Onions can be grown all over the country, as long as the weather permits and you use a soil that is well-drained, fertile and rich in humus. Some varieties work better in specific regions of the country, but there are still risks in planting onion that are experienced by most gardeners, such as certain types of weather, rot, disease or insects.
Onion thrips are pests that can cause a lot of damage without leaving a trace behind and they are also only a fraction of an inch long and light brown, blending in with the onion easily. These damage the surface of the onion leaf so it leaks out juices for the pests to feed upon. Thrips can be taken care of with insecticides as long as you aren't going to harvest the onions within the next seven days. The most common insecticides are malathion or diazinon to treat thrips.
If you live in a region where hail is common or large, your onion crop is in danger of being ruined. Hail damages the onion leaves, making them grow more slowly and smaller. When hail is expected, cover the onions with a protective sheeting so they aren't harmed.
The disease smut is especially common in northern onion crops. Smut appears on young seedlings and is recognizable by the black spots that appear between the onion layers and on the leaves. The disease is in the soil, so it is important to use a fungicide in the planting site before you plant the crop. If onions survive this disease, they are often deformed and smaller.
Blight and Purple Blotch
These two diseases are some of the most common of onion diseases. Blight turns onion leaves light green then yellow, while purple blotch causes small purple sores on the leaves. These two diseases usually hit when small jumps in warm weather happen suddenly. Since they can be so destructive, it is important to do the best you can to prevent them both. This includes making sure the onions are in well-draining soil and using a fungicide if symptoms appear, like daconil that is applied over a week-span.
Onion root maggots affect the bulb and are 1/3-inch-long worms as babies, then grow to winged gray or brown adult flies. The worms bury into the onion bulb, thus destroying the onion. This occurs most often in wetter years with lots of rain or moisture. To control this pest, spray the foliage with a malathion or diazinon application when you first see the insects.
Cutworms affect the foliage of the onion plant and can be identified when you notice the plants are chewed off at about ground level. These 1-1/2-inch-long caterpillars are typically green brown or gray and coil up when disturbed. They usually stay in the soil line if the ground is wet, than move over the surface when the soil is dry. To help get rid of cutworms, add a cardboard collar around the onion plants (found at local nurseries), going 2 inches into the soil for protection.
Pink rot and neck rot are two common types of rot the onion plant runs into. For pink rot, the roots turn a pinkish hue. The only way to help prevent this is to plant the onion bulbs in sterilized soil that isn't infested (sterilize it with a solution such as Vapam before hand). Neck rot affects the bulb of the onion, and you will notice water-soaked spots in the neck area that are a yellow hue. Gray mold will develop and the bulb will dissolve. Neck rot occurs most often right before harvest time. To prevent rot, try not to use too much nitrogen fertilizer and let the onion tops mature thoroughly.