Vegetable Seed Production - "Dry" Seeds
Onion
Introduction
Onions originated in Iran and Pakistan and were established staple foods of Egypt and India around 1500 B.C. By the middle ages, they were in Europe and brought to North America by Spanish settlers.
The unique flavor and odor of onions have made them an excellent food source that provides zest to many dishes. Moreover, the numerous ways in which onions are prepared from boiling, frying, stewing, baking, pickling, to eating raw make them a versatile food source. The recent popularity of a health conscious world for salad bars has further increased their agricultural importance. Nutritionally, onions are low in calories (approximately 40 calories for an average size onion) and high in ascorbic acid. They are classified into three agriculturally important groups: common, aggregation, and proliferous. The common onion is the most agriculturally important group. It is a bulb producing type reproduced by seed. The aggregation group is characterized by formation of many lateral bulbs or shoots and is reproduced vegetatively. An example of this is shallots. The proliferous group has bulblets form on the inflorescence and is vegetatively reproduced. An example of this is top-set onions.
Plant Development
Vegetative. Onions are a member of the Liliaceae family and are considered a cool-season crop. They are herbaceous biennial plants. The first year of development results in the establishment of a fibrous, shallow root system attached to a base plate of fleshy leaves arising around a small conical terminal stem, a structure botanically termed a bulb. Because the root system is shallow, onions are particularly prone to dehydration injury. The leaf sheaths that project above the ground are sometimes called a false stem to distinguish them from the true stem at the bulb's base. Bulb formation is initiated by changes in day length. Very early types require a 12 hour photoperiod and later types initiate bulbing with a 15 hour photoperiod.
Reproductive. Flowering and seed formation occur in the late Spring or early Summer in response to a vernalization treatment during the Winter. The onion inflorescence is an umbel that produces 50 to 2,000 flowers. Flowering can be as long as two weeks and is not uniform since the umbel actually consists of aggregations of smaller 5 to 10 flowered inflorescences called cymes. From one to over a dozen flower stalks can be produced on a single plant dependent on its development. More flower stalks are produced on plants grown from bulbs than those grown from seed. The stigma of the flower becomes receptive to pollen after the anthers have released their pollen thereby reducing self-fertilization. However, open-pollination primarily by insects such as honey bees and leafcutter bees carrying pollen from one flower to the next is common. Optimum seed production occurs when 12 to 24 hives per hectare (5 to 10 hives per acre) of hives are moved to the edge of seed fields. In some years, when dry and hot weather prevails, flower nectar production becomes concentrated and unattractive to bees. In these instances, the bees move to other crops and pollination does not occur. High temperatures also lead to death of flowers and seed abortion. To minimize this problem, misting of onion seed crops by overhead sprinklers increases seed yields when temperatures exceed 38oC (100F). In 1925, male sterility in onions was discovered and this finding quickly led to the development of a hybrid onion seed industry.
In the production of hybrid seed, the ratio of female to male rows varies but usually is 8 to 2 or 10 to 2. For pollination to be successful, the female and male lines must flower at the same time. Two approaches are used to ensure that this occurs: 1) the parent lines can be planted at different times or bulbs can be harvested and 2) differences in storage temperature can alter the time of flowering after replanting of the bulbs in the Spring. After pollination, the male plants must be removed by mowing or disking to ensure that seed heads from the male and hybrid do not become mixed during harvest as a consequence of lodging.
Seed
The seed is three sided, 3.0 to 3.5 mm (0.12 to 0.14 inches) long. Its surface is wrinkled, dull and greasy looking, black, and has a depressed scar at one end (Figure 00.1). Within the seed, the embryo is 6.0 mm (0.24 inches) long and 0.4 mm (0.02 inches) in diameter and is curved in a complete circle. Most of the embryo is comprised of a single cotyledon to which is attached a very small shoot apex, first foliage leaf, and short primary root. It is embedded in a tough thick-walled endosperm. The 1,000 seed weight of onion is 3.6 g (126,000 seeds per pound).
Seed Production
Dry onions are produced throughout the United States. However, the climatic conditions for onion seed production are more demanding than those for bulb production and are concentrated in the dry areas of the West. Most of the onion seed produced in the United Sates comes from the Snake River Valley of Idaho and Oregon, the Willamette Valley and Madras regions of Oregon, the Columbia Basin of Washington and the Salinas Valley of California. In 1991, 1.6, 3.2, and 10.5 hectares (4, 8, and 26 acres) of foundation, registered, and certified seeds were produced, respectively.
Onion seed production can be open-pollinated or hybrid. Since hybrids have greater uniformity, increased yields, and greater disease resistance, most seed produced today is hybrid. These can be produced either by a bulb-to-seed or seed-to-seed method. The bulb-to-seed method is when onion bulbs are grown, harvested in the fall, stored, and replanted in the Spring. The cold vernalization requirement is experienced during bulb storage in the Winter. This method is more expensive than the seed-to-seed method but it also permits a grower to easily discard off-types, diseased or otherwise undesirable bulbs. It is practiced primarily for maintenance of onion stock seed. The seed-to-seed method is when seed is planted in the Summer, carried over through the Winter as bulbs in the field, followed by flowering in the Spring. This method results in higher seed yields than the bulb-to-seed method because there are more plants per hectare (acre) and less time is invested in bulb establishment. While overall costs are less than the bulb-to-seed method, careful attention must be given to pest control. Roguing programs to eliminate off-type plants are also required. Because this method is the most common approach to onion seed production, it will be described in the following discussion.
Tillage. An onion field should not be used for seed production more than once every four years. The field must be free of perennial weeds and soilborne diseases. The soil should be friable, fertile, well supplied with humus, and well drained. The soil should be plowed to a depth of 15 to 20 cm (6 to 8 inches), harrowed, and worked until a uniform firm seedbed 10 to 15 cm (4 to 6 inches) deep is established.
Planting. Onions seeds are planted 1 to 3 cm (1/2 to 1 inch) deep in rows 50 to 75 cm (20 to 30 inches) apart dy drills at a rate of 4.5 to 6.7 kg per hectare (4 to 6 pounds per acre). Higher seeding rates and closer row spacings have, in some cases, led to higher yields. Planting should occur as soon as the danger of frost is over. If the fields are irrigated or rainfall is common, row spacings of 30 to 45 cm (12 to 18 inches) can be used.
Fertilization. The objective of fertilizing onion seed crops is to ensure that the onion bulbs reach adequate size before the onset of Winter in order that they can produce vigorous seed stalks the following Spring. Onions respond well to fertilizers because the root structure is limited and shallow. They grow best on soils that are slightly acidic (pH 6.0 to 6.5). Mineral soils should be amended with organic matter to assist in moisture retention and improve the soil physical structure. The equivalent of 56 kg per hectare (50 pounds per acre) of nitrogen should be residually available in the soil or the equivalent applied broadcast before planting. Additional applications of nitrogen per hectare (acre) can be applied in one or two sidedressings to make it easily available to the roots. Both phosphorus and potassium should be broadcast at the rate of 67 kg per hectare (60 pounds per acre) prior to planting. Later applications, if needed, can be sidedressed. Fertilization of nitrogen and potassium is not recommended at flowering because these make the flower nectar unattractive to bees.
Minor element deficiencies occur in onion, particularly on muck soils. Copper deficiencies result in thin, poorly colored scales that have a poor bulb storage life. This can be corrected with the application of 224 kg per hectare (200 pounds per acre) of copper sulfate. This treatment lasts for several years. Magnesium deficiencies result in leaf chlorosis and are often encountered on muck soils that are alkaline or have recently been limed. Manganese deficiencies cause a gradual interveinal yellowing, twisting, curling, and stunting of leaves. Manganese sulfate at 168 kg per hectare (150 pounds per acre) mixed with the broadcast fertilizer alleviates this problem.
Irrigation. Seed onions produced in the dry West require irrigation because of their shallow roots and high demand for water. An onion crop can use 1 1/2 to 2 1/2 acre feet of water during its development and maturation. Soil moisture should always be above 65% field capacity. In areas where irrigation is practiced, seedbeds are usually irrigated immediately after planting. Then, one to three irrigations are necessary after seedling emergence to ensure rapid, continued growth. As the plants initiate bulbing, irrigation is stopped and the soil allowed to dry.
Weed and Pest Control. Onions are poor competitors against weeds because of their slow growth, small stature, shallow roots, and lack of dense foliage. Weeds interfere with harvest and can become contaminants of the crop seed. The most effective weed control measures include crop rotations, cultivation, and use of selective herbicides. A short fallow period before planting also permits an opportunity to use tillage and non-selective herbicides to control weeds. Cultivation is one of the most successful weed control techniques in onion. Weeds can be destroyed during seedbed preparation. After planting, cultivation every one to two weeks until bulbing occurs is useful. This practice not only suppresses weeds but also loosens the soil after it has hardened following irrigations and rains. Initial cultivations should be shallow since the onion roots are close to the soil surface and damage to the crop can occur.
Winter annuals, summer annuals, and perennials are troublesome weeds in onion fields. Winter annual weeds are London rocket, shepherdspurse, sowthistle, prickly lettuce, and wild oats. Summer annuals include barnyardgrass, yellow mustard, lambsquarters, pigweed, purslane, and sunflower. Persistent perennial weeds are yellow nutsedge, quackgrass, Canada thistle and field bindweed.
Onions are subject to many of the same fungal and insect pests that invade other crops. Scape and umbel blight caused by Botrytis allii infects onion seed stalks just below the umbel causing it to be girdled and topple which reduces seed yield and quality. Downy mildew (Peronospora destructor) is a problem when cool, wet weather prevails during flowering. It causes chlorotic lesions on the leaves and seed stalks. Pink root caused by Pyrenochaeta terrestris is a soilborne pathogen that infests onions at any stage of development and causes the roots to turn pink followed by brown to black and then death. It is suspected when the above ground portion of the onion plant shows stunting and reduction in vigor. Fusarium basal rot (Fusarium oxysporum) is detected when plants show a progressive yellowing and die back from the tops of the leaves. Infected plants have roots that are dark brown, flattened, hollow, and transparent and the stem plate is brown, rotted, and discolored. This disease reduces seed yields.
There are two common insect pests of onions: thrips and maggots. Onion thrips (Thrips tabaci) feed on leaf surfaces during warm, dry weather causing the leaves to turn white or silver. They also feed on the flowers of seed crops causing reduced seed set and smaller seeds. When thrip damage is suspected, it is best to examine the angles of leaves since this is a common hiding place for these small insects. Control of thrips is difficult in seed crops because use of insecticides can simultaneously harm pollinating bees. Onion maggots (Delia antigua) are the larvae of a small fly that lays its eggs in the soil. The eggs hatch to produce the larvae which attack the onion plants at any age but do most damage in the Spring by invading germinating seeds and seedlings. Seedlings are initially stunted, then yellow and die.
Harvesting, Drying, and Threshing. Although all seed heads on an onion plant do not mature simultaneously, there still is usually one harvest in the seed field. This is accomplished by harvesting the seed heads at about 30% moisture (the heads have some opened capsules with black, ripened seeds exposed) by hand cutting 10 to 15 cm (4 to 6 inches) of the flowering stem below the umbel. When cutting, the umbel is supported in the palm of the hand and held between the fingers to avoid seed shattering. The seed heads are placed into a burlap sack, loaded onto trucks, and hauled from the fields. They should not be left in the sacks for more than 1 to 2 days since heating of the mass occurs which is detrimental to seed quality. The heads are dried by forced air in boxes or large bins or by spreading in a shallow layer on a clean surface exposed to the sun. The seeds are dry enough for threshing when the capsules and small seed stems are brittle and readily break when rolled in the palm of the hand. If seeds are dried with heat, 32oC (90F) until the moisture content is below 18%, then at 38oC (100F) until the moisture content is below 10%, and at 43oC (110F) until the storage moisture content is attained. Threshing is done with a combine. Yields range from 560 to 784 kg per hectare (500 to 700 pounds per acre) for open pollinated lines and 336 to 1,120 kg per hectare (300 to 1,000 pounds per acre) for hybrid lines.
Conditioning. After threshing, onion seeds are conditioned to eliminate weed seeds, light seeds, and chaff. Initial cleaning is with an air-screen cleaner followed by a gravity table. In some cases, the light separated seed is washed. Heavy, good seeds sink while light, poor seeds float. The washing treatment should not exceed three minutes since the seeds rapidly absorb the water and quality is subsequently reduced. The heavy seeds are retained and immediately spin dried in a centrifuge and further air-dried. The seed is dried to a moisture content of 12% or less prior to storage.
Storage. Onions possess one of the most rapidly deteriorating seeds among major crops. They quickly lose complete viability in less than a year when stored under hot, humid conditions. If properly stored, however, viability of onion seed can be retained for long periods and low seed moisture content and low storage temperature favor longer storage life. Of these two parameters, seed moisture content is the easiest to adjust. Onion seeds stored at 6.0% moisture content have retained viability up to three years. Small lots of valuable breeding onion seed material are often stored in desiccators over calcium chloride. Under these conditions, their moisture content will equilibrate to about 2.0% and germination can remain high for as long as nine years in storage.
Bassett, M. J. 1986. Breeding Vegetable Crops. Avi Publishing, Westport.
Comin, D. 1946. Onion Production. Orange Judd Publishing, New York.
George, R. A. T. 1985. Vegetable Seed Production. Longman Press, Essex.
Jones, H. A., and L. K. Mann. 1963. Onions and Their Allies. Botany, Cultivation, and Utilization. Leonard Hill, New York.
Rabinowitch, H. D., and J. L. Brewster (eds.). 1990. Onions and Allied Crops. CRC Press, Boca Raton.
Cabbage
Introduction
Most wild forms of cabbage are found along the coasts of the Mediterranean Sea which is considered their point of origin. Greek and Roman civilizations were the first to recognize the value of cabbage. Its ease of growth and excellent storability ensured its place as an important crop. Initially, these cultures used cabbage for medicinal purposes. Cabbage juice was employed as a gargle against hoarseness and the leaves covered wounds and ulcers to hasten healing. Later, cabbages were eaten raw as well as cooked. By the time the Greek and Roman empires expanded into Europe, cabbage was considered a staple that was carried with them and introduced throughout the continent. European settlers brought cabbages in the 16th and 17th centuries to North America. By the 1700s, cabbage was a common crop grown by the colonists.
There are three major types of cabbage classified according to market demand: fresh market, storage, and sauerkraut. Fresh market types usually form conical to globe heads and have a range of maturities from 50 to 95 days. Storage types are slow in growth and have a firm head with thin, finely veined leaves. Sauerkraut types form large heads that mature in about 130 days. Cabbages are used in a variety of ways from raw in salads and soups, boiled or steamed in mixtures of vegetables and cabbage rolls, to pickled in sauerkraut. Nutritionally, cabbage is a good source of vitamin A and possesses moderate amounts of vitamin C and the B-complex vitamins. It also has high levels of potassium and some calcium and phosphorus.
Plant Development
Because cabbage is a biennial plant, the first year of growth requires the production of a vegetative head followed by flowering and seed production in the second year. It is considered a cool-season crop which grows best between temperatures of 10 and 25C (50 to 77oF).
Vegetative. The cabbage seedling often produces a red hypocotyl with two notched cotyledons and a tap root with lateral roots. The first three leaves have a petiole while later leaves are completely sessile. The leaves are unique from many other crops because they are coated with a layer of wax. The first leaves unfold normally but later leaves only partially unfold and do not expand as much as earlier leaves ultimately forming a head (Figure 00.4) that varies from flat-topped to long-oval in shape.
Reproductive. Cabbage is not photoperiodically sensitive but the apical meristem of plants that have begun to form a loose head must be exposed to four to six weeks of 4 to 7C (39 to 45oF) for vernalization to occur eliciting production of a reproductive apical meristem. Vernalization of the seed or seedling does not cause flowering. To permit the seed stalk to penetrate the tightly oppressed leaves of the cabbage head, the head is often quartered by a knife. The seed stalk then attains a length of 1 to 2 meters (39 to 78 inches). The inflorescence is a raceme on which are born four petaled, yellow flowers typical of the Brassicaceae (Figure 00.5). The flowering process starts from the bottom of the raceme and moves to the top. This often results in racemes on which mature siliques are found on the bottom with unopened flower buds on the top causing an uneven setting and ripening of seed on the plant. The flowers are insect pollinated. When hybrid seed is produced, this normally occurs in a 2:2 ratio of male:female parent rows. After pollination, elongation of a dry pod called a silique containing 10 to 30 seeds is completed after three to four weeks. Dehiscence occurs when the two silique valves split from the point of attachment to the plant upward leaving the exposed seeds attached to the placenta.
Seed
The cabbage seed is almost round, 2.0 to 3.0 mm (0.08 to 0.12 inches) in diameter, reddish brown to grey to black, with a discernable radicle groove and hilum. The seed coat possesses a small reticulated surface (Figure 00.6). The embryo is large, folded and little endosperm is present in the mature seed. The 1,000 seed weight is 3.6 g (126,000 seeds per pound).
Seed Production
Cabbage is produced throughout the United States with Texas, Florida, California, and Georgia leading the production of fresh cabbage and New York, Wisconsin, and Ohio producing the largest quantity of cabbage for sauerkraut. Seed production in the United States occurs principally in the Skagit Valley of the Puget Sound in Washington state. This area is not only noted for its mild climate and soils favorable for cabbage seed production, but also the abundance of syrphid flies and bees which are excellent pollinating insects.
Tillage. Cabbage requires an abundant supply of water. As a result, it typically grows best on soils that are heavy and contain a high level of organic matter. The pH should be between 6.0 to 6.5 for optimum growth. The planting bed for cabbage should be deep and well worked to enable rapid seedling root development and access to water. This can be accomplished by disking the soil. Disk cultivators are useful in preparing seed bed rows about 7 to 10 days before planting because they further pulverize the soil. Additional smoothing of heavy soils, if required, can be done with a harrow.
Planting. Normal cabbage seed production begins with planting in the Summer followed by the half-grown plants over wintering, flowering in the Spring, and maturation and harvest of the seeds in the Summer. All stock seed is tested or hot water treated for black rot prior to planting. Cabbage is planted initially in beds or the greenhouse. The seedlings are then transplanted 45 days later in the field in rows 107 cm (42 inches apart) with a spacing between plants of 40 cm (16 inches) to produce a population of 3,645 to 4,050 plants per hectare (9,000 to 10,000 plants per acre).
Hybrid seed production is more labor intensive. Each parent is normally staked and stringed to allow improved pesticide spraying and facilitate subsequent hand cutting of the inflorescence. The stakes and strings insure that the parents stay separated. The nick between female and male parents must be managed to minimize the number of inbreds and maximize the number of hybrids produced. To accomplish this, bees are placed in the field at 5% bloom. In some cases, plants are topped if one parent comes into bloom earlier than the other.
Fertilization. Nitrogen during the establishment of cabbage plants leads to a softening of tissues and an increased susceptibility to frost. As a result, nitrogen is usually omitted at planting with phosphorus and potassium applied at rates of 100 and 150 kg per hectare (90 and 134 pounds per acre), respectively. If nitrogen is necessary, it is split as a sidedress in the Fall and added in the Spring just before bolting. This ensures maximum growth of the sidebranches which enhances seed yield. Too much nitrogen in the Spring or Fall enhances soft tissue development, increases the risk of lodging, and promotes extended flowering which may cause increased inbreds in hybrid seed production. Cabbage is not as sensitive to micronutrient deficiencies as are other members of the Brassicaceae.
Weed and Pest Control. Chemical and mechanical weed control programs are practiced with cabbage. Seedbed preparation should eliminate most weeds. When weeds are persistent, however, a preplant or preplant incorporated application of herbicides can be used. The extensive and shallow rooting system of cabbage limits deep cultivation to control weeds, particularly when the soil is hard. Only when weeds are present should large plants be cultivated and all cultivation should be discontinued after heading to avoid further plant damage. The most serious weeds include tiny vetch, wild geranium, and wild turnip which must be controlled or removed prior to harvest.
The cabbage plant is subject to a host of diseases. The most serious of these are downy mildew and alternaria which must be controlled up to harvest. This is accomplished by spraying the hybrid seed fields 3 to 5 times with appropriate fungicides. Other diseases include black rot, yellows, club root, and blackleg. Black rot (Xanthomonas campestris) first appears as a yellowing of leaves and blackening of veins. Later, the plant becomes dwarfed and the heads one-sided. Yellows (Fusarium oxysporum f. conglutinans) occurs in plants grown in warmer climates and is indicated when the plants turn yellow, the leaves fall off, and the plants die. Club root (Plasmodiophora brassicae) produces malformed, club-like roots. Blackleg (Phoma lingam) attaches to the stem of young plants causing dark shrunken areas that leads to plant wilting and death. Black rot and blackleg are diseases carried on or in the seed. These can often be hot-water treated to eliminate the pathogen(s). Care must be taken not to kill the seed in the hot water treatment. Seed treatments, where approved, can also be very effective against these diseases.
Cabbage is susceptible to an array of insects and other pests. Cabbage aphids and pod borers are the most serious insect pests of cabbage seed production. These sucking insects possess a protective waxy coating similar to cabbage leaves. Insecticides must contain an appropriate detergent or sticker to control these pests. Other insects include cabbage worms and the cabbage looper. The harlequin bug is another sucking insect that is about 1.0 cm (0.4 inches) long and is mottled red, black, or yellow in appearance. Root knot in cabbage is caused by a parasitic eelworm that produces irregularly shaped galls on the root that sometimes can be confused with club root. Rotations or soil fumigations are the best control measures for this problem.
Irrigation. Cabbage demands a high and consistent quantity of water for optimum development. This is attributed to its large leaf surface area and resultant loss of water through evapotranspiration. The soil water level should be maintained at 60% field capacity or higher throughout crop growth. After bolting, irrigation should be discontinued.
Harvesting, Threshing, and Drying. Generally, each parent is harvested separate from each other. However, in some cases when both parents are stongly self incompatible, they are harvested together. The uneven maturation of siliques on the cabbage raceme and the strong tendency for the seed pods to shatter creates harvesting problems for obtaining maximum yields. For these reasons, harvesting of hybrids is done by hand by cutting the seed stalks with a knife or shears. Open pollinated varieties are swathed by machine. In both cases, this is started when the basal siliques turn yellow and the enclosed seeds are brown. Harvesting in the morning after a dew tends to reduce the level of seed shattering. The seed stalks are placed in windrows for continued drying or on canvas to collect the fallen seeds from dehisced siliques. After the plants are dried for 10 to 14 days, they are threshed in a combine. Combine cylinder speed and concaves should be adjusted to minimize damaged seed. Cylinder speeds should not exceed 700 rpm because of the tendency for dried seeds to easily split.
Conditioning. Removal of remaining chaff and straw from the cabbage seed can be done on an air-screen cleaner. Separating splits from whole seeds is accomplished with a spiral separator. In addition, cabbage seed is normally sized into several sizes for better precision planting.
Drying and Storage. Cabbage seed can remain germinable for four to six years if properly dried and stored at a low moisture content in a low relative humidity (50% or less) environment. The seed should be at a moisture content not exceeding 6% at storage. When drying is necessary, the temperature should not be above 45C (113oF) with lower moisture seeds more resilient to higher temperatures.
Bassett, M. J. 1986. Breeding Vegetable Crops. Avi Publishing, Westport.
Nieuwhof, M. 1969. Cole Crops. Botany, Cultivation, and Utilization. Leonard Hill, London.
Lettuce
Introduction
The exact origin of lettuce is unknown. Paintings of a cos-like lettuce are found in Egyptian tombs constructed about 4500 B.C. From Egypt, it is believed that lettuce quickly spread around the Mediterranean basin and was adopted as a popular vegetable by the Greek and Roman civilizations. The Romans further developed differing lettuce types that were broad-leaved, nonheading, nonspiny, had decreased latex content, and were resistant to early seed stalk formation. Eventually, lettuce spread throughout Europe where the present six types of lettuce were selected and produced. It was brought to the New World by the Spanish settlers and was marketed as a vegetable crop in seed catalogues by 1806. By the 1880s, over 20 lettuce varieties were available for production in North America.
Lettuce is unique among vegetable crops because it is used exclusively as a fresh, unprepared component of salads. There are six morphological types of lettuce: crisphead, butterhead, cos, leaf, stem, and Latin. Crisphead lettuce, often called iceberg, is most common in the United States. This type of lettuce produces large, heavy (up to 1.0 kg/2.2 pound) heads with tightly folded, green outer and white or yellowish inner leaves. Butterhead lettuce forms relatively small, loose heads with broad, oily, crumpled, soft-textured leaves and is commonly consumed in northern Europe. Cos lettuce, often called romaine, has elongated leaves that are dark or light green with heavy ribs forming a loaf-shaped head and is most popular in southern Europe and the Mediterranean basin. Leaf lettuce produces a variety of leaf types and color but all are characterized by forming a rosette of leaves that do not form a head. This lettuce is used primarily in home gardens because of its ease of production. Stem lettuce is one of the few lettuce types that is not consumed raw. It produces leaves that are coarse, peeled, and cooked. This lettuce is grown primarily in the Orient. Latin lettuce resembles butterhead lettuce in forming a loose head but the leaves are elongated and similar to cos lettuce. This lettuce is popular in the Mediterranean countries and South America.
Lettuce is relatively low in essential nutrients compared to other vegetables. It is known for its low calorie level which is attributed to a very high (94 to 95%) water content per leaf. Lettuce contains moderate amounts of vitamins A and C and calcium as well as some phosphorus, iron, sodium, and potassium. Despite its low nutritional value and lack of diversity in food preparation, lettuce is so important as a main component of salads that it ranks fourth in vegetable popularity behind only tomato, citrus, and potato.
Plant Development
Like sunflower, lettuce is a member of the Asteraceae family. The cultivated lettuce is closely related to and can easily cross with the common wild lettuce (Lactuca serriola). Both produce high levels of latex in the stem and leaves and this trait is responsible for the derivation of the generic name Lactuca meaning "milk-forming". Lettuce is a cool-season crop that requires abundant moisture and ample sunlight for optimum development. Cool nights are important to establish normal head development and high temperatures promote seed stalk formation.
Vegetative. Lettuce is an herbaceous annual that differs from its wild-type relative by producing a rosetted, short-stemmed vegetative plant. The leaves vary widely in shape from round to obovate with varying degrees of cupping and indentation. The root system develops a deep, penetrating taproot with the major absorbing surface being in the top 30 cm (12 inches) of soil.
Reproductive. Lettuce varieties are either long-day sensitive or day-neutral and high temperatures (above 18C/64oF) promote bolting (a change from the vegetative state to flower formation). In most cases, the seed stalk or inflorescence rapidly emerges from the head or leaf structure. In some cases, particularly with crisphead lettuce, the head is so tight and firm that the seed stalk is unable to emerge from the head. To enhance seed stalk emergence, three methods are used: 1) deheading - physical removal of head leaves surrounding the seed stalk, 2) slashing and quartering - cutting the seed head at its apex in an "x-shape" that loosens the tight-fitting leaves over the seed stalk, 3) application of growth regulating chemicals - application of gibberellic acid (20 to 500 ppm) to lettuce leaves at the three to five-leaf stage promotes bolting.
Lettuce produces a panicle with many individual compound flowers formed into a capitulum inflorescence. As a result, many flowers are formed at differing stages of development. Over 90% of the seed yield comes from flowers that open during the first 35 days of the 70 day flowering period. Seeds produced during this time are bigger and heavier than those produced later. The flowers are yellowish or white yellow and consist of several florets borne on a flat, naked receptacle. The corolla rays are five toothed and truncate at the end. The flowers contain fused stamens that release pollen at the time of style elongation through the anther sheath ensuring self pollination. Flowers open only once for a short period in one morning and then never reopen; a process that also reduces access to foreign pollen.
Seed
The lettuce fruit (seed) is an achene. It is lanceolate, flat, and the cream- to back-colored fruit wall has about 8 lengthwise ridges. The achene is 4 to 5 mm (0.16 to 0.20 inches) long, beaked, and topped with a number of large soft, capillary, white or brown, delicate pappi (Figure 00.7). The 1,000 seed weight of lettuce seed is 0.6 to 1.0 g (453,000 to 755,000 seeds per pound) dependent on cultivar.
Seed Production
Approximately 70% of the lettuce seed in the United States is produced in California. This is due to the availability of favorable soils, climate, dependable water supply, and access to available labor since this crop requires a high labor input.
Tillage. Lettuce can grow on soils that range from fine sandy loams to clays to mucks but they must have a high organic matter content and adequate nutrients available in the upper 25 to 30 cm (10 to 12 inches) of soil. Since calcium is an important element for rapid leaf growth, the soil should be at a pH of 6.0 to 6.5 to ensure its availability. Typical vegetable tillage practices should be followed. The soil should be plowed to 30 cm (12 inches) as early in the season as possible. The seedbed is then smoothed by disking or harrowing and then leveled with a spiked harrow. When irrigation is necessary, elevated seed beds are prepared and then dragged flat.
Planting. High quality lettuce seed should be used in planting for greater uniformity of growth. It is often pelleted to facilitate precision planting. The minimum soil temperature required for germination of lettuce seed is 1.7C (35oF) and the optimum temperature is 24C (75oF). High temperatures of 33 to 35C (91 to 95oF) inhibit germination. When lettuce is planted under high temperatures, it is often planted at night and irrigated to induce germination before the high day temperatures are experienced. The seeds also can be osmoconditioned in growth regulators such as kinetin and gibberellic acid and redried, or pregerminated in a gel then planted by fluid drilling. Lettuce seed is planted 1 cm (0.4 inches) deep at a rate of 2.4 kg per hectare (2.0 pounds per acre) and spaced 5.1 to 7.6 cm (2 to 3 inches) apart in rows 38 cm (15 inches) apart. The plants are thinned four to six weeks after emergence to 25 to 30 cm (10 to 12 inches) apart to give a population of 74,000 plants per hectare (30,000 plants per acre).
Fertilization. On mineral soils, nitrogen is usually limiting and should be applied at planting, after thinning, and at bolting. Phosphorus and potassium should be applied at planting because of the crop's inability to extract these elements from deeper portions of the soil. Muck soils have a lower nitrogen requirement but require phosphorus and potassium. Thus, while it is difficult to generalize on fertilizer application rates, a ratio of 3-2-2 usually provides suitable crop growth. Other elements do not often limit lettuce growth. However, under high or low moisture situations, a condition known as calcium-related leaf tip burn (similar to blossom-end rot in tomatoes) occurs.
Weed and Pest Control. Crop rotations with tomatoes, cucurbits, sweet corn, spinach, beets, and carrots assist in minimizing the buildup of lettuce weeds and diseases. Weed control is accomplished by both cultivation and herbicide application. Since herbicides are increasingly regulated, particularly in California, good cultivation practices should be encouraged and practiced. These begin with proper land preparation before planting. After planting, if weeds are present, a shallow cultivation not more than 5.1 to 7.6 cm (2 to 3 inches) deep may be necessary. This practice cuts the weeds at or immediately below the soil surface, leaves a shallow mulch on the soil, and protects the small developing roots of the lettuce plant from mechanical damage. It may still be necessary for hand weeding between rows at later stages of crop development. Common weeds of lettuce fields are lambsquarter, pigweed, henbit, chicory, groundsel, sowthistle, and chickweed.
Lettuce is prone to a host of pathological disorders. From the perspective of the seed producer, lettuce mosaic virus is the most serious of these because the virus is carried in the seed. Infected seeds germinate to produce plants that are stunted, have yellow leaves, and often fail to produce a harvestable head. More importantly, sap sucking insects such as aphids feed on infected plants and serve as vectors for dissemination of the disease to healthy plants throughout the field. Seed production on infected plants can be reduced by 68% and seed weight by 62%. The virus is transmitted to 1 to 15% of the seeds maturing on the infected plant. As a result, basic seed stocks are often produced in insect-free structures or areas where temperatures are too high for aphid attack. Most major lettuce producing areas require planting only of mosaic-indexed seed set at some local arbitrary level such as 0.5%. This level is determined by seed companies planting the seeds in flats in greenhouses and growing the seedlings to the three to four leaf stage. Infected seedlings are stunted and lighter than healthy seedlings. The use of mosaic-indexed seed has been very successful. Estimates of reductions in losses of 95 to 100% have been reported as a result of this practice
Other common diseases of lettuce are found. Downy mildew occurs on lettuce plants grown at low temperatures and high humidity. Symptoms appear first on the lower leaves as large yellow to brown lesions. Sclerotinia rot is common under cool, moist growing conditions or when the crop has been excessively irrigated on heavy soils. It is characterized by an initial wilting and collapsing of lower leaves followed by the entire head which eventually rots at the base. Botrytis or grey mold can cause damping-off of seedlings or head rotting in more mature plants. Powdery mildew occurs under warm, humid conditions and is evidenced by powdery lesions on the leaf surfaces that ultimately cause leaf death. Aster yellows is a mycoplasma disease that causes a blanching and yellowing of the heart leaves and infected plants often bolt to produce a seedstalk that has bushy outgrowths of abnormally developed buds and sterile flowers.
Insects can also create problems in lettuce production. The cabbage looper is a 2 cm (0.8 inch) long green caterpillar that feeds on leaf tissue. The green peach aphid is not only the principal vector of lettuce mosaic virus but it also feeds on the plant and reduces its vigor and yield. This pest is difficult to control because it also colonizes a number of weeds common to lettuce fields. Other aphid pests are the root aphid and lettuce seed stem aphid. The corn earworm, beet armyworm and yellow striped armyworm also are destructive insects.
Irrigation. From seedling to harvest, lettuce demands a high quantity of water. This can be provided either by sprinklers or irrigation. The usual practice is to supply early water by sprinklers and then shift to furrow irrigation when the plant reaches the rosette stage. This allows less use of water since the sprinklers can be metered to permit moisture penetration to 30 cm (12 inch) depth per setting while the lettuce plants are small. Irrigation, however, enhances the level of water related pathogen problems. Irrigated fields should be tiled so that the water can drain through the soil thus minimizing waterlogging conditions.
Irrigation for lettuce seed production is generally stopped after flowering. In particular, sprinkler irrigation after flowering causes loss of seed yield since the falling water droplets dislodge the maturing seed off of the flower receptacle.
Harvesting and Threshing. A period of 12 to 21 days is required from flowering to mature seed formation. However, because of the nature of the flowering response, not all seeds mature simultaneously on the plant. As a result, lettuce plants are harvested when approximately half of the seed heads have "feathered" (seed pappus is fully expanded and dry). They can be either machine or more commonly hand harvested. If machine harvested, the plants are cut and placed in windrows until the majority of the seeds have matured and then threshed by combine. Cutting the plants by machine is best done in the morning while dew is still on the flowers. This minimizes seed shattering and reductions in yield. Hand harvesting is accomplished by cutting the seed stalks and vigorously shaking the seed heads in a canvas bag or plastic bucket. This process can be repeated in two or three days to permit additional seeds to mature and increase yield.
Conditioning. Harvested lettuce seeds can be as much as one and a half times their dry weight due to contamination with high moisture flower and vegetative plant parts. These must be removed by cleaning as soon as possible to minimize seed deterioration. Lettuce seed is often cleaned on an air-screen cleaner followed by upgrading on a disk separator or an indent cylinder.
Storage. If properly stored, lettuce seeds retain their viability for four to five years. Seeds should be at a moisture content no higher than 7%. Storage deterioration of lettuce seeds is often manifested by an abnormality known as "cotyledonary necrosis". Deteriorated seeds exhibit reddish necrotic spots on the cotyledons. The cause of cotyledonary necrosis is still not known.
Bassett, M. J. 1986. Breeding Vegetable Crops. Avi Publishing, Westport.
George, R. A. T. 1985. Vegetable Seed Production. Longman Press, Essex.
Grogan, R. G. 1981. Control of lettuce mosaic with virus-free seed. Plant Disease Reporter 65:5.
Ryder, E. J. 1979. Leafy Salad Vegetables. Avi Publishing, Westport.
Carrot
Introduction
Carrots originated in Afghanistan where they expanded to the eastern Mediterranean area. Arabs brought carrots with them during their conquests of Europe in the 10th to 12th centuries. European settlers brought carrots to the New World in the 1600s. Today, carrots are grown worldwide and its wild relative, Queen Anne's lace, is equally well known.
In early history, many of the most important medicinal plants came from the Apiaceae family of which carrot is a member. Resultantly, it is not surprising that carrot was originally used as a medicinal rather than food source. Historical writings in the 12th century extolled the virtues of carrots as a sweet root crop that could also be prepared as a juice. By the 16th century, breeding of carrots had produced a crop that showed extreme variation in size, color, and shape.
Carrots can be prepared in a variety of ways ranging from baked, boiled, steamed, and diced but their primary consumption is as a fresh product. Nutritionally, they are an excellent source of vitamin A and have moderate amounts of potassium.
Plant Development
Carrots can be grown almost anywhere as long as the growing season is relatively cool. Their optimum growth occurs between 16 to 18C (61 to 64oF). Botanically, the carrot is an herbaceous biennial that is characterized by vegetative production in the first year of growth followed by reproduction growth after vernalization in the second year.
Seed. The seed is fawn to brown, oblong, 3 to 4 mm (0.12 to 0.16 inches) long and 1 to 2 mm (0.04 to 0.08 inches) wide with one side flattened and the other possessing three to five ribs that are barbed (Figure 00.10). The embryo is extremely small relative to the size of the seed. The 1,000 seed weight is 0.8 g (567,000 seeds per pound).
Vegetative. Carrots form a vegetative structure consisting of an above ground rosette of leaves and a large, deep tap root. The leaves are double compound with the lower leaves of the rosette being more divided and larger than the upper leaves.
Reproductive. The carrot plant generally requires vernalization for flowering. This effect is not related to root size since even small rooted plants, if vernalized, will bolt and flower. After vernalization, the rosette bolts to form a stem that is 61 to 122 cm (2 to 4 feet) high with branches that produce flowering heads or umbels. The appearance of umbels is not uniform. The first and largest umbel to flower is the last umbel on the main flowering stalk and is known as the primary or king umbel. Secondary umbels are formed at the terminus of branches from the main flowering stem and flower in a sequence from the top to the bottom of the inflorescence. Tertiary umbels originate on secondary umbel stems. Figure 00.9 illustrates this flowering sequence.
Seeds produced on primary and secondary umbels vary in quality. Primary umbels produce seeds that are heavier, more mature, and of higher quality (germination percentage is higher) and vigor (germination rate is faster) than secondary umbels. As a result of this relationship, many seed companies have increased planting density in order that more primary umbels are produced for harvest. In other cases, to enhance seed yields, seed companies have established management practices that focus on harvesting the king, secondary, and tertiary umbels. This allows them to grade seeds into various sizes from large to medium to small.
Hybrid carrot seed production is possible. Male sterile lines have been discovered and in hybrid seed production the common ratio of female to male rows is 6:2 or 6:4. Unfortunately, the production of hybrid seed has required the intensive selective inbreeding of carrots for several generations resulting in inbreds with reduced plant vigor. As a consequence, yields of planted hybrid seed have not been as high as open-pollinated lines. In addition, the male sterile flowers produce smaller petals that are less attractive to pollinating bees causing lower seed set and reductions in seed yield. These factors have led to higher costs for hybrid carrot seed. This, in conjunction with lower crop yield, have continued to make open-pollinated lines the preferred choice of planting seed among carrot growers. However, the commercial fresh market acreage in the United States of the Imperator carrot varieties is almost entirely with hybrid seed. Processing carrots are still dominated by open-pollinated varieties.
Seed Production
Carrot production occurs throughout North America. In the winter, Florida, Texas, California, and Arizona have ideal conditions for carrot production. In the summer, carrots are grown on the muck soils of New York, Wisconsin, and Michigan. These soils are noted for producing straighter, cleaner, and higher yielding roots than inorganic soils.
Carrot seed production used to be located primarily in California. However, some years prevailed when the necessary vernalization temperatures (below 7C/45oF) to induce flowering were not experienced or high temperatures were encountered during seed set that led to empty seeds. Today, most of the carrot seed production occurs in the Columbia Basin of Washington, Madres area of Oregon as well as Idaho which are ideal desert environments for maximum seed yield and quality.
Carrot seeds are produced by two methods: seed to seed and root to seed. In the seed to seed method, the seed is planted direct in late Summer, vernalized in the Winter, flowers initiated in the following Spring, and seeds harvested in late Summer. In the root to seed method, roots raised in beds usually in an area away from commercial production are removed from the soil in the Fall, examined for trueness to type and stored at 1C (35oF) at 95% or higher relative humidity to reduce storage rots in the Winter, and then replanted in the Spring. The plant bed is used as a backup for direct seeded production in the event of Winter kill. Some types of carrots such as high sugar types do not overwinter well so they may be grown in a plant bed. In both cases, the process of removing plants, checking for off-types, storage, and then replanting is labor intensive and expensive. Even so, the root to seed method is required by most seed companies for new varieties, new inbreds and stock seed to allow selection for root characteristics. The seed to seed method, however, is preferred among seed companies for large commercial seed production.
Tillage. Fields are placed according to isolation distances established for each type of carrot. Carrots grow well on most soils but the best growth occurs on deep, friable soils such as sandy loams. Carrots tolerate a wide range of soil pH. Because seedling establishment is a problem in carrot seed production due to the seed's small size and slow germination rate, particular attention is give to tillage operations prior to planting. The seedbed must be clean of weeds, well pulverized, and compact to ensure ready flow of soil water to the seed and yet minimize puddling or crusting after a heavy rain.
Planting. If planting is done for the seed to seed method, row spacings of 50 to 90 cm (20 to 35 inches) are used and 2 to 3 kg of seed planted per hectare (1.8 to 2.7 pounds per acre). If the seed is mechanically precision planted, it is often coated using a ratio of 3:1 finished weight over the raw seed. Seeds should be planted 1.3 to 1.9 cm (0.5 to 0.7 inches) deep every 7 to 10 cm (3 to 4 inches). If the root to seed method is used, the transplants are placed every 20 to 30 cm (8 to 12 inches) apart in rows that are 75 to 90 cm (30 to 36 inches) wide.
Higher plant densities are desired in carrot seed production. Such a practice results in a concentration of umbels in the upper part of individual flower stalks causing a higher proportion of primary to secondary umbels and a more uniform period of seed maturation from umbel to umbel. Normally, bees are used for open pollinated varieties and are required for hybrid seed production. Usually 2 hives per hectare (5 hives per acre) are required.
Fertilization. Because of the wide range of soil types in which carrots grow, a soil analysis provides the best recommendation for fertilizer requirements. As a general rule, the relative proportions of 1-2-2 are used. Less nitrogen is needed on muck soils compared to mineral soils. The nitrogen should be applied as ammonia since other forms cause serious burning. The fertilizers are best applied in a band and to the side of the seed at planting. Vegetative growth is further encouraged, particularly under irrigated conditions, by a second side dressing later in the Summer. In the Spring of the second year, another nitrogen application will stimulate vigorous plant growth prior to flowering.
Weed and Pest Control. Weed control in carrots is almost totally dependent on chemicals. This is because any physical alteration of the firm seedbed dislodges the small seed from the soil and results in poor emergence. In the past, herbicidal solvents such as Sotddbard's solvent at 702 to 749 liters per hectare (75 to 80 gallons per acre) were applied after the first true leaves were formed and before the four-leaf stage. This treatment delayed weed growth until the carrot's foliage was large enough to shade competitors. Another approach was to use shielded flame throwers to burn the weeds. Today, weed control in carrots is practiced using a combination of approved herbicides. Weed seeds that are particularly troublesome to remove during conditioning include canada thistle, nightshade, and barnyardgrass.
Because carrots are subject to an array of diseases and insects, crop rotation is one of the most effective production practices to minimize the buildup of these organisms. Common diseases include leaf blights, carrot yellows caused by a leaf hopper transmitted virus, and brown roots which usually infects feeder roots. Of particular note are two distinct diseases caused by the related species of the fungus Alternaria, Alternaria leaf blight and black rot. Both diseases are found wherever carrots are grown and because these fungi survive between crops in residues, both diseases cause significant yield losses where continuous cropping of carrots is practiced. More importantly, from a seed production perspective, both fungi can be carried on or in the seed. Another seedborne disease is caused by the bacterium from Xanthomonas. As a result, most seed companies routinely use standardized seed assays to determine the level of infection of these devastating fungi and bacteria. When infected seed are commercially planted, they can cause 100% crop failure. Troublesome insects are leaf hoppers, aphids, tarnish plant bug, carrot rust fly, and carrot weevil. The major insect pest is lygus bugs which decrease seed germination. In all of these cases a comprehensive integrated pest management strategy is necessary to control lygus bugs, weeds, and fungicidal sprays for alternaria.
Irrigation. All carrot seed production is irrigated. Carrots require an abundant and well distributed water supply for optimum growth. The primary approach is to use rill (flood) irrigation. Fields irrigated overhead should be evaluated for the presence of additional fungicidal sprays. The water table should be kept at 76 to 91 cm (30 to 36 inches) below the soil surface. In inorganic soils, the water can be supplied by sprinklers and furrows. Sprinkler irrigation is most effective during the early stages of germination and emergence when the water supply does not have to be extensive. With later vegetative growth, sprinkler irrigation should be discontinued because of the increase in foliar diseases encouraged by the wet leaves.
Harvesting and Threshing. For hybrid seed production, the male rows are removed and turned into the soil prior to swathing. Harvesting of the seed crop is done by machine. This is initiated when the earliest maturing seed on the primary umbel turns brown and the umbel is brittle. Because of the uneven maturity of the seeds on the umbels and the tendency for shattering, umbels can be sprayed with a polyvinyl acetate adhesive to reduce seed loss. This process, however, is not done in large scale seed production. Harvesting early in the day while dew still remains on the umbel is also helpful. The fields are swathed by machine into windrows. After 3 to 5 days of drying, the crop is havested with modern combines at a seed moisture content of 7%.
Drying and Cleaning. The small size of the carrot seed makes it very responsive to equilibration with a low relative humidity environment. As a result, carrot seed is seldom dried unless wide ranges in seed maturity exist. Since the barbs on the seed reduce flowability and increase seed volume, they are often removed by a debearder. Cleaning of the seed is accomplished using an air-screen cleaner and indent cylinders. Carrot seed is sized into varying lengths and widths for improved precision planting.
Storage. Carrot seed should not be stored at moisture contents above 7.0%. At this moisture content and under appropriate temperature and relative humidity conditions, the seeds remain viable for 3 to 5 years.
Bassett, M. J. 1986. Breeding Vegetable Crops. Avi Publishing, Westport.
George, R. A. T. 1985. Vegetable Seed Production. Longman Press, Essex.
Sugarbeet originated in the Mediterranean. It belongs to the same species as the red or gardent beet. In the middle of the 18th century, it was discovered that sugar from sugarbeet was identical to that found in sugarcane. From that point, countries reliant on sugarcane as their sole source of sugar begainefforts to develop this as an alternative sugar source. France was the first country that to breed a new variety of sugarbeet that could be economically processed for profitable yield. Initially, sugarbeet was introduced in 1831 into the United States in Massachusetts, but this effort was not successful. Later production of sugarbeet in California, however, did succeed and ensured that this crop remains a sable agricultural commondity in North America.
Most sugarbeet seed in the United States is produced in the Willamette Valley of Oregon and in areas of the Intermountain West, particularly Utah, Arizona and Idaho. The primary reason for this location is the favorable harvesting weather and isolation from other sugarbeets which might be a source of genetic outcrossing and contamination. The seed is produced by specialized seed companies associated with different sugar companies under contract with seed growers by methods and standards prescribed by the seed company.
Plant Development
Sugarbeet is a biennial plant that grows vegetatively the first year and produces seed the second year after being vernalized by thermal induction during the Winter. However, when grown as a sugar crop, it is harvested after the first year during which the leaves and the fleshy root are formed. In the first year, it produces a dense canopy of leaves and a large root in which the sugar is stored. In the second year, the sugar is used to form the flowering stalk, flowers, and seed. The leaves are simple with a large blade and petiole. The root is divided into three zones: The top from which the leaves are produced in a dense spiral, the thick fleshy hypocotyl where most of the sugar is found, and the root which becomes a long taproot. The flowers are perfect but incomplete; they have a five part calyx but the corolla is absent. They are produced on a panicle-like spike.
Traditionally, seed was produced by digging up the roots after the first year's growth and storing them in cool, damp storage over the Winter. These propagules, called stecklings, were replanted the following spring to produce seed. However, it was discovered that thermal induction could also be obtained by growing the crop as a Winter annual. By planting in the late Summer or early Fall, seed could be produced the following Summer. This method is used today in North America and most areas which produce sugarbeet seed.
Seed
The sugarbeet "seed" that is planted is actually a single achene, which is a one-seeded fruit with a somewhat inflated, corky outer layer, or pericarp (Figure 00.11). The traditional natural sugarbeet seed unit was a multi-germ "seedball," or a cluster of two to five achenes formed by the joining of several floral units. When planted, each of the achenes germinated, resulting in three to five plants. The resulting stand of plants was much too thick for optimum root and sugar production and had to be manually thinned at great expense to the grower.
In 1948, a single plant was identified which produced single monogerm seeds or a single achene. From this and other sources, new monogerm sugarbeet varieties were rapidly developed and by early 1960 had almost completely replaced multigerm seed. By 1960, open pollinated varieties had also been largely replaced by single cross, three-way cross or double-cross hybrids.
Seed Production
It is important that sugarbeet seed producers have a contract with seed companies or sugar companies for seed production. This assures access to varieties that will be in demand by growers. It also assures them of parent seedstocks (i.e., inbred lines) needed to produce seed of different hybrids. Furthermore, it insures seed growers of technical advice needed for managing the planting of parental and restorer lines. Most importantly, it assures them of a market for their harvested seed.
Hybrid seed is produced in two different ways. Most commercial seed is produced by mixing the male-sterile F1 line as the primary female parent with seed of a multi-germ pollinator in a 10:1 proportion. The resulting crop contains a low percentage of seed that is not hybrid. However, since these are large and shaped differently, they are easily separated from the small flattish monogerm seed during conditioning.
The second method is more expensive, but results in a completely hybrid crop. It is used primarily to perpetuate parent stock seed which will be used in three-way or double-cross hybrids. It is produced by growing the multi-germ pollinator in about four separate rows separated by a skip-row from 16 to 20 female male-sterile rows. The male rows are destroyed after pollination and before harvesting the hybrid seed produced on the male-sterile parent.
The male-sterile lines are perpetuated for further use by backcrossing with their type "O" equivalent. This process insures that the progeny will be male sterile. The type O lines are male fertile lines that do not restore pollen production to the offspring when crossed with male-sterile lines. They are comparable to maintainer lines used in producing hybrid corn.
Tillage. Sugarbeet grows best on slightly acidic fertile loamy soils. Sandy soils do not retain adequate moisture between irrigations and heavy clay soils cause problems at harvesting. Fields are plowed to 20 to 30 cm (8 to 12 inches) deep in the late Summer. If sprinklers are not used for irrigation, the fields are leveled for surface irrigation and a firm seed bed prepared by disking, harrowing, and rolling to minimize soil crusting.
Planting. Planting occurs in late August or early September. Seed is planted with a standard drill in rows spaced 51 to 60 cm (20 to 24 inches) apart at a rate of 13.4 to 16.8 kg per hectare (12 to 15 pounds per acre). Seeds are platned from 2.5 to 3.8 cm (1.0 to 1.5 inches deep. The optimum temperature for germination is 15oC (60oF). Temperatures below -4oC (25oF) kill beet seedlings.
The fields must be isolated from other fields of sugarbeets or garden beets that are allowed to produce seed. This assures genetic purity of the resulting seed. For varieties that are similar in characteristics, isolation of 0.8 km (0.5 miles) may be adequate. However, for elite seedstocks or very dissimilar varieties, isolation of 3.2 to 4.8 km (2 to 3 miles) may be desirable. It is important that the guidelines of the seed company for each variety be closely followed.
Fertilization. Fertility practices vary greatly between seed production areas. In Oregon, the usual practice is to apply 268 kg per hectare (240 pounds per acre) of nitrogen and 112 kg per hectare (100 pounds per acre) of phosphate, along with 56 kg per hectare (50 pounds per acre) of sulfur and 5.6 kg per hectare (5 pounds per acre) of boron. Another 179 kg per hectare (160 pounds per acre) of nitrogen are side-dressed in the Spring, often in two applications.
In Arizona about 56 kg per hectare (50 pounds per acre) of nitrogen are applied prior to planting, along with 67 kg per hectare (60 pounds per acre) of phosphate. Another 112 to 168 kg per hectare (100 to 150 pounds per acre) of nitrogen are added during the Fall and an additional 224 to 336 kg per hectare (200 to 300 pounds per acre) are side-dressed in the early Spring or added with the irrigation water. In Utah 269 kg per hectare (240 pounds per acre) of phosphate and 140 kg per hectare (125 pounds per acre) of phosphate are applied prior to planting and another 67 kg per hectare (60 pounds per acre) of nitrogen are side-dressed in the Spring.
Weed and Pest Control. Good weed control includes cultivation and chemical weed practices in addition to hand weeding. Sugarbeet grows rapidly and soon shades most weeds and prevents them from establishment. Cultivation should be used wherever possible. However, chemical control is the most common method and many chemicals are registered for all seed production areas. Finally, hand weeding is an important additional weed control practice. The most common weeds of sugarbeet seed fields are pigweed, lambsquarter, barnyardgrass, mustard, and wild oats.
Rarely can sugarbeet seed crops be grown continuously on one field. Rather, they are often rotated with cereals, beans, vegetables such as tomatoes and lettuce, and forage legumes to control diseases and root-knot nematodes. Black root is a fungal disease that kills germinating seedlings and reduces plant vigor so that uneven stands and poor yields result. Seed treatment is effective against this disease. The virus disease, curly top, is a particular problem for sugarbeet seed producers in the intermountain area. This disease is controlled almost exclusively through the use of disease resistant varieties. Virus yellows is another disease which occurs throughout the West that may also be destructive to both yield and quality of seed. These diseases are spread by leafhoppers or aphids. Although neither disease is seedborne, both can be destructive and reduce seed yield. A variety of insects attack the sugarbeets including beet armyworms, beet webworms, beet leafhoppers, and grasshoppers. Most losses in seed yield are attributed to these insects feeding on the foliage of mature plants. The root-knot nematode is a microscopic eelworm that persists in the soil, feeds on the root, and can cause serious losses in seed yield. Control is accomplished by soil fumigations or crop rotation. Seed producers will make 5 to 6 applications of chemicals to control weeds, diseases, and insects throughout the Fall and Spring prior to harvest.
Harvesting. Sugarbeet seed shatters readily as it approaches maturity. Shattering is minimized by windrowing the crop before the seed reaches full maturity and allowing it to cure a few days before combining. Windrowers are equipped with vertical cutter bars on each side to enable the crop to be cut with a minimum of shattering. In drier areas such as Arizona and Utah, this requires about 7 to 10 days compared to about 10 to 14 days in the Willamette Valley. The cutter-bar should be raised high enough so the plants are gently laid on the ground so air can circulate through the windrow and allow aeration and drying. Some of the mature seed will still shatter during windrowing, however most will fall onto the windrow and can be recovered during combining.
Sugarbeet plants tend to be large and viny, thus the size of the windrow feeding into the combine is substantial. However, this should not cause a problem if the crop has been adequately dried and the combine properly adjusted. Each windrow may consist of three to six rows, depending on the size of combine used for threshing. The direction of the combine should be the same as that of the windrowing operation because the seedheads are laid down to go into the cylinder head first.
Some of the small, lighter seed will be lost from the back of the combine with the tailings. These fall to the ground and provide a source of volunteer plants in subsequent crops which can be eliminated by light tillage which encourages their germination so they can be destroyed by further tillage.
Conditioning. The harvested seed is first conditioned over an air-screen cleanter to remove inert material as well as multi-germ seeds which contaminate the lot. This is followed by a draper belt which further removes stems and remaining inert matter. The remaining multi-germ seed is removed by width or thickness graders which allow the small monogerm seed to fall through, but reject the large multi-germ seed. The monogerm seed are further conditioned by passing between two horizontal disks, one of which rotates. This action rubs the corky tissue from the seeds and reduces them to small, uniform size and shapes. Further screening can remove all chaff and inert material. Finally, the seeds are conditioned over a gravity table which removes light weight, low quality seed from the seedlot. The seed is then treated with a fungicide and an insecticide.
1. Figure 1. Monogerm vs. multigerm seed.
2. Figure 2. Swathing or harvesting photo.
Bassett, M. J. 1986. Breeding Vegetable Crops. Avi Publishing, Westport.
Cooke, D. A., and R. K. Scott (eds.). 1993. The Sugar Beet Crop. Chapman and Hall, New York.
George, R. A. T. 1985. Vegetable Seed Production. Longman Press, Essex.
Johnson, R. T., J. T. Alexander, G. E. Rush, and G. R. Hawkes (eds.). 1971. Advances in Sugar Beet Production. Iowa State University Press, Ames.