Genetic Diversity Analysis Reveals High Heterozygosity, Low Clonality, and Distinct Populations in Peronospora...

Basil (Ocimum basilicum) is one of the most widely grown herb crops for fresh consumption, processing, and essential oil production. These markets are reliant upon high-quality, pathogen-free material, as disease renders fresh material unsellable and may alter taste and essential oil profiles for industrial producers. Meeting high-quality standards has become increasingly difficult with the introduction of basil downy mildew caused by Peronospora belbahrii into the United States and globally. Basil downy mildew is a highly destructive foliar disease of basil resulting in chlorosis, large lesions, gray-black spores, and plant death. Fungicides have shown variable efficacy between field seasons, are expensive, and require diligent and repeated applications for complete control. Repeated applications of high-risk fungicides provide selective pressure for pathogen evolution, exemplified by the appearance of mefenoxam-resistant populations of P. belbahrii. Resistant sweet basil varieties, though initially successful, have begun to break down under rapid pathogen race differentiation. This adaptability makes understanding the population genetics of the pathogen critical. This study, utilizing simple sequence repeats, examined the genetic diversity of 92 P. belbahrii isolates collected during 2018 to 2024 from Israel, Italy, Hawaii, Florida, Kansas, and the U.S. Northeast. Genetic diversity statistics indicate a highly heterozygous, largely non-clonal population structure in both the United States and Israel. These two populations appear to be at least partially genetically distinct and indicate differentiation along geographic lines. Significant genetic differences were also detected between race 0 and race 1, though a distinct set of classifying markers was not found.