Sunlight causes the formation of chlorophyll in the outer scales, which results in the scales turning green.
Conditions for Disease Development
Excessive or late season nitrogen applications can delay maturity and enhance the greening of onion bulbs. Greening can occur if onion bulbs are exposed to sunlight during the growing season or the bulbs are allowed to cure for extended periods under moderate light.
An early fertilization program that promotes foliar development can reduce losses from greening at bulb maturity. Avoid excessive and late season nitrogen application. Do not cure bulbs for extended periods in the field.