Identification of Host Resistance to Basil Downy Mildew (Peronospora belbahrii)

In addition to being the most widely cultivated and economically important basil species, sweet basil (Ocimum basilicum L.) is also the most susceptible to downy mildew (Peronospora belbahrii). Significant crop losses have been experienced annually since the first reports of this disease in Europe and more recently in the United States.  Although chemical control is available, registered products remain limited and costly. Thus, there is a hastened need for identification of host resistance. Two commercial varieties, Ocimum americanum ‘Spice’ and Ocimum citriodorum ‘Sweet dani’, have been previously reported as potential sources of resistance to basil downy mildew in a 2009 field trial. A leaf inoculation assay was developed for detection of basil susceptibility to downy mildew in controlled environmental conditions. Three commercial varieties, representing three Ocimum spp., were screened at the cotyledon, first true leaf set, and second true leaf set growth stages. All plants were evaluated for the presence of sporangiophores on the abaxial leaf surface daily for fifteen days. O. basilicum ‘DiGenova’ was completely susceptible at all growth stages, while O. americanum ‘Spice’ displayed no signs or symptoms at any growth stage. O. citriodorum ‘Sweet dani’ exhibited a decreased mean disease incidence (DI) with increasing growth stage. Breeding and genetic implications are discussed.