As the onion begins to bulb the soil around the bulb should be loose so the onion is free to expand. Do not move dirt on top of the onion since this will prevent the onion from forming its natural bulb. Start early with cultivation practices. The two major diseases that will affect onions are blight and purple blotch. Should the leaves turn pale-green, then yellow, blight has probably affected the plant.
Purple blotch causes purple lesions on the leaves.
Heavy dew and foggy weather favor their rapid spread, and when prolonged rainy spells occur in warm weather, these diseases can be very destructive. Should conditions persist, a spray with a multipurpose fungicide such as daconil can be applied on a 7 to 10 day schedule. The insect that causes the most damage is the onion thrip. They feed by rasping the surface of the leaves and sucking the liberated juices. They are light-brown in color and are approximately 1mm long.