Onion plants are shallow-rooted and plant growth is good when nitrogen is carefully applied with irrigation water. It is important that excellent plant development is achieved before bulbing starts. After bulbing begins, high temperatures and low humidity continuing through the harvest process are important. Through the entire growing process, adequate soil moisture must remain constant. Back to Top of Page Diseases and Pests Growers must closely monitor onion plants for pests and diseases.
Onion thrips are a widespread pest.
Onion maggots are another problem pest. Diseases include pink root, botrytis blast, downy mildew, purple blotch, white rot and neck rot. This term describes the effect of day length on bulb formation. Some onion varieties have a short day length response, forming bulbs when the days are 12 hours or less. Other varieties have a long day length response, forming bulbs when there are 15 or more hours of daylight.