Basil (Ocimum basilicum) is one of the most widely grown herb crops for both 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 US and reported globally. Basil downy mildew, is a highly destructive foliar disease of basil resulting in chlorosis, large lesions, grey-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 Peronospora 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 Peronospora belbahrii isolates collected during 2018-2024 from Israel, Italy, Hawaii, Florida, Kansas, and the US Northeast. Genetic diversity statistics indicate a highly heterozygous, largely non-clonal population structure in both the USA 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.
Barrett et al. (Mon,) studied this question.