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 commercially available since 2018. However, new pathogenic races that overcome host resistance were observed after each new cultivar introduction, suggesting rapid pathogen evolution under selective pressure. To track the race structure and to monitor the shift of pathogen populations, we established a differential panel of distinct basil cultivars that carry varying sources of genetic resistance. Testing against P. belbahrii strains obtained from geographically diverse multi-year collections from basil-producing locations in the US, Israel, and Italy, this differential panel revealed three distinct virulence profiles that distinguish race 0, race 1, and race 2. Race 0 isolates are avirulent on cultivars with resistance loci Pb1 and Pb2, while races 1 and 2 were defined by virulence in cultivars with loci Pb1 and Pb2, respectively. Compared to major gene resistance, evaluation of cultivars with quantitative disease resistance revealed more variation in susceptibility (or intermediate resistance), suggesting race-nonspecific resistance and highlighting the potential durability of utilizing diverse sources of resistance in future breeding approaches. With the dynamic nature of the pathogen and the continued efforts in adding disease resistant basil cultivars to meet the market demands, the denomination of P. belbahrii races is likely to expand, and this differential panel will continue to develop. Most importantly, 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.