Morphological and Molecular Identification of Peronospora destructor Causing Downy Mildew on Onion and Garlic

Abstract

Downy mildew symptoms observed on onion (Allium cepa L.) and garlic (Allium sativum L.) were investigated to determine the causal pathogen through integrated morphological and molecular approaches. Field surveys revealed chlorotic streaks, pale patches, and characteristic gray-violet sporulation on the abaxial leaf surface, leading to progressive blighting and reduced bulb development under humid conditions. Microscopic examination of infected tissues showed branched sporangiophores emerging through stomata and producing ovoid to ellipsoid sporangia, consistent with Peronospora destructor. Pathogenicity tests conducted under controlled humidity conditions successfully reproduced downy mildew symptoms on both onion and garlic seedlings, confirming host susceptibility and fulfilling Koch’s postulates. Molecular identification using ITS and cytochrome oxidase subunit II (cox2) gene sequencing confirmed the isolates as P. destructor, with high sequence similarity to global reference isolates. Phylogenetic analysis placed all isolates within a well-supported clade corresponding to P. destructor, showing limited genetic variation among populations infecting different Allium hosts. Observed sequence homogeneity suggests a conserved pathogen population structure with potential for cross-infection between onion and garlic crops. Disease development was strongly associated with prolonged leaf wetness, moderate temperatures, and high relative humidity, which favored sporulation and secondary spread. The study highlights the importance of accurate molecular diagnostics for early detection and differentiation of downy mildew pathogens in Allium production systems. These findings provide a baseline for epidemiological monitoring and support the development of integrated disease management strategies, including resistant cultivars, optimized irrigation practices, and targeted fungicide applications to reduce disease incidence and improve crop productivity.