BRUSSEL SPROUTS Brassica oleracea var. gemmifera

FAMILY CHARACTERISTICS
    The Brassicaceae family includes about 350 genera with approximately 3,200 species of pungent or acrid herbs.  Of this family, two species, B. oleracea and B. campestris, are the source of the edible crops.  They originate from Europe through  the Asian countries of Iran, Afghanistan and Pakistan.   Both of these species tend to be biennial plants, with some annual B. campestris, differing in stem hardiness;  B. oleracea most often has a woody basal stem and B. campestris, a fleshy basal stem.
    Vegetables in B. oleracea include the cabbages, collards, cauliflower, broccoli, brussel sprouts,, the kales, and kohlrabi; B. campestris includes bok choy, pak choi, the Chinese cabbage, Siberian kale, turnip, mustards, rape, rutabaga, and radish.  Pungency from various sulfur compounds varies throughout the crops. All have an abundance of fiber and vitamin C, and most are also high in folate, potassium, and calcium.  The cole crops are hardy cool season plants that are similar in cultural requirements, morphology, disease and pest susceptibilities.
Family members include:
        Armoracia rusticana Gaertn., Mey., Scherb.; Horseradish
        Barbarea vema (Mill.) Aschers; Upland Cress
        Brassica carinata A. Br.; Abyssinian Mustard
        Brassica campestris L. (Chinese group); Pak-Choi,  Chinese Mustard,  Celery Mustard
        Brassica campestris L. (Pekinensis group); Pe Tsai,  Chinese Cabbage,  Celery Cabbage
        Brassica campestris L. (Perviridis group); Spinach Mustard,  Tendergreen Mustard
        Brassica campestris L. (Rapifera group); Turnip
        Brassica campestris L. (Ruvo group); Broccoli Raab,  Rapa,  Italian Turnip
        Brassica juncea (L.) Czern. & Coss.; Leaf,  Brown and Indian Mustard
        Brassica napus L. (Napobrassica group); Rutabaga,  Swede,  Swede Turnip
        Brassica napus L. (Pabularia group); Siberian Kale,  Hanover Salad
        Brassica nigra Koch.; Black Mustard
        Brassica oleracea var. acephala D.C.; Collards, Scotch Kale
        Brassica oleracea var . botrytis L .; Cauliflower
        Brassica oleracea var. capitata L.; Cabbage
        Brassica oleracea var. fruticoca Metz.; Thousand-headed Kale
        Brassica oleracea var. gemmifera, D.C.; Brussel Sprouts
        Brassica oleracea var. gongylodes L.; Kohlrabi
        Brassica oleracea var. italica Plenck.; Broccoli
        Crambe maritima L.; Sea Kale
        Eruca sativa Mill.; Roquette or Rocket Cross
        Lepidium meyenni Walp.; Maca
        Lepidium sativum L.; Land Cress,  Pepper Grass
        Raphanus caudatus L.; Rat-tail Radish
        Raphanus sativus L.; Radish
        Raphanus sativus L. (Longipinnatus group); Daikon or Chinese Radish
        Rorippa nasturtium-aquatkum (L. ) Hayek; Water Cress

CROP HISTORY AND DEVELOPMENT
    Brussel sprouts, Brassica oleracea var gemmifera, are known to be native to cool regions in northern Europe.  They were a popular vegetable crop in Belgium during the sixteenth century from which they were spread to the surrounding countries throughout temperate Europe.  French settlers in Louisiana extensively cultivated brussel sprouts for its continuous production of miniature cabbages throughout the growing season.
    The origin of Brasssica oleracea var gemmifera is thought to be the result of a  mutation from the savoy cabbage, Brassica olearcea capitata L. sabuda subgroup.  Two main types of brussel sprouts have arisen:  the tall variety, standing 2 to 4 feet tall, and the short variety, growing to a maximum height of 2 feet. The preferred size of the sprouts varies with Europeans opting for sprouts ½ inch in diameter, while Americans prefer sprouts 1 to 2 inches in diameter.
    Most of the breeding work with brussel sprouts has occured in Europe. US hybrids are primarily from European introduction focus on uniformity, vigor and disease resistance.  Some of the standard varieties grown today have been in production for a long time.  ‘Catskill’ (also known as Long Island Improved) is a dwarf variety with medium sprout size coming into maturity 85-95 days after transplanting.  ‘Jade Cross’ (F1 hybrid) is a compact variety of a bluish-green color, medium size sprouts come into harvestable maturity 85-90 days after transplanting.  ‘Jade Cross E’ (hybrid) is taller and has more uniform sprout growth than the F1 hybrid.  Some other dwarf cultivars are ‘Early Morn Dwarf Improved,’ and ‘Friither Zwerg Kvik.’
    In regions with a longer growing season, taller cultivars are preferred. Many of these are specific to European countries.  Some of the earlier cultivars are ‘Breda’ and ‘Weibulls Rapid'.  ‘Red Vein,’ ‘Hild’s Idea,’ and ‘Deroshy Amager’ are some of the later maturing, more hardy cultivars.
    Commercial production of brussel sprouts began in the US in 1925 in the Louisiana delta, and by 1939 had moved to mid coastal California with limited production in New York state. In 1997, 4,200 acres of brussel sprouts were harvested in California.  Total weight harvested was 630,000 cwt (150 cwt/acre) with a total value of $26,800,000.

PLANT CHARACTERISTICS
    Overview. Brussel sprouts are a cool season biennial.  Axillary buds are borne in the leaf axils during the first year of growth.  A seed head is produced in the second year after vernalization has occured.
    Root System.   As with other members of this family, brussel sprouts have a shallow root system.
    Because transplants usually have a broken taproot, a fibrous system of adventitious roots forms.  If the taproot is not damaged, it is dominant.  Seventy to 80% of the roots grow in the upper 8 to 12 inches of soil.
    Stem. Brussel sprouts have a light grayish-green stem.  Axillary buds are produced in nodes of the elongated stem beneath the leaves.

Leaves. The round to heart-shaped leaves are simple with lengthy petioles and alternate along the upright stem.  The coloration varies from light green to deep grayish-green.  Leaf texture is glabrous (waxy). The sprouts are modified leaves forming a "head."     Flower. A period of vernalization is required for flower stalk initiation.  Temperatures below 45°F for a period of 1 to 2 months promotes flowering (bolting).    Perfect flowers (with both male and female parts) are borne in terminal racemes.  The calyx has four sepals, the corolla, four yellow petals (if any at all) two long and two short.  The superior ovary has two locules with many ovules.

    Pollination occurs with insects.  To grow crops for seed production the desired variety must be at least ¼ mile from other B. olearcea species and gemmifera varieties.
    Seed. There are approximately 9,000 seeds per ounce.

PROPAGATION METHODS
    Overview.  Seeds are usually planted in a field nursery and the seedling transplanted to the field for production.  Seedlings are transplanted to fields prepared as flat beds or ridges and furrows.
    When direct seeding is used, soil preparation is important to obtain a uniform stand.
    Generally 1 pound of seed per acre is used if direct seeding.  Transplanting rates are 5 ounces per acre.  Primed and coated seed is becoming popular.  Priming seeds will ensure a more rapid and uniform crop establishment
    Seed borne diseases must be prevented by using hot water and/or fungicide treated seed. This practice is important to produce a uniform stand of healthy seedlings.
    Seed germination occurs best at temperatures above 50°F.
    Direct Seeding.   Selection of size, seed and precise spacing of seed for late summer and fall crops held in obtaining a uniform stand.  Soil moisture required for germination is low. Protection from such pests as the flea beetle is necessary to prevent seedling damage that affects yield.
    Transplants.  Transplants are the primary method of field planting. Production of transplants is done in both seed beds and greenhouses.  Greenhouse production allows for an early crop.  Seeds can be planted in flats using a peat vermiculite mix or sterilized soil. Greenhouse temperature should be maintained between the extremes of 45 and 85°F in the production of transplants.
    Outdoor seed beds are used when the weather is appropriately warm for seed germination.  Soil in the seed beds should be fumigated or sterilized and fertilized with a seed starter fertilizer. Clean plastic cover over the seed bed can be used to maintain the proper temperature for germination.
    Transplants are put out in the field after 5-6 weeks of growth in the seed bed or greenhouse, when plants are about 6 inches tall. They should be planted at densities of 16 to 18 inches in the row with rows 36 to 40 inches apart.  Apply a transplant starter fertilizer high in phosphorus at time of planting.  After planting irrigation is helpful in establishing transplants.
    Diseases may be more prevalent in transplant fields versus direct seeded fields.  Keep them separated.

CULTURAL PRACTICES     Overview.  All Brassica crops are susceptible to diseases and other related problems. It is important to plant brussel sprouts on land that has not been planted with other Brassica crops (cabbage, cauliflower, broccoli, kale, kohlrabi, Chinese cabbage, all mustards, turnips, rutabagas, radishes, brussel sprouts, etc.) nor has been infested with related weeds (wild radish, wild mustards, etc.) for at least 2 years, preferably 4 years. Soils should be maintained at a pH 6.5 or higher to avoid specific diseases (club root in particular).     The highest quality and yields of brussel sprouts are obtained in cool temperatures of 60 to 65°F. In warm climates seed are sown in mid-summer, in cooler climates, early to mid spring.  Compact brussel sprouts occur best in the coolest part of the growing season.

    In fall production,  the top growing point is cut off in August or September (3 weeks before harvest) to force the plant’s energy into developing sprouts, encouraging a uniform sizing and maturity of sprouts.
    Soil Type.  Soils that are well drained with high organic matter is best for optimal growth. However, brussel sprouts may be grown on a variety of soils. Earlier crops are planted on light, sandier soil, while high yielding crops are planted in heavier soil. Liming the soil to a pH to make nutrients most available is recommended; however, brussel sprouts can tolerate a more acidic environment than other vegetables of this family.
    Temperature. Brussel sprouts are a cool season crop that grows best between 45 and 75°F with optimal growth at 60 to 65°F. Brassicas can withstand some freezing.
    In warm weather, sprouts become loose, forming more open heads, while cool weather firms the sprouts and leads to a milder flavor.
    Exposure to temperatures below 50°F for an extended period promotes a shift from vegetative to reproductive growth.
    Fertility. Side dressing of N during the growing season  is essential, though heavy use of nitrogen results in loose sprouts. As with other vegetable crops in this family, Ca and B levels  must be maintained to avoid physiological disorders.
    Generally, a total N application is made in split applications throughout the growing season; broadcasting or banding at time of seeding or transplanting, side dressing the remainder later in season.  Phosphorous is best banded at time of planting, 2 to 3 inches to the side and 3 to 4 inches beneath seed or plant.  Broadcasting and incorporation or banding of potassium  prior to planting is suggested.
    Sulfur is often applied and can be incorporated into soil at time of planting. The form of S is important, so as to not overly acidify the soil.
    The application of B fertilizer is through broadcast or foliar applications.
    Liming the soil to correct acidic soil pH prior to planting will supply Ca and Mg.
    Additional fertilizer applications should be determined by foliar analysis taken from mature leaves off new growth.  Nutrient sufficiency range is as follows:
Macronutrients

	N	P	K	Ca	Mg	S
	----	----	-%-	----	----	----
Low	3.1	0.3	2.0	1.0	0.25	0.3
Sufficient	5.5	0.75	4.0	2.5	0.75	0.75

Micronutrients

	Fe	Mn	B	Cu	Zn	Mo
	----	----	ppm	----	----	----
Low	60	25	30	5	25	0.25
Sufficient	300	200	100	15	200	1

    Weed Control.   Weed control can be obtained through the use of chemical herbicides and/or cultivation, or black plastic mulch.  Most often, commercial operations utilize herbicides and cultivation, limiting cultivation to early growth period so as to avoid root damage.  Herbicide materials are available for preplant,  preemergence, and postemergence or post-transplant applications.
    Irrigation.   Brussel sprouts are relatively heavy water users.  During the growing season, growers in California supply over 6 inches through furrow or overhead sprinkler irrigation to avoid water stress. In most regions 1-1.5 inches of water is provided every seven days.  Erratic soil moisture restricts uptake of nutrients, depresses yield, and may cause physiological disorders.  It is best to keep soil moisture at 60 to 100% field capacity.  Soil conditions and amount of organic matter are good factors in determining water requirements.   The breaking of a hardpan, if it exists, is essential to allow for proper drainage, as this crop does not tolerate standing water.
    Brussel sprouts generally require 15 to 20 inches of water through the growing season, varying according to planting date, seasonal variation and variety.  Frequency and amount of water applied during irrigation is affected by the soils type, with lighter soils needing lighter, more frequent applications than heavier soils.

INSECTS
    Insect pests of brussel sprouts are those of other crops in this family.  The most significant in terms of damage are the larvae of butterflies and moths.  Removal of alternate host plants and isolation from other Brassica sp will reduce insect infestation.  Infestation can occur early in the season, crop must be frequently checked to avoid heavy losses.
    Cabbage Looper (Tricheplusia ni).   This little green larvae is identified by its characteristic looping mode of locomotion.
    Diamond Back Moth larvae.  Similar to cabbage looper in appearance, damage, and control.
    Imported Cabbageworm (Picris rapae).  This small, green larvae of the butterfly Pieris rapae devours plant foliage.  The female moth or butterfly flies from plant to plant laying eggs on leaves.  Eggs hatch in 3 to 5 days and young worms begin feeding on foliage.  Egg laying is difficult to control with chemiclas used to control the young larva giving the best results. Insecticide applications should be made immediately upon finding young worms. Once worms reach ½ inch in length, they are more difficult to control.
    Cutworm. Cabbage Maggot (Hylemyia brassicae).  A root feeding insect pest, destroys plant’s ability to take up water and nutrients.
    Aphid (Brevicoryne brassicae).  Extensive damage can be done to foliage if populations go uncontrolled.
    Some other insects include Harlequin Bugs, thrips and webworm

DISEASES
    Brussel sprouts are susceptible to all the diseases common to members of this family.   The most serious diseases are Fusarium yellows, clubroot, black rot, black leg, and downy mildew.   Other significant diseases are black blight (ring spot), Alternaria, soft rot, and mosaics.  Best control is through proper sanitation practices, crop rotation, alternate host eradication, and use of various fungicides (carbamates) or bactericides.
    Downy Mildew (Pernospora parasitica).  This fungus can be brought in from transplant bed and may be introduced to new field plantings by wind blown spores. Plants can be infected at any time of growth.  The pathogen favors temperatures of 50 to 60°F.   It is predominant in humid coastal regions, causing damage both to young seedlings and to produce in transit.  It over winters on perennial plants or on infected plant debris.  Young plants infected early show a white mildew effect mostly on the underside of the leaf, eventually chlorosis occurs on the upper leaf in corresponding areas.  Young chlorotic leaves drop off, while older chlorotic leaves persist and the infected areas enlarge turning tan in color and papery in texture.
    Fusarium Yellows. Fusarium oxysporum f. sp. conglutinans is the soil borne causal agent for this vascular disease.  The first symptom is a yellowing of the foliage.  Leaves soon become distorted, prematurely dropping after a gradual browning.  Once soil is infested with F.oxysporum, adequate control is obtained through  use of resistant cultivars.
    Clubroot (Plasmodiophora brassicae). As soil temperatures rise, or the soil becomes more acidic, this soil borne fungus becomes active and aggressive.  Germinated zoopsores enter young plants through root hairs or wounds.  The fungal hyphae proliferates through the root system, stimulating cell division and enlargement, producing knots, or club-like growths that restrict the uptake of water and nutrients.  Above ground symptoms may not be apparent until wilting occurs during periods of intense sunlight.  Recovery from wilting does not occur, causing stunted growth and eventual death.   Inoculum transmission occurs through transplants, by equipment, by windblown dust, or by manure of animals that fed on infected plants.  Control is very difficult as there is not genetic resistance and soil persistence is great.  To limit infestation, measures to maintain the pH at 7.3 or higher, rotating with non-susceptible crops, use of clean transplants, and sanitation and disinfection of all tillage equipment may be most effective.
    Black rot  (Xanthomonas capestris).  This bacteria is seed borne.  Care should be taken to get only disease free seed.  Disease may be spread to the field by planting infected transplants or seeds.  Black rot is more severe under warmer and more humid conditions.  Plants are infected through the pores at the margins, spreading the infection towards the midrib in a V pattern.  Necrotic areas brown and dry up.  The midrib and large veins may become black, and discoloration may extend to the root.  Use of disease free seed, a three year rotation of crops, and proper sanitation practices reduce inoculum levels.
    Black Leg (Phoma lingam).  This fungal disease is often spread through hardening off processes of transplants.  Topping of young plants and dipping in water before transplanting causes more susceptibility.  Cooler temperatures increase pathogen activity.   Fungal matter can infect and persist in seeds, proliferating after germination.  Plant symptoms include lesions bearing pycnidia, the dark-colored fungal fruiting bodies.  Propagules can disperse through water or air, having a wide range of potential infestation.  First symptoms usually appear 2 to 3 weeks before field planting when plants become covered with circular black spots.  Stunting of growth becomes apparent.  Eradication of cruciferous weeds from surrounding areas, and isolation from other Cole crops reduces disease pressure.  Proper field and equipment sanitation and crop rotation are also helpful in reducing inoculum.

HARVESTING
    Harvest occurs 90 to 100 days from transplanting.  Early maturing crops are harvested several times. Sprouts lowest on the plant are removed at each picking.  Sprouts are harvested when the lower leaves begin to turn yellow. Waiting beyond this time will result in sprouts that are tough and lose their delicate flavor.  In picking the sprouts, the leaf below each sprout is broken off and the sprout is removed by breaking or cutting it off at the stem. Plants are capable of producing 2 ½ to 3 lbs of sprouts, but commercial production is 2 lbs.  Sprouts are generally hand picked into baskets or other containers and directly sent to the packing house.  Multiple hand harvests have been replaced by mechanical once-over harvesters.   Hybridization has developed cultivars that ripen uniformly and are responsive to mechanical harvesting.
    Lightly processed sprouts are harvested by the stalk.  Leaves are removed and the stalk is passed through a stripper which detaches the sprouts, releasing them into a receptacle.

POST HARVEST
    Once removed from the field, brussel sprouts may be forced-air-cooled or vacuum-cooled to remove field heat.   Brussel sprouts should be cleaned, trimmed of loose leaves, and sorted to remove those that are soft, damaged, or too large size.
    Quality factors for brussel sprouts are the brightness in color and the firmness.  Off color, wilted, or puffy sprouts tend to be woody in texture and have an off flavor.  Sprouts with small, loose leaves at the base are considered poor in quality.
    Brussel sprouts can be stored for 3 to 5 weeks if kept at 32°F and 95 to 100% relative humidity.  Longer storage may result in black speckling of the leaves, loss of fresh bright green color, decay, wilting, and discoloration of the surface.  Rate of deterioration is twice as fast at 40°F as at 32°F.  At 50°F and above, deterioration (yellowing of the sprouts and discoloration of the cut surface) is rapid, and yellowing becomes evident within 1 week at 50°F.
    Controlled atmosphere of 2.5 to % O₂  and 5 to 7% CO₂  retards deterioration of sprouts held at 40 or 50°F but not at 32°F.  Use of packing film is useful in retaining moisture, yet should be perforated for gas exchange of respiration volatiles and air circulation.  Storage with ethylene producing fruits should be avoided.
    Brussel sprouts for processing are cleaned of outer loose leaves, flash frozen, and packaged in plastic bags.