The onion, like its close cousins garlic and leeks, has a very weak, shallow root system. In hot dry weather the bulbs stay small and often have stronger flavour. The neck is the portion of the stem immediately above the bulb. A few days later, you can pull the onions out of the ground and leave them to dry someplace out of the rain like a porch. Firm, clean onions with well-sealed necks will keep for a few months in a cool room.
Onions seem to fend off most pests and diseases.
In hot, dry weather the onion thrip, a tiny slender brown insect, leaves small silver or white streaks on the leaves. In many areas there could be a problem with onion maggot. This happens most often if starting from seed or if you've been growing onions for a couple of years and have not cleaned them out at the end of the season. Maggots can be controlled by spreading a granular form of diazonon on the soil when you plant seeds or underneath bulbs when you set them out. Botrytis leaf blight will dry up and kill the tips of the leaves and possibly the entire plant.