Onion

ONION
Allium cepa

FAMILY CHARACTERISTICS
    Family members include:
        Allium ampeloprasum L. (Great-headed garlic group); Great-headed Garlic
        Allium ampeloprasum L. (Kurrat group); Kurrat
        Allium ampeloprasum L. (Leek group); Leek
        Allium cepa L. (Aggregatum group); Shallot, Potato or Multiplier Onion
        Allium cepa L. (Common onion group); Onion
        Allium chinese G. Don.; Rakkyo
        Allium fistulosum L.; Welsh or Japanese Bunching Onion
        Allium sativum L.; Garlic
        Allium schoenoprasum L.; Chive
        Allium tuberosum Rottler: Chinese Chives, Oriental Garlic

CROP HISTORY AND DEVELOPMENT
    One out of close to 300 species in the genus Allium, the onion has been dated to 2800 BC and played a significant part in the development of our world. Many beliefs were associated with the onion, ranging from curing baldness to curing rabies. The onion was brought to the Americas by early explorers and was accepted quickly. It is not known exactly when the onion first arrived and by whom, because of how rapid its acceptance and popularity grew.
    The onion is a major vegetable in the American diet with close to 39 million cwt. being produced on 123,000 acres annually. The total value of the onion crop is about $400 million per year. Trends in production have led to a general decrease in total acreage in production but a significant increase in yields per acre. Most of the production area is concentrated in California, Georgia and Texas for sweet types of onion, while New York and Oregon capture most of the market for the more pungent types. Because of storage techniques, onions are available year round in most countries. Because of different climatic zones, onions can be produced almost year round in the United States. Very mild forms of onions are produced in Georgia and Hawaii that are considered as premium in taste.

    Bulbing onions are classified into three categories: short, intermediate, and long day types. Short day onions are mild, soft fleshed and cannot be stored for long periods. These types are grown south of 35° latitude and require 12 to 13 hours of light to initialize bulbing. Some short day cultivars include 'Awahia', 'Red Granex', 'Red Creole', 'White Granex', 'Crystal Wax', 'Grano', 'Granex', and 'Yellow Creole'. Long day onions require over 14½ hours of daylight to bulb and are very pungent, hard, and store well. Long day cultivars include 'Australian Brown', 'Carmen', 'Southport Red Globe', 'White Lisbon', 'Ivory', 'Fiesta', 'Sweet Spanish', 'Autumn Spice', and 'Downing Yellow Globe'. Intermediate onions are also fleshy and used primarily in fresh market produce and require 13 ½ to 14 hours of daylight. 'Cochise Brown', 'Stockton Early Red', 'Fresno White', and 'Rialto' are common intermediate cultivars.
   ‘Vidalia,’ ‘Walla Walla,’ and ‘Maui’ are three mild salad onions that have a specialized market developed due to the adaptation of the onion with the location that it is grown in.
Vidalia onions

PLANT CHARACTERISTICS
    Onion is a cool season biennial that is tolerant of frost. Optimum temperatures for growth and development are between 55 and 75° F. However, optimum temperatures for seedling growth are much narrower, with 68 to 77°F being most productive.

    The bulb constitutes the edible part of the onion. The bulb is comprised of concentric, enlarged fleshy leaf bases, also called scales. The outer leaf base dries and becomes scaly and the inner leaf bases thicken as the bulb develops. The stem of the onion is made up of hollow green leaves composed of a blade and a sheath. The sheath develops to completely encircle the growing point and forms a hollow tube that encloses the younger leaves and apex of the shoot. From here, the young leaf blades will arise, growing from the tip of the blade and sheath of the older leaves. As more new leaves are initiated and grow, the older leaf bases get pushed further away from the apex.
    Temperatures below 50° F initiate bolting of the onion, or flowering. The flower is composed of a compound umbel that is formed on an elongated stem called a scape arising from the center of the bulb. It was first thought that flower initialization was a photoperiodic response, but is now known that the length of day only affects the rate of seed stalk development.

    The root system of onion is fibrous and shallow, spreading just beneath the soil from 12 to 18 inches in depth. Root growth is not considered aggressive; therefore onions can tolerate crowding, but cannot tolerate competition from aggressive weed growth. The roots are initiated at the base of young leaves and push downward through the outer layers of the stem to emerge. New root formation occurs at the apex, and older roots get pushed further away from the apex.

Umbel of the onion

Flowering onions in the field

PROPAGATION METHODS
Onions can be established one of three ways: direct seeding, sets and transplants. Direct seeding gives excellent results where the season is sufficiently long enough to provide the early prebulbing growth needed before bolting and is the preferred system for most acreage in the U.S. Sets and transplants are used in areas where the season is not long enough for proper bulb development. The use of sets and transplants ensure large bulb sizes and uniform maturity dates. Sets are small dry bulbs that are produced in the previous season by overseedng and growing under conditions that promote rapid bulbing. Transplants are grown in the south and have three to five leaves at the time of transplanting. The leaves are pruned prior to field planting to facilitate the handling and to harden off the plants.
Direct seeding planting rates are at depths of ¼ to ¾ of an inch with spaces from 1” to 3” depending on the product being produced. There are approximately 9500 seeds per ounce, and with good germination, 3 to 4 pounds of seed per acre are required for a good stand. Rows should be 18 to 26” apart.

Young onion field

Maturing onion field

Onion sets

Transplant production

Setting transplants

CULTURAL PRACTICES
    Soils Onions can be successfully grown on any soil that is fertile and well drained. Onions do not do well on soils that can crust easily. Clay, alluvial, sandy loam, and muck are the soil types best suited to onions, with the last two being preferred. Muck soils are considered best for production in the North. A soil pH of 6.0 to 7.0 is recommended. Onions are only slightly tolerant of acidic soils. Onions are not at all tolerant of salinity and yields are severely reduced when subjected to saline soils.
    Temperature Optimum temperatures for plant growth are from 55° to 75°F. High temperatures favor bulbing and help with the curing process. At 77°F, seedlings emerge in 4 days.
    Fertility Fertilizer is applied as a broadcast or in a band 2 to 4 in. directly below the seed at planting. Onions utilize large amounts of nutrients, so fertilization is extremely important. If heavy fertilization is called for by soil tests, the material should be incorporated into the plow layer or banded 6 in. to the side of the row. Minor element deficiencies are often encountered with this crop, especially zinc and copper.
    Nutrient deficiencies (Photos provided by APS Press)

Zinc deficiency (right)
Normal onions (left)

Molybdenum deficiency -
dieback and wilting of leaves

Center row stunted with
Mo deficiency - outer rows
sprayed with sodium molybdate

Copper deficiency (right)
White and spiral twisting leaf tips

    Irrigation Onions require even moisture throughout the season and are usually irrigated either by furrow irrigation or by overhead irrigation, depending on soil type.
    Weed Control Onions do not compete well with weeds. Mechanical cultivation should be shallow to prevent root damage and most growers lean towards chemical control.

INSECTS
    Maggots and thrips are responsible for most of the losses attributed to insects. The maggot is a pest of northern onions where it may destroy up to 90% of the crop. Maggot infestations are common in wet springs. The larvae bore through the stems into the bulbs, causing plants to turn yellow. Small bulbs will be completely ruined, while large bulbs are damaged and often rot in storage. The insect over winters in soil, old cull piles, or plant debris.
    Thrips are small insects that cut the epidermis of leaves or stems and feed on the plant sap, resulting in white blotches on the leaves. Severe infestations cause leaf blasting and collapse. Bulbs are often distorted in shape and undersized. Unlike the maggot, thrip infections are more severe in dry seasons. The insect has a wide variety of host plants and adults over winter on plants, plant debris, or in weeds in or along side of the field.

DISEASES
    Field diseases include smut, downy mildew, pink root, smudge, leaf blight, and several basal rots. Some of the storage diseases that affect onions are common field rots, botrytis neck rot, and bacterial soft rot.
    Smut attacks immature tissue only. Infested plants show gray streaks on leaves, sheaths, and bulbs. Plants become twisted and eventually die. The fungus over winters in the soil and will infect new plants in cool, damp conditions the next spring. Seed treatment provides excellent control against smut.
    Downy mildew also attacks immature tissue. Symptoms are white specks, confined to the oldest leaves of the plant. A grayish white mycellium develops rapidly down the sheath in cool damp weather, causing the plants to fall over and dry out. Young plants should be treated with mancozeb every week until bulbing begins.
    Pink root is a root disease that eventually kills infected roots, with new roots continually being infected. This results in small bulb sizes. The fungus is soilborne and persistent and the best method of control is through plant selection.
    Smudge causes black or dark green areas on the bulb or neck of the onion that appear as concentric rings. Both wind and water, along with tools and clothing spread spores. The disease over winters in the soil and in sets. Temperatures between 75 and 85° F favor infection. Affected bulbs may shrink and also may sprout in storage. Proper selection of cultivars and proper culling of plant material is the best method of control.

Botrytis leaf blight, also known as blast, first appears as white specks on leaves that expand to cause a die back of the leaf. Tops can be killed within a week of infection. Blight often follows previous damage from insects, diseases, or mechanical damage. Air pollution has also been shown to promote leaf blight.

Botrytis

Bulb rots may occur in the field or in storage, and the best method of control is careful grading.
Other diseases

Pseudomonas on onion
HARVESTING
The tops of onion plants fall over when bulbs are ready for harvesting. Bulbs that will be stored can be harvested when 50% or more of the tops have fallen, while fresh market bulbs can be harvested when 15 to 20% of the tops have fallen. The onion bulbs are cut from the root system with a knife or lifter that is drawn underneath the row. The bulbs are then dug and either packaged to be cured or cured in the field. In dry conditions, growers can leave the bulbs in the field to cure, either where they were dug, or in windrows. If conditions do not favor curing in the field, bulbs are brought out of the field to the curing facility.
Curing allows the natural dormancy of the onion to develop and also dries out the onion to protect against disease organisms that may be present. An onion that has been cured correctly will have a dry, shrunken neck and dry outer scales. The respiration rate of the bulb will also be lowered. The curing process exposes the bulb to temperatures up to 95°F in low relative humidity. Air movement of 1 cubic foot per minute per cubic foot of onion bulbs must be provided.

Undercutting field

Onions that have been undercut left for drying

Onions curing in the field

Harvesting

POST HARVEST
After curing, onions must be placed into cold storage lowering the temperature of the onion gradually to 32°-34°F with the RH at 60 to 70%. Air exchange in the cooling facility is extremely important to inhibit the build up of condensation on the bulbs. When removed from storage, the crop should be gradually conditioned for several days at 68°F with 50% RH.

Bagging onions

Ready for shipment