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

Abstract

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. A sample was deposited in the local herbarium (KUS-F23241). Conidiophores were subhyaline, 230­ 460 X 7-11 pm, straight, monopodially branched, in 4-6 orders, and emergent from stomata. Ultimate branchlets were mostly in pairs, slightly curved, 10-25 (-30) pm long and had subtruncate tips. Conidia were broadly ellipsoidal to subglobose, greyish brown, and measured 24*3- 32-5 X 22-5-26-5 pm (length/width ratio = 1-06-1-23). This pathogen is unequivocally in the genus Peronospora, and well concordant with characters ofthe unnamedPeronospora speciesreported in basil (Belbahri et al., 2005).