Downy mildew is a plant disease, caused by a fungal-like water mould known as Peronospora destructor. The disease thrives in cool, wet conditions and can cause significant yield and quality losses if not managed.

Early symptoms include pale yellow patches on leaves, followed by the development of greyish-purple spores under moist conditions. When spores occur, the disease can quickly spread through the crop. Initial sources of the pathogen can include infected bulbs, seeds, and plant debris. Onions are most susceptible to downy mildew during the early vegetative growth and seedling stages.