Basil Downy Mildew (Peronospora belbahrii): A Major Threat to Ocimum basilicum L. Production

Basil (Ocimum basilicum L.), a key herb in Mediterranean cuisine, holds substantial economic and cultural value due to its aromatic and medicinal properties. Cultivated globally, particularly in Italy’s Liguria region, basil is consumed both fresh and processed, with pesto sauce as its most notable derivative. Despite its commercial success, basil production is significantly constrained by a broad spectrum of fungal pathogens, with Peronospora belbahrii, the causal agent of downy mildew, posing the most severe threat. This study aims to provide a comprehensive overview of basil’s disease susceptibility and control. Special emphasis is placed on the biology, epidemiology, global spread, and diagnosis of P. belbahrii, which has become a critical challenge for both conventional and organic farming systems. Disease management strategies, including cultural practices, genetic resistance, fungicide applications, resistance inducers, and biocontrol agents, are reviewed in detail. The development of downy mildew-resistant cultivars—although limited for PDO-designated Genovese basil—has emerged as the most sustainable control measure; however, the increasing genetic variability in P. belbahrii underscores the ongoing need for integrated pest management and resistant cultivar development. Seed health and quality remain the starting points of any fully integrated approach, although the suggested management measures for basil production should be combined with appropriate cultivation techniques aimed at reducing the relative humidity of the environment, while taking into account whether basil production takes place in open fields or under protection.