Abstract: The interactive effect of three different levels of NPK fertilizer, plant population and irrigation was studied on downy mildew severity and yield of onion. The lowest disease severity was recorded in treatment where NPK 120:90:60 kg/ha, coupled with plant population 0.5 million/ha and eight irrigations. In this treatment the yield and bulb size were also the highest. However, number of bulbs was lower than some other treatments indicating the non-significant effect of disease.