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Extension > Garden > Yard and Garden > Vegetables > Growing broccoli, cabbage and cauliflower in Minnesota

Growing broccoli, cabbage and cauliflower in Minnesota

Vincent A. Fritz, Carl J. Rosen, Michelle A. Grabowski, William D. Hutchison, Roger L. Becker, Cindy B.S. Tong, Jerry A. Wright, and Terry T. Nennich

Success or failure in producing broccoli, cabbage and cauliflower commercially is measured by the quality of the end product - the broccoli and cabbage heads or cauliflower curds. A high quality product sells at a premium because all buyers seek growers who produce a superior product. The challenge of producing quality cauliflower, cabbage and broccoli in Minnesota can be met with experience, by growing and observing available cultivars, studying available production information and improving cultural practices. Knowing the production requirements and the response of broccoli, cabbage and cauliflower cultivars to local growing conditions is essential in producing a consistently high quality product.

Climatic requirements

Weather is one of the most limiting factors in producing cole crops. Broccoli and cauliflower perform best with cool daytime temperatures (70°-85°F), lots of sun, and moist soil conditions. Growers need to provide the best possible growing conditions during the summer so that plants are the maximum possible vegetative size when head or curd (cauliflower) development starts, since the size of the plant limits the potential size of the head or curd. Cauliflower produces best in a fertile, moist, well-drained soil which includes both soils high in organic matter and sandy soils. Broccoli and cabbage require the same conditions as cauliflower but are less exacting and can accommodate a slightly wider range of soil and climatic conditions. Plants grown on droughty, sandy soils in Minnesota will most likely benefit from supplemental irrigation by either a sprinkler or drip irrigation system to maintain adequate soil moisture especially during head development.

Broccoli, cabbage and cauliflower do not tolerate frost in the early seedling stage of growth (before the plant has three or four pairs of true leaves). Early maturing varieties are more sensitive to low temperature damage than are those that mature later. Consequently, the initial planting date in Minnesota is seldom earlier than mid-April, and there is some risk of cold temperature damage until mid-May. The earliest maturity possible for cauliflower is approximately 50 to 55 days from transplanting; for broccoli and cabbage, it is between 60 and 65 days. Fall season plantings should be made in early July, since the end of the season in Minnesota is approximately November 1, although you could extend it using season extension strategies (row covers or high tunnels).

Long days and hot weather in the summer cause broccoli to bolt and go to seed and cause cauliflower curds to develop a red-purple discoloration and leaves through the center of the head. Broccoli, cabbage and cauliflower will tolerate some frost and freezing conditions to near 20°F late in plant development. The degree of tolerance depends on how fast frost or freezing conditions occur and on the conditions that exist prior to their occurrence.

Varieties for Minnesota

For recommended varieties, consult the Midwest Vegetable Production Guide for Commercial Growers (BU-07094-S). This guide is a multi-state publication that is updated each year.  

Starting plants from seed

Transplants can be produced as containerized plants that are planted with roots and growing media intact or as bare root transplants in flats, coldframes, hotbeds, or in the field. Seedlings produced in the field need a loose, well-drained soil that is fertile and does not crust. Plant seed shallowly (1/4-1/2 inch) and thin early to allow seedlings to develop in ample space (~12 plants/ft). Seedlings should be kept in the greenhouse or hotbed until field conditions permit transplanting. The most desirable growing temperature is 60° to 70°F. High temperatures result in too rapid a growth rate and tall, weak plants that are difficult to handle without causing damage. The use of a shade cloth or lath fencing draped over the seedlings will slow the growth rate and help produce more stocky transplants. Plants can be transplanted to the field when they are 25 to 35 days old.

Maintaining seedbed sanitation is important in hotbed transplant production. To ensure that flats, seedbeds, hotbeds, or media are free of club root, black leg, black rot, ringspot, and damping-off organisms, remove old plant debris, fumigate or steam sterilize soil or soil-less mix, and use seed that has been treated with fungicides unless you are going to grow organically. (Note: Copper materials should not be used on broccoli.)

You can construct a simple cold frame using wood or concrete framing material. Avoid using wood products that have been treated with creosote. Research conducted at the University of Minnesota using chromated copper arsenate (CCA) pressure treated lumber showed some plant uptake of arsenic but the amounts accumulated were well within U.S. Public Health Service standards. The frame should be ten to twelve inches high on the south-facing side with the north side six inches higher. The frame should extend two or more inches into the ground with soil banked up around the frame to conserve heat. Concrete frames should be three inches thick and extend at least six to eight inches into the ground. To make your cold frame a hotbed, place a sheet of insulation on the ground inside the frame and cover it with approximately one inch of soil. Place electric heating cables at one-foot intervals and cover them with 1/2-inch mesh screen. Then cover the screen with four inches of clean sand. Transplants can be grown either directly in the sand or in flats placed on top of the sand. In either case, the bed should be heated prior to planting to ensure that the soil temperature is about 70°F. The frame should have a glass or plastic sash cover to retain the heat. The sash should be removable so ventilation can be provided on hot days. The frame can be five to six feet wide, depending on how easily the sash can be manipulated.

Apply fertilizer to developing seedlings beginning when the first true leaves appear. Use a half-strength starter solution once a week. After two true leaves are present, apply fertilizer twice a week and monitor regularly for insect and disease problems that may occur.

Transplanting

In Minnesota, transplanting to the field normally begins late April to early May and may continue until approximately mid-July. Plants are normally set into rows spaced 30 to 36 inches apart with in-row spacing of 8 to 12 inches for broccoli and 15 to 18 inches for cabbage and cauliflower. Increased spacing within the row will improve ventilation within the plant canopy and reduce foliar disease development. If possible, avoid using transplants that are older than six weeks. Use a transplant starter solution that is high in phosphorus and low in nitrogen and potassium. If necessary, use irrigation to establish the planting, especially if you don't use a transplant solution. Include an insecticide in the transplant solution for maggot control (check pesticide label for rates).

Transplants must be handled carefully. Transplant shock can be prevented by using containerized transplants, by shortening the time between seeding and transplanting, and by using transplants that have been properly hardened. Effective hardening can be achieved by progressively exposing plants to less protective conditions (temperature and soil moisture) one to two weeks before transplanting. Early varieties should be transplanted no later than 28 days after seeding. Remove plants from the seedbed, flats or containers very carefully, leaving as many roots as possible on each plant. Be extremely careful not to break the growing point of the plants, since this results in barren (non-heading) plants.

To ensure rapid vegetative growth after transplanting, follow these recommended practices:

  1. Do not transplant before the last killing frost in the spring. Cold temperatures may slow growth or injure transplants.
  2. Use transplants that are less than four inches tall and have good root systems.
  3. Use starter solutions high in phosphorus with an insecticide to control cabbage maggot.
  4. Irrigate after transplanting and as necessary to ensure that transplants receive at least an inch of water weekly.
  5. Starting approximately the third week after transplanting, sidedress with 40 to 60 pounds actual nitrogen (depending on the native fertility of the soil) at two-week intervals for a total of three sidedressings. For very sandy soils low in clay and organic matter content, one or two of the sidedressings should include an equal amount of potassium.

Direct Seeding

Broccoli, cabbage and cauliflower plantings scheduled for harvest in late summer or fall can be seeded directly in the field. Direct-seeded crops require approximately three weeks longer to mature than crops transplanted at the same time, although there are some differences among varieties. Plantings can be either precision-seeded to the desired stand or overseeded and thinned to the desired stand after emergence.

Plant seed shallowly (1/4 to 1/2 inch) and use irrigation to ensure adequate moisture in the seed zone. Avoid soil crusting by using one of the many available anticrustants or by keeping the soil moist during emergence with irrigation. Do not use cultivation to break crusts.

Fertility Requirements

Soil testing in production areas is important because deficiencies of macro- and micronutrients can lead to an unmarketable product. Most cole crops are susceptible to calcium disorders and boron deficiencies and some may require manganese, magnesium, molybdenum and copper depending on soil conditions. Soil application of required fertilizer is preferable during seedbed preparation or as a sidedress for nitrogen and potassium, but foliar sprays can be used for the micronutrients to correct deficiencies based on soil test results, tissue analysis or visual symptoms. Plant tissue samples can be analyzed to determine nutrient deficiencies, but these tests are somewhat more costly than soils tests and usually provide only an after-the-fact analysis of the problem.

Deficiencies can be recognized by the following plant symptoms:

Nitrogen deficiency

Causes buttoning of premature head formation (also caused by lack of water, poor drainage and cold shock to transplants), pale green leaf color and poor growth. (see photo)

nitrogen-deficiency

Nitrogen deficiency, cauliflower Carl Rosen, University of Minnesota

Boron deficiency

Causes hollow stem in cauliflower, and curds become discolored and deformed; in broccoli, florets turn brown. (see photo)

boron-deficiency

Boron deficiency, cauliflower Carl Rosen, University of Minnesota

It is important to note that cole crops are also sensitive to boron toxicity. Toxicity symptoms appear as scorching on the margins of older leaves. (see photo

boron-deficiency

Boron toxicity Carl Rosen, University of Minnesota

Magnesium deficiency

Older leaves lose their green color except in the veins. (see photo)

magnesium-deficiency

Magnesium deficiency, cabbage Carl Rosen, University of Minnesota

The specific fertility for an individual field program should be based on recommendations from soil tests. In general, fertility requirements are 100 to 180 pounds of nitrogen, 0 to 150 pounds of phosphorus, and 0 to 250 pounds of potassium per acre. The amount of nitrogen and potassium applied at or prior to planting and the number of sidedressings and amount per sidedressing depend on the texture and native fertility of the soil. Cole crops grown on sandy soils should receive lower amounts of nitrogen and potassium at more frequent intervals than those grown on finer-textured soils where a preplant fertilizer application is generally sufficient. Phosphorus usually is applied in a single application at or before planting. Specific recommendations based on a soil test can be found in Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota.

Important disorders of broccoli, cauliflower, and cabbage

The most important disorders are hollow stem, buttoning, blindness, ricing, whiptail, browning and leaf tipburn.

Hollow stem (broccoli and cauliflower)

Causes: Heavy nitrogen fertilization, which results in rapidly growing plants, especially during warm weather (no internal discoloration); or boron deficiency (dark brown internal discoloration)
Symptoms: Hollowing of the stem and branches of the head or curd
Remedy: The hollow stem resulting from rapid growth can be reduced somewhat by spacing plants closer together or by reducing the amount of nitrogen fertilizer applied. Hollow stem resulting from boron deficiency can be controlled by soil (preferably) or foliar application of low rates of boron. Boron deficiencies are more common on sandy soils and may be detected in a soil test. Be careful not to over apply boron as high rates can cause toxicity - cupping and scorching of leaves.

Buttoning (broccoli and cauliflower)

Causes: Nitrogen deficiency, cold temperature shock to young transplants, drought stress or other factors that markedly restrict vegetative growth
Symptoms: Development of small heads of curds (buttons) on immature plants. Plants that develop buttons are small and have small leaves that do not cover the developing head.
Remedy: Follow practices that will result in rapid vegetative growth and delay planting until the danger of fronts has passed (varieties differ in the amount of cold they can tolerate).

Blindness (broccoli, cabbage, and cauliflower)

Causes: Damage to terminal growing point due to low temperature, cutworm damage or rough handling of transplants
Symptoms: Plants have lost their terminal growing points. The leaves that develop are large, dark, green, thick and leathery. The plant does not produce a marketable head or curd.
Remedy: Handle transplants carefully, control cutworms and avoid low temperatures.

Ricing (broccoli and cauliflower)

Causes: Due to high fertility (particularly nitrogen) and high temperatures combined to favor very rapid growth
Symptoms: Small flower buds develop in the curd creating a fuzzy or velvety appearance.
Remedy: Managing both soil moisture and fertility during curd development can reduce the incidence of ricing.

Whiptail (mostly cauliflower)

Cause: Deficiency of molybdenum in the soil, or growing these crops on very acid soils.
Symptoms: Leaf blades do not develop properly and may be straplike and severely savoyed (crinkled). In severe cases only the midribs develop, which accounts for the name whiptail. The growing point is usually severely deformed and does not produce a marketable head. In severe cases there is a stimulation of sprouts on the base of plants.
Remedy: Lime the soil to pH 6.5 and use soil applications of low rates of molybdenum where deficiency exists. Deficiency seldom occurs on soils with pH 6.0 to 7.0.

Browning (heads and curds of broccoli and cauliflower)

Cause: Boron deficiency (also causes uneven head formation in broccoli) and exposure of cauliflower curds to light during development.
Symptoms: Water-soaked areas develop in the center of the stem and the branches of the head and curd stem; stems may become hollow and black inside. The first external appearance in cauliflower is on the surface of the curd. Later, these areas change to a rusty brown that often is associated with hollow stem. Curds affected are bitter, both in the raw and cooked state. Other symptoms of boron deficiency are changes in color of foliage and thickening, brittleness, and downward curling of the older leaves, which is followed by the development of blisters on the upper side of the midrib. In severe cases the small leaves and the growing point may die.
Remedy:Use soil (preferably) or foliar application of low rates of boron. Deficiencies are more common on sandy soils and may be detected in a soil test.

Leaf tipburn (cabbage and cauliflower)

tipburn-cauliflower

Tipburn, cauliflower Carl Rosen, University of Minnesota

Cause: localized calcium deficiency due to water stress or uneven watering even with adequate levels of soil calcium present
Symptoms: Younger leaves show signs of browning at the tips (see photo). In cabbage, the browning can be seen only after the head is cut open.
Remedy: If soil pH is less than 6.0, use lime to bring the pH up to 6.5 to 7.0. Avoid using only ammonium forms of nitrogen, and ensure an adequate and even supply of water. Adjust planting date so that head maturation occurs during cooler temperatures. Plant a cultivar that is less susceptible to the disorder. In general, calcium foliar sprays have not been shown to be effective for controlling tipburn incidence. Soil testing in production areas is important because deficiencies of macro- and micronutrients can lead to an unmarketable product. All cole crops are subject to calcium and boron deficiencies and all require manganese, magnesium, and molybdenum; cauliflower also requires copper. Soil application is preferable during seedbed preparation, but foliar sprays can be used for calcium and the micronutrients to correct deficiencies based on soil test results, tissue analysis or visual symptoms. Plant tissue samples can be analyzed to determine nutrient deficiencies and toxicities, but these tests are somewhat more costly and usually provide only an after-the-fact analysis of the problem.

Weeds and pests

Weeds, insects and diseases should all be managed within an Integrated Pest Management (IPM) approach. With IPM, correct pest identification is always the first step. Once pests are identified, all feasible control tactics (e.g., crop rotation, resistant varieties, biological control) should be used as much as possible. Only when these control methods are ineffective or not feasible for a given farm should growers consider pesticide solutions. Pesticides available and application rates for controlling weeds, insects and diseases change frequently, and should always be used after consulting the label for current rates, pre-harvest intervals, re-entry times and other safety information. Always read pesticide labels carefully before using a product. Refer to the Midwest Vegetable Production Guide for Commercial Growers for updated information.

Carefully timed shallow cultivation can effectively control many weeds. Crops should be rotated on land to be used for broccoli and cauliflower production to prevent the buildup of weed species that tolerate or are resistant to specific herbicides, or the buildup of weed species that are adapted to crop-specific cultural practices such as season of planting, competitiveness of the crop, etc. Careful, long-term records should be maintained for farmland used for vegetable crops to develop a history of weed infestations as well as other problems. A history provides a valuable source of crop production information, and if herbicides are used, helps maintain preharvest intervals and helps to avoid injury to rotated crops.

Following is a summary of herbicides for use on broccoli and cauliflower. Refer to the Midwest Vegetable Production Guide for Commercial Growers for additional information. Always read labels carefully to determine specific use rates, application timing and method of application. The product label must be in possession at the time of application and supersedes anything in this bulletin. Currently, Aim, Dacthal, Devrinol, glyphosate products, Goal, Poast, Prefar, Select, and trifluralin products are labeled for use on broccoli and cauliflower.

Before planting to emerged weeds:
Glyphosate products (Roundup Original, numerous others) can be used to manage emerged weedy vegetation prior to seeding or transplanting.

Pre-plant incorporate:
Devrinol and trifluralin products (Treflan and numerous others) can be applied and incorporated before seeding or transplanting the crop to control annual grasses and some annual broadleaf weeds. Prefar and Dacthal can be pre-plant incorporated to control annual grasses.

Preemergence (before weeds emerge):
Apply Prefar after seeding, or Dacthal after seeding or transplanting to control annual grasses that have not yet emerged. Dacthal can be applied after transplant too, but will not control grasses that have already emerged. Goal can be applied after seedbed preparation, but before transplanting to control annual broadleaf weeds that have not yet emerged.

Postemergence (after weeds have emerged):
Glyphosate products and Aim can be used between rows with hooded or shielded sprayers that prevent spray contact with the crop. Roundup will control most emerged grasses and broadleaf weeds; Aim will control some emerged broadleaf weeds. Poast and Select can be broadcast sprayed or applied in a band over the crop row with excellent crop safety to control annual grasses, and to suppress perennial grasses.

Insects

All of the cole crops, including broccoli and cauliflower, are subject to attack by several kinds of insects throughout the growing season. Repeated insecticide applications may be necessary to produce a high yielding, quality crop. However, if fields are scouted as part of an Integrated Pest Management (IPM) program during the growing season, the number of treatments often can be reduced and application timing can be improved (see Hutchison et al. 2006). The following insects cause losses in broccoli, cabbage and cauliflower via damage to the plant by reducing yield, lowering quality, or contaminating the product, making it unsalable:

Because insecticides are available in a variety of concentrations, the insecticide application rates referred to in the following paragraphs are expressed in pounds of active (AI) ingredient.

As a general rule, a treatment for cabbage maggot control is advisable at planting time. During the larval stage, this insect feeds on the plant roots and, when abundant, causes plants to grow slowly and produce poor quality heads. Plants may be killed during serious infestations. Cabbage maggot damage is most common on soils that are high in fresh organic matter (e.g., through the use of turkey manure). Infestations intensify during extended periods of cool, wet weather. Recommended insecticides include diazinon at two to three pounds AI per acre, either as a furrow drench or in the transplant water, and chlorpyrifos (Lorsban) at one pound AI per acre incorporated into the soil around the plant roots.

Cutworms, flea beetles and aphids tend to be most damaging to young plants. Cutworm feeding results in plants that are cut off at or just below the soil surface. Often this type of feeding is preceded by some leaf chewing. If you notice cut plants or leaf feeding, check under dirt clods and crop debris to confirm that cutworms are present. Note that most of the feeding occurs at night, and larvae hide near the base of the plants during the day. Use a recommended insecticide treatment promptly. Use carbaryl (Sevin) at two pounds per acre, or trichlorofon (Dylox) at one pound per acre.

Flea beetle injury is characterized by a "shot hole" type of feeding wound. Infestations often are restricted to spot locations in a field. Seeding plants are particularly susceptible to this injury; insecticide treatment occasionally is warranted. Carbaryl (Sevin) at 1 1/2 pounds per acre, endosulfan (Thiodan) at 3/4 pound per acre, or diazinon at 1/2 pound per acre are recommended spray treatments. One of the registered pyrethroids will also work well. Beyond the seedling stage, only extremely severe infestations will cause yield reductions.

Several species of aphid (e.g., cabbage aphid, turnip aphid and green peach aphid) commonly are found on broccoli and cauliflower. When abundant on new growth, they can cause curling and other deformations of the expanding leaves. Economic infestations of aphids often are associated with extended periods of cool weather, or they may be induced by repeated applications of insecticides that destroy natural aphid enemies. Aphid injury to mature plants is rare. Recommended aphid control treatments include diazinon at 1/2 pound per acre, dimethoate (Cygon, Defend) at 1/4 to 1/2 pound per acre, endosulfan (Thiodan) at 3/4 pound per acre, malathion at 1/2 to 3/4 pound per acre, or mevinphos (Phosdrin) at 1/4 pound per acre. Thorough coverage of the plants is necessary; the addition of a wetting agent often improves control.

Several species of caterpillars can be found feeding on the foliage of broccoli and cauliflower. Usually imported cabbageworm, which is the larval stage of the familiar cabbage butterfly, is the most abundant species. Mid- to late-season, cabbage looper may be important. Zebra caterpillar and the smaller larvae of the diamondback moth and webworms also may be present. Feeding injury may be economically important on young plants, although it occurs rarely. Established plants (beyond the four-five true leaf stage) can tolerate 50-percent defoliation until the preheading stage without yield loss. If serious infestations threaten, recommended treatments are: azinphosmethyl (Guthion) at 1/2 to 3/4 pound per acre. Bacillus thuringiensis formulations are labeled, carbaryl (Sevin) at 1 1/2 to 2 pounds per acre, fenvalerate (Pydrin) at 0.05 to 0.1 pound per acre, or permethrin (Pounce or Ambush) at 0.05 to 0.1 pound per acre. When cabbage looper is present in large numbers, fenvalerate, permethrin or spinosad can be used.

As heading of plants begins, these insects become more important as potential contaminants of marketed vegetables. Caterpillars are particularly common in broccoli heads. Two insecticide applications spaced four to seven days apart prior to harvest are suggested to eliminate or minimize insect contaminants. In cauliflower, insect contaminants are less frequent, but insect excrement may fall on the head from larvae feeding on the wrapper leaves. A spray application made immediately prior to blanching and again seven to ten days prior to harvest should eliminate this problem. The insecticides recommended above are suitable for preventing insect contaminants.

Diseases

General disease control guidelines

Successfully avoiding disease problems in broccoli and cauliflower requires careful attention to good management practices. The following production practices will reduce the risk of disease epidemics:

The symptoms, methods of dissemination, and control of the more serious diseases are described below.

Black rot (Xanthomonas campestris)

black-rot

Black rot David B. Langston, University of Georgia, Bugwood.org

black-rot

Black rot Langston, University of Georgia, Bugwood.org

Black rot, which is distributed worldwide, is the most serious disease affecting broccoli and cauliflower. This bacterial pathogen can infect all crucifers including crops and related weeds. Disease symptoms usually appear first on the edges of the leaves. The bacteria enter through leaf pores and wounds, eventually producing a yellow V-shaped area on the margin of the leaf, with the point of the V following a vein towards the leaf petiole. The veins within infected areas eventually turn black, and the leaf tissue becomes dry, brown and brittle. The bacteria then move systemically throughout the plant. When infected petioles are cut open, veins are clearly black and may ooze a sticky yellow bacterial slime. Eventually plants become stunted, wilt and die. High temperatures and humid conditions speed the disease process, but unfavorable environmental conditions may slow symptom development and prevent detection of the disease.

Black rot is transmitted by infected seed. The pathogen is spread in the field by equipment, people, animals, and rain or irrigation. The black rot bacteria may survive from year to year on weeds or on plant residues.

Management:

Black leg (Phoma Lingam)

Cabbage and Chinese cabbage are fully susceptible to black leg, whereas cauliflower and broccoli are considered only moderately susceptible. This fungus is transmitted on infected seed, and occasionally through airborne spores. Once in a field it survives three years on crop debris and moves from plant to plant by splashing rain or irrigation.

Infection starts on young seedlings as a bluish black discoloration on the stem. This lesion grows into a sunken brown lesion with a purple black border. Small black spots may be seen in the center of the lesion. These are spore producing structures and release a coil of pinkish spores in wet conditions. The stem lesion extends into the soil and may cause discoloration and death of plant roots. The lesion eventually girdles the entire stem, causing the plant to wilt and lodge.

Control measures for black leg are the same as for black rot. In addition, crucifers should not be planted adjacent to or downwind from fields that were planted in crucifers the previous year.

Club root (Plasmodiophora Brassicae)

club-root

Club root Robert Wick, University of Massachusetts, Bugwood.org

Club root is a serious disease that occurs worldwide. The infection process begins when the resting spores germinate and enter the plants through root hairs or wounds. The fungus then increases and infects other roots cells, stimulating cell growth and division. Eventually the roots form large club-like masses that crack, dispersing the spores into the soil. The development of this disease is favored by warm temperatures, high soil moisture and an acid pH.

The initial symptoms of club root are difficult to detect. Later symptoms include pale yellow leaves and a tendency to wilt during hot, sunny days. Young plants may be killed by the disease within a short time after infection, whereas older plants may survive but fail to produce marketable heads. Club root is transmitted by infected transplants, equipment, windblown dust and irrigation. Club root can be managed by maintaining soil pH at 7.3 or above by additions of lime. Avoid planting any cruciferous plant in the same fields for long time periods. The resting spores can survive in the soil for many years, so the effect of rotation is not great. Always use plant transplants that are disease-free, and do not move equipment used in diseased fields to clean fields.

Black leaf spot and gray leaf spot (Alernaria Spp)

black-leaf-spot

Black leaf spot Michelle Grabowski, University of Minnesota

Black and gray leaf spot are caused by related fungi. These fungi survive in seed and in infected plant debris. Leaf spots are gray to black, round and often have concentric rings within them. As leaf spots mature, the tissue becomes dry, brittle and often falls out, resulting in a 'shot hole' appearance of the leaf. In many cases, several spots grow together resulting in large dead areas of the leaf. Dark brown irregular sunken areas can also form on broccoli and cauliflower heads.

Abundant spores are formed within leaf spots in response high humidity or moisture on the foliage. These are spread through the field through wind, rain or irrigation. Lesions on cabbage heads can continue to grow in storage and may serve as an entry for secondary rot organisms.

Management:

Downy mildew (Peronospora Parasitica)

mildew

Mildew Colucci, North Carolina State University

Downy mildew can infect all cruciferous plants including crops and weeds, although there is some host specificity among different strains of the fungus. Leaf spots are sunken gray-white areas that may appear angular due to growth restriction by the leaf veins. Leaf tissue around the spots turns yellow, and fuzzy gray fungal growth can be seen on the underside of the leaf spot.

The fungus moves through the plant systemically, resulting in dark colored leaf petioles and black or brown streaks in the veins of broccoli heads. On mature cabbage heads and cauliflower curds, the infection may occur as dark sunken spots. Infected crops are more susceptible to rot through secondary fungi or bacteria.

The downy mildew fungus can survive in soils and plant debris, and is spread by wind, rain and possibly by seed. Disease development is optimal at 50° to 60° F in very wet conditions.

Management:

Harvest and handling

Broccoli

Cut broccoli, with eight to ten inches of stem, before the individual florets in the head open enough to show yellow. When mature, the central heads usually are three to six inches across. Over-maturity causes woodiness in the stems, loose heads, and reduced market value. In many varieties you can get follow up harvests of side shoots that continue to develop after the central head has been cut. Side shoots measure one to three inches across and are generally very good quality.

Cabbage

Cabbage is cut generally when heads are firm and of the appropriate size and density. An early harvest may be warranted to capture better market prices. The harvest window is somewhat flexible due to the crop's field holding capacity. However, delaying harvest at peak maturity will increase the risk of head splitting, particularly after late season rainfall. This will drastically reduce marketability and storability.

Cauliflower

To prevent discoloration, cauliflower curds must be shielded from the sun as soon as they start to develop (when they are two to three inches in diameter). Tie leaves together to protect and blanch the developing curd. Tie the leaves close to the top of the plant to allow adequate space for curd expansion and air circulation and to avoid riciness. Tying is commonly done with color-coded rubber bands to indicate the day on which the plant was tied. Several cultivars show a semi-upright (White Empress) or upright (Stovepipe) leaf habit with leaf-free curd and tolerance to high temperatures at maturity. Self-Blanche and other early-maturing, self-blanching types also are currently on the market. Varietal selection to improve the ease of cauliflower curd blanching greatly reduces the amount of labor necessary to produce a superior product.

In warm weather, curds may develop in as little as three to five days after blanching, but with cool temperatures they may take as long as two weeks. Ideally, curds are mature, clear white, compact, and six to eight inches in diameter. Over-mature curds are somewhat loose and unmarketable. It is better to cut a little early than risk losing the curd to over-maturity. Harvest should begin when curd diameter reaches six inches. Curds need to be trimmed and cooled immediately after harvest. Hydrocooling, or forced air cooling, is necessary to remove field heat during hot weather. Short-term storage is possible under the proper low temperature, high humidity conditions.

Packing

Proper cleaning, size grading and packing are essential in marketing cauliflower and broccoli. It is important that growers be aware of, and strive to meet, specifications of the market they serve. In Minnesota, cauliflower curds should be closely trimmed, wrapped in perforated film, and packed in cartons with nine, twelve, or sixteen heads per carton. If a crate is used, allow a few leaves to remain attached. Cut them off just above the curd, leaving a jacket of stalks and part of the leaf blades to protect the heat. Tie broccoli in bunches and pack in crates or in wax-impregnated cartons. Broccoli is very perishable. Cool broccoli promptly after harvest, and put ice in the carton before closing it.

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