Downy Mildew of Onion (Allium cepa): An Overview of the Pathogen, Disease Cycle, and Management

Abstract Onion downy mildew (ODM) is one of the serious threats to onion cultivation worldwide, caused by an oomycetes Peronospora destructor (Berk.) Casp. ex Berk. It is particularly troublesome in subtropical and dry temperate regions, where environmental conditions favor its rapid development. Cool, humid weather promotes its development, leading to significant yield losses and deterioration of bulb quality. This paper summarizes the pathogen’s biology, life cycle, and epidemiology. It also describes the disease symptoms and outlines integrated management strategies. Particular attention is given to sustainable approaches to minimize economic losses and safeguard onion production.The disease spreads rapidly, especially in cool, humid climates, making it a persistent challenge for growers. Downy mildew affects all known onion varieties and can cause yield losses of up to 75%. Severe infections lead to early leaf loss, smaller bulbs, and reduced quality. The pathogen, which has been reported in over 70 countries, primarily infects leaves, which decreases the plant's photosynthetic activity. Under favorable conditions, the pathogen can defoliate entire fields in a single week.P. destructor survives as oospores in plant debris and soil, or as mycelium within onion bulbs. The oospores are the primary source of infection. When cool temperatures (10–20 °C) and high humidity (>90% RH) are present, oospores germinate and produce sporangia, which are then carried by the wind to infect new plants. Effective management requires an integrated approach that includes using disease-free bulbs, practicing crop rotation, ensuring proper plant spacing, and applying fungicides preventively during favorable weather conditions. These strategies are essential for minimizing disease incidence and ensuring sustainable onion production.