Pink root symptoms are frequently misdiagnosed as drought stress or a nutrient deficiency. Pink Root rarely results in the death of the plant and the disease is limited to roots and bulb, which basically are the Onion crop. Severe cases will reduce the bulb size, all cases will result in diminished quality.
Crop rotation with unrealted non host crops is an important step in curtailing its spread. Grasses, grains, corn are host crops - avoid them in the vicinity of where you plan to grow , or are growing Onions or other alliums.
Fungicides are somewhat effective, and not always worth the expense for smaller plantings given the success rate they have displayed. Fungicides that contain chloropicrin are effective against the widest range of pink Root strains. Unfortunately it is a restricted use chemical, unavailable to Home Gardeners.Products containing boscalid are less effective - but readily available.
Proper cultural practices are a defense against onion pink root.Remove and destroy all plant refuse in the fall and use deep cultivation to bury any remaining refuse.