ONION HEALTH MANAGEMENT & PRODUCTION
OTHER CULTURAL PRACTICES By M. E. Bartolo, C. Cramer, and D. Drost
Dry bulb storage onions are often grown in 3- to 4-year rotations with other crops in the western U.S., but rarely rotated on muck soils in the Great Lakes region. Rotations differ among the growing regions. In general, the main rotational crops include potatoes, carrots, and sweet corn. Other rotation partners may include field corn, wheat, peas, cole crops, and beans. Alfalfa, sugar beets, and grass seed are also sometimes included in the rotation. Rotational restrictions are very important in making herbicide decisions. Volunteer potatoes from a previous potato crop can become a serious weed in an onion field; therefore, when possible, growers schedule rotations so that potatoes are at least two crop years away from onion production. Onion fields may be fumigated during each rotation cycle. If the rotation includes lower-value crops, the fumigation is done just prior to planting onions. If the rotation partners are carrots or potatoes, fumigation may occur the year before or the year after onions are planted as carrots and potatoes are also high-value crops and successful management of the pests of those crops typically entails fumigation. Cover crops, e.g., wheat or other cereals, are usually planted prior to planting onions in the western U.S.; however, pre-formed beds with no cover crop are also used on heavier soils that are less prone to wind erosion. In the Great Lakes region, onion is co-planted with barley, which is either planted in rows between onions or broadcast in the field. In both cases, the cover/companion crop is utilized as a means to reduce soil erosion and to protect young onions from wind and blowing soil damage. The most successful programs employ wheat planted in late summer, which is generally killed by fall herbicide or soil fumigant application. In the western U.S., overwintered cover crops are commonly sprayed with herbicides in spring, before the cover crop gets large enough to compete with onion seedlings. The herbicide application usually occurs before the onion crop is planted, but may be applied after the onion crop is planted if the cover crop is not tall enough to protect onion seedlings at the time of planting. Spring-planted cover crops have proven more problematic due to the difficulty of killing the cover crop before significant competition occurs with the onion crop. This is typically attempted with selective grass herbicides. Cover crops may also have the benefit of providing some weed suppression and harborage for beneficial insects and arachnids. Crop Rotation Onions are used in a rotation once every three to four years (or more) depending upon the history of diseases in the field and region. A history of outbreaks from soil-borne problems such as pink root and Fusarium basal rot may require that the field be rotated out of onions for at least four years, if not longer. Common rotation crops include sweet corn, field corn, alfalfa, potatoes, dry beans, melons, sugar beets, and small grains such as oats, barley and wheat. Large-rooted crops like corn or alfalfa are not recommended in the year before the onion crop because of the potential debris problem in the seed-bed. Small grain crops are often preferred before the next season of onions because of softerr ground and reduction of soil-borne pathogens. Sanitation Good overall sanitation will help reduce pest problems. Eliminating culls and onion residue from previous crops will help protect the current crop from disease infection. Before setting in the field, transplants should be inspected carefully for diseases and insects. Weed pressure should be reduced by screening surface irrigation water. Small mesh screens can remove some weed seeds before they are deposited in the field.
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