Development of a differential panel to identify Peronospora belbahrii races in sweet basil (Ocimum basilicum)

First reported in the United States in 2007, downy mildew of sweet basil (Ocimum basilicum), caused by the oomycete pathogen Peronospora belbahrii, has become a major challenge in global basil production. Downy mildew-resistant cultivars have been developed and commercially available since 2018, but observations of isolates overcoming resistance were reported after each new introduction. The rapid development of pathogens overcoming genetic resistance suggests the evolution of new races under selective pressure. In this study, we report on the development of a differential panel of distinct basil cultivars with varying and differential sources of genetic resistance tested against P. belbahrii strains obtained from geographically diverse multi-year collections from basil-producing locations in the US, Israel, and Italy. Screening multiple P. belbahrii isolates revealed three distinct virulence profiles that distinguished race 0, race 1, and race 2. Races 1 and 2 were defined by virulence in cultivars with single gene resistance loci Pb1 and Pb2, respectively. Evaluation of cultivars with quantitative resistance revealed more variation in susceptibility or partial resistance, suggesting "broad spectrum" (or race-nonspecific) resistance. With continued isolate collection and screening efforts, the denomination of P. belbahrii races may be further expanded, and newly developed resistant cultivars can be added to the proposed panel to definitively evaluate novel P. belbahrii strains. This study lays the groundwork to further explore dynamics of race evolution and mechanisms of resistance breakdown, which will be essential for discovering new sources of resistance for controlling basil downy mildew.