited in the local herbarium (KUS-F23241). Conidiophores were subhyaline, 230-460 × 7-11 μm, straight, monopodially branched, in 4-6 orders, and emergent from stomata (Figs. 3 & 4). Ultimate branchlets were mostly in pairs, slightly curved, 10-25 (-30) μm long and had subtruncate tips (Fig. 5). Conidia were broadly ellipsoidal to subglobose, greyish brown, and measured 24.3-32.5 × 22.5-26.5 μm (length/width ratio = 1.06-1.23) (Figs. 6 & 7). This pathogen is unequivocally in the genus Peronospora, and well concordant with characters of the unnamed Peronospora species reported in basil (Belbahri et al., 2005).

The amplification and sequencing of the ITS rDNA were performed with procedures outlined by Cooke et al. (2000), and the sequence of the region was deposited in GenBank (Accession No. EU622314). Comparison of the sequences available in the GenBank database revealed that the ITS sequence is identical to those of Peronospora sp. found on Ocimum basilicum (AY831719 ex. Switzerland, AY919301 ex. Italy, and DQ479408 ex. South Africa), but shares three base substitutions with the sequences (EF153666-EF153670) from Iran.

Based on morphological and molecular approaches, the assumed pathogen was identical to the unnamed Peronospora species, reported on basil in Italy and Switzerland (Belbahri et al., 2005). Downy mildew of sweet basil has been previously recorded only from Asia (Iran) (Khateri et al., 2007), Africa (South Africa, Tanzania, Uganda) and Europe (France, Italy, Switzerland) (Farr et al., 2008). This is the first record of a downy mildew epidemic on sweet basil in the Americas and suggests that the pathogen is capable of rapid spread into countries outside of Africa and Europe. Since sweet basil is cultivated on a commercial scale in Argentina, the downy mildew poses a serious threat to production of the herb.