Crop Profile for Broccoli in Minnesota

Prepared: August, 2001

Revised: October, 2001

Executive Summary

In 1997 approx. 45 ac. of broccoli were harvested in Minnesota. Another 700 acres of related crucifer crops, including cabbage, cauliflower, and Chinese cabbage (e.g., bok choi, nappa, etc.) are also produced annually in Minnesota, most of which is sold for fresh market. Cabbage and cauliflower are covered in separate crop profiles for Minnesota. Despite the relatively few acres of broccoli in the state, the high value of the crop (ca. $3,600/ac), and traditionally high insecticide use of about 5-6 sprays/season, continues to create a demand for effective integrated pest management (IPM) programs. In response to new IPM information needs generated by the Food Quality Protection Act (FQPA), this crop profile was developed to a) summarize current IPM practices for insect, disease and weed pests, b) highlight pesticides under review by US-EPA, c) estimate the impact of the loss of selected pesticides, and d) assess alternatives for such losses.

In most years, insect pests are usually the most damaging and difficult to control in Minnesota broccoli. The most important insect pests include: cabbage looper, imported cabbageworm, diamondback moth, cabbage maggot and flea beetles. With high parasitism rates of diamondback moth (typically 80-100%), and in the absence of insecticide resistance with this pest, cabbage looper is often the most common and most difficult insect pest to control (10). Black rot is probably the most damaging disease problem, but like most diseases of crucifers, few therapeutic treatments are available. Many broadleaf and grass weed species are potentially very damaging. However, in tandem with timely cultivation, most weeds are still controlled well with currently labeled herbicides.

Insects: The recent success of a cabbage IPM program in Minnesota, in commercial production fields, shows considerable potential for extending this program to broccoli (see MN Cabbage Crop Profile). Three of the new "reduced-risk" insecticides (SpinTor, Proclaim and Avaunt), with good activity on the Lepidopteran pest complex, are also labeled for broccoli. During the next 5 years, the primary concern will be insecticide availability for cabbage root maggot, with the potential loss of Diazinon and/or Dyfonate. However, based on recent EPA rulings (2000), Lorsban should still be available for cabbage root maggot. Sevin insecticide remains a popular choice by many small-scale growers for flea beetles and other insects, because of its broad-spectrum efficacy and non-restricted use status. Future FQPA review of this carbamate insecticide and potential loss, would create new challenges for many growers.

Diseases: With a few exceptions, most of the damaging diseases in broccoli are not treatable with fungicides or bactericides. For example, black rot, a bacterium, can be treated with copper (e.g., Kocide), but only for low-level infestations. Thus, growers must rely on crop rotation, clean cultural practices, resistant varieties and timely irrigation (where possible), to help manage most crucifer diseases. The primary materials available for broccoli diseases include Bravo, Rovral, Maneb, Terrachlor and various copper formulations. Where possible, fungicides such as Ridomil Gold Bravo for downy mildew control, are critically important, and should remain available for selected diseases. When these products come under review, with respect to FQPA, there will be a renewed need to assess risks/benefits.

Weeds: For both broadleaf and grass species, trifluralin (e.g., Treflan) has typically been the most commonly used herbicide on broccoli in Minnesota. To date, we are not aware of weed resistance problems as a result of this use. This is likely due to the fact that it is only used once/planting, and may not be used significantly on surrounding crops on a given farm. Unlike some of the other herbicides, Treflan can be used for both direct-seeded and transplanted broccoli. However, Treflan and Dacthal are not recommended for use on muck soils. In addition, Goal is not recommended for use on direct-seeded broccoli. Post-emergence herbicides (Poast, Gramoxone and Glyphosate) are typically limited to weed escape situations, in unusually high weed pressure fields. Pyridate (lentagran), another broad-spectrum post-emergence herbicide is no longer manufactured. Potential additional losses of herbicides would limit grower options. If and when these products come under review, with respect to FQPA, there will be a renewed need to assess risks and benefits of each.

Minnesota harvested 45 acres of broccoli on 1997, of which, 13 acres were irrigated. Nearly all broccoli produced in Minnesota is destined for fresh market sale. The national average (1997-1999) for production of fresh market broccoli was 134 cwt/acre (6.7 tons/acre) with an average value of $27.43/cwt ($550/ton). The 1997 Minnesota fresh market cabbage crop had a total market value of $165,403 (1).

Broccoli is a cool season crop that may be planted early or late in the growing season. The best quality broccoli is produced with daytime temperatures ranging from 70-80º F, sunny conditions, and moist, fertile, well-drained soil. Soil pH should be maintained between 6.0 to 6.5. Broccoli needs at least 1 inch of either rainfall or irrigation water each week. Irrigation may be needed to obtain good quality and high yields. Broccoli’s marketable parts are the flower stalks and buds. Typically, broccoli form heads after 26 to 29 leaves have developed. Several days of cold temperatures (e.g., <50° F) can cause premature heading in broccoli. Premature heading is also caused by inadequate water, inadequate amounts of nitrogen, excess salt, or weed competition. High temperatures can also damage broccoli by causing the flower buds to enlarge and elongated pedicels and slow the heading (3, 6, 13).

Cole crops generally need 80-120 pounds of nitrogen depending on the amount of organic matter present in the soil, 25-200 pounds of phosphorous, and 0-250 pounds of potassium per acre. The addition of phosphorous and potassium should be based on soil tests. When broccoli is transplanted, a small amount of water soluble fertilizer in the transplanting water may be beneficial. All cole crops are susceptible to deficiencies in calcium and boron and also need the micronutrients of manganese, magnesium, and molybdenum. Two or three weeks after transplanting or after rapid growth has begun, application of 60 lb. of nitrogen is often recommended. An additional sidedress application of 30 lb. of N on sandy soils, where leaching may occur may be necessary. However, excess fertilization can cause broccoli to grow too quickly and have a hollow flower stalk. Throughout the growing season, foliar tissue tests can be performed to determine if a nutrient deficiency exists (3, 6, 13).

Broccoli is hand-harvested before the flowers open and the head is still compact. Sometimes, a second crop of sideshoots is harvested. If broccoli is harvested too late, the stems tend to be woody. The quality of the broccoli is based on the degree of compactness, leafiness, trimness of heads, and absence of insect damage and disease (6, 13).

Although quality standards may vary somewhat depending on whether broccoli is being produced for the fresh market or processing, consumers have a low tolerance of insect damage or excrement on the broccoli flowers. Growers may have more options to control insects early in the production that may allow survival of more beneficial insects but as harvest time approaches, the control options are more limited due to the concern about insect damage. The same pests typically attack all the cole crops although particular insect species may prefer one type of cole crop (5, 13).

The cabbage looper (CL) is a major economic pest of broccoli in Minnesota and the upper Midwest. The cabbage looper does not overwinter in the upper Midwest but migrates into the region from southern states from mid-June through September. There are 1-3 generations depending on temperature and summer wind patterns. Larvae are pale green with narrow white lines running along each side. Larvae have a characteristic looping motion as they move across vegetation (5, 10, 13, 15).

Larvae feed for 2-4 weeks after hatching from eggs. When they initially hatch, they feed between the veins on the underside of the lower leaves producing small holes that generally do not break through to the upper leaf surface. Later instars, however, chew large, ragged holes in the leaves and often move to the center of the plant to feed. Loopers are able to bore directly into broccoli florets resulting in unsightly holes and large amounts of frass. Damage can be severe enough that plants are severely defoliated, stunted, and more susceptible to invasion of disease pathogens (5, 10, 13, 15).

Also see fact sheet: http://vegedge.umn.edu/vegpest/colecrop/looper.htm

The diamondback moth (DBM) is another economically important pest in broccoli in the upper Midwest. Adult moths may survive the winters in protected locations. However, DBM may also migrate to the upper Midwest via southerly winds, or may be shipped to the upper Midwest on broccoli transplants that are shipped from the southeastern U.S. DBM larvae are small (5/16 inch long), light green, tapered at both ends, and wiggle vigorously when touched. Because of their relatively small size, DBM larvae may also be present in broccoli heads but not detected until harvest (5, 11, 13, 15).

Initially, DBM are leaf miners. As they grow larger, they feed on the leaf surface, eating all the layers except the outer layer, resulting a characteristic windowpane look. The larvae also feed on the developing broccoli heads causing them to look deformed and encouraging the invasion of soft rot (5, 11, 13, 15).

Also see fact sheet: http://vegedge.umn.edu/vegpest/colecrop/diamond.htm

The imported cabbageworm (ICW) is a day-flying butterfly that overwinters in the upper Midwest. ICW is also an economically important pest that annually infests broccoli. The larvae are velvet green, about 1-inch long in the last instar, and move sluggishly when touched (5, 12, 13, 15).

ICW usually feed on the upper surface of leaves, leaving irregularly shaped holes. As the caterpillars become larger, they move towards the center of the plant. They tend to feed on the edges of the leaf, leaving the large veins intact. They also contaminate the leaves with large fecal pellets (5, 12, 13, 15).

Also see fact sheet: http://vegedge.umn.edu/vegpest/colecrop/cabbworm.htm

The cabbage maggot is a sporadic pest of broccoli. The adults look like small houseflies. The larvae are legless, yellowish-white, and reach the maximum length of ¼ inch (2, 5, 13, 15).

Seedlings and transplants are more susceptible to broccoli maggot injury during cold wet springs, with most of the damage limited to the first plantings. Transplants or seedlings planted later in the growing season may also be susceptible to maggot damage. Broccoli maggots chew into the fine root hairs and create extensive, slimy tunnels on and throughout the broccoli root. Maggot feeding can cause the broccoli to look off-color, sickly, and stunted. Extensive feeding can even wilting and even death. The feeding also provides entry points for fungal pathogens (2, 5, 13, 15).

Also see fact sheet: http://vegedge.umn.edu/vegpest/colecrop/cabmag.htm

Both cabbage and green peach aphids are found on broccoli. The green peach aphid has numerous host plants whereas the cabbage aphid is usually only found on cole crops. Cabbage aphids are serious pests only occasionally. Turnip aphids typically prefer mustard, turnip, and radish but they occasionally damage cole crops (5, 8, 13, 15).

Aphid populations tend to increase and cause more damage during hot, dry weather. Under cool and humid conditions, beneficial insects usually keep aphid populations in check. Feeding injury can kill seedlings or young transplants. Injury on older plants can result in yellowing and curling leaves, stunted growth, and deformed heads (5, 8, 13, 15).

Also see fact sheet: http://vegedge.umn.edu/vegpest/colecrop/aphid.htm

Flea beetles occasionally feed on cole crops, usually causing most damage to early (spring) plantings. Although several flea beetle species feed on cole seedling crops, the striped flea beetles, the western black flea beetles, and the crucifer flea beetles are the most common flea beetles found on cole crops. Flea beetles are small with large hind legs that enable them to jump a considerable distance when disturbed (5, 13, 15).

Although some larvae may feed on the roots, the adult flea beetles cause more damage when they feed on cotyledons, stems and foliage. The beetles gouge out small (usually less than 1/8 inch) holes that result in a "shot hole" appearance in the foliage. A heavy flea beetle infestation on seedlings may cause stunted growth, invasion points for fungal pathogens, wilting, and even death. Transplants and older plants tolerate more damage than young seedlings (5, 9, 13, 15).

Also see fact sheet: http://vegedge.umn.edu/vegpest/colecrop/flea.htm

Onion thrips are slender, minute insects (1/16 inch) that are sporadic pests of broccoli. Thrips tend to be more of a problem in hot, dry weather. Heavy rains may wash thrips off plants. They usually overwinter in clover, alfalfa, wheat, and other grasses. They may move to other vegetable crops if a wheat or alfalfa field is harvested. Growers should avoid planting broccoli next to, or immediately downfield, from a small grain or alfalfa field (5, 6, 13, 15).

Thrips’ rasping mouthparts cause whitish marks or brownish patches on the foliage and heads. Although the feeding damage on the outer leaves can be tolerated, extensive damage on the heads results in an unmarketable product. Large thrips populations within a broccoli head may result in distortion (5, 6, 13, 15).

Also see fact sheet: http://www.gov.on.ca/OMAFRA/english/crops/facts/99-027.htm

Insect Control Options

The diamondback moth (DBM), imported cabbageworm (ICW), and the cabbage looper (CL) are the most important economic pests of broccoli and other cole crops in the upper Midwest. Several species of wasps and flies parasitize the eggs, larvae, or pupa of the caterpillar pests. Avoiding use of broad-spectrum insecticides during the early growth stages of broccoli when DBM and ICW have low populations and cause moderate damage may help to conserve populations of natural enemies that may help to suppress caterpillar populations later in the season. Some natural enemies occur in high enough numbers to provide good control whereas other species need to be mass released to provide control. Only some parasitoids, however, are commercially available which are listed in the table below (5, 13, 17).

Some carabid ground beetle species eat maggot eggs. In addition, parasitic wasp species and a rove beetle species parasitize the egg or the larval stages. The nematode Steinernema carpocapse populations that are usually present in the soil provide some control of a variety of soil-inhabiting insects, including cabbage maggots. Moreover, S. carpocapse are commercially available. The nematodes must be mass released under moist conditions to provide control. None of the other natural enemies, however, are commercially available. The naturally occurring populations of predators and parasitoids enemies usually do not provide sufficient control to prevent economic damage (5, 13, 17).

A small aphid wasp, Diaeretiella rapae, is the most common parasite of the cabbage aphid. Although D. rapae is very common, it usually does not effectively control the cabbage aphid. By the time wasp populations have increased in sufficient numbers, the aphid population has often exceeded threshold levels. Moreover, D. rapae is often killed by hyperparasites. Augmentative releases of this commercially available wasp could provide effective control (5, 13, 17).

In addition, some of the generalist predators such as ladybird beetles (family Coccinellidae), syrphid fly larvae (family Syriphidae), green lacewing larvae (Chrysoperla sp.), minute pirate bugs (Orius sp.), and damsel bugs (Nabis sp.) provide some control of aphids especially when the aphid colonies are small. Some ladybird beetles species, along with lacewing larvae, and minute pirate bugs are commercially available (5, 13, 17).

Few natural enemies of the cabbage flea beetle provide substantial control. The commercially available nematode Steinernema carpocapsae infects flea beetles but encapsulation methods need to be improved to increase nematode persistence within the soil (5, 13, 17).

No effective means of biological control of thrips specifically on broccoli are currently available. Although some predatory mites are commercially available for control on some greenhouse and cucumber outdoor crops, more research is needed to increase better control in broccoli crops (5, 13, 17).

Destroying crop residue immediately after harvest eliminates breeding sites where populations can build up and move to newer broccoli plantings. Clean cultivation also destroys potential overwintering sites.

In well-drained fields, overhead irrigation may wash off DBM larvae and disrupt adult activity to reduce DMB populations by as much as 80%.

In small fields, floating row covers can prevent adult moths from laying eggs on plants (5, 13, 17).

Since the first generation of root maggots is the most damaging, planting seeds or transplants after the peak of adult emergence and egg laying in the spring may provide the best control. A grower may predict the peak egg laying period using degree days. Other cultural control practices consist of avoiding plowing fresh animal manure, weeds, green manure or other cover crops in spring because root maggots are often attracted to rotting organic matter and rotating broccoli to avoid infestations (2, 5, 13, 17).

Aphids are less attracted to crops when a cover crop mulch is planted between the rows than when the crop is highlighted against a bare soil background. Moreover, some cover mulches may provide additional food sources to the aphids’ natural enemies. However, some weedy plots may increase flea beetle populations. Destroying and removing crop residue after harvest provides fewer overwintering sites for aphids, along with the DBM, ICW, and cabbage maggot (5, 8, 13, 17).

Planting early in spring may avoid high populations of flea beetles when plants are small and most susceptible to damage. In addition, flea beetles tend to have higher populations in weedy fields. Clean sanitation practices in and around the field may help to reduce flea beetle populations. Weedy plots may provide food sources for the natural enemies of some of the other broccoli pests (5, 9, 13, 17).

Avoid planting next to or down wind from wheat or alfalfa fields. Thrips often build up large populations in wheat or alfalfa and may move to broccoli crops when the wheat or alfalfa is harvested (5, 13, 17).

Alternaria leaf spot may be more prevalent during moist, warm conditions. Seedlings may be especially susceptible although this disease is also a problem in storing broccoli. On seedlings, small black dots appear on the stems, often causing their collapse. Leaf spots on the broccoli head begin as small, dark dots that can enlarge to dark, circular, water soaked lesions. Large masses of spores are produced in the infected areas.

Moist wind currents can carry spores between fields. Rain and equipment can also disseminate the spores. The spores can also overwinter in old, infected plant debris and within the seed coat.

Also see fact sheet: http://www.extension.umn.edu/distribution/horticulture/DG1169.html

Black leg can be a serious disease of broccoli but applying preventive measures can control it. Cool, humid conditions provide the most favorable condition for its growth. Black leg can infect broccoli during any of its growth stages. The first symptom that typically appears is a depressed canker at the base of the stem that may eventually surround the entire stem. At first, the black, circular spots or lesions on the foliage are inconspicuous. As the spots get larger, the yellow spots develop gray centers filled with small black dots (the fungal structures). The spots on the lower leaves are more linear shaped with purplish margins and many small black dots filling the center. If the disease spreads throughout the plant into the root system, the entire plant may collapse. The leaves wilt but generally do not fall off.

The disease remains dormant in plant debris and seed coats. If a plant or seedling is infected, spores can be spread to other susceptible plants through splashing and running water, insects, animals, and equipment. The fungus can survive for 2 winters in plant debris.

Also see fact sheet: http://www.extension.umn.edu/distribution/horticulture/DG1169.html

Black rot, which is caused by the bacterium, Xanthomonas campestris pv. campestris, is a very serious disease on broccoli crops. Losses from this disease tend to be higher in years when moisture is plentiful and the average temperature ranges from 60-70° F.

The bacterium often enters the pores on the leaf or through holes caused by hail or insects. The bacterium then spreads throughout the water conducting tissue of the broccoli. The leaf turns yellow in a v-shaped pattern with the wider area at the fringe of the leaf. The veins often turn black within the yellow area. The yellow sections then typically turn brown and brittle. The entire leaf may fall off. When the stem is cut, a black discolored ring is visible throughout the vascular region.

The disease is transmitted via infected seeds. Bacteria can also overwinter in infected cruciferous weeds or in field debris. Infected seeds may produce diseased heads which can infect surrounding plants. Splashing water from rain or irrigation, large animals, insects, or farm machinery infected with black rot can also spread the disease.

Also see fact sheet: http://www.extension.umn.edu/distribution/horticulture/DG1169.html

Club root, caused by the fungus Plasmodiophora brasicae, can seriously damage the current year’s broccoli crop. In addition, the resting spores can survive within the soil for many years and infect any subsequent crucifer crops. Acidic soil and cool weather are favorable environmental conditions for its growth.

Club root causes the roots to become enlarged and distorted which decreases the ability of the plant to take in water and nutrients. Yellowing and wilting, especially on a hot day, may occur some time after the roots are distorted. Younger plants may die whereas older plants may have stunted growth or never develop marketable heads.

The spores invade the plant by entering its root hairs or wounds. The spores stimulate root growth which resembles large knots or clubs. Eventually, the club root release spores and infects the surrounding soil. Spores are also spread by splashing and running water, farming equipment, animals, and humans that carry spores to an uninfected field.

Also see fact sheet: http://www.extension.umn.edu/distribution/horticulture/DG1169.html

Downy mildew (Peronospora parasitica)

Downy mildew is an important disease that is typically more prevalent on early or late maturing crops in moist, cool conditions. The disease typically attacks seedlings but it may it may also attack in a later growth stage, causing discolored broccoli.

The first symptom that typically develops is a grayish white, fluffy growth on the underside of the leaves. Irregular yellow and brown spots may then develop on the upper yellow leaf surface. The spots often develop in into purplish, sunken spots on the broccoli. If the lower leaves are infected, the fungus can invade the plant systemically. Moreover, soft rotting bacteria may also invade the lesions.

The spores are spread between plants through air currents and rainstorms. The fungus overwinters as resting spores in plant debris left in the fields.

Also see fact sheet: http://www.extension.umn.edu/distribution/horticulture/DG1169.html

Fusarium yellows (Fusarium oxysporum f. sp. conglutinans)

Fusarium yellows, which is a soilbourne fungus, can be a problem for susceptible broccoli cultivars. Fusarium yellows is most severe when temperatures exceed 70° F.

The first symptom is yellowing of one of the lower leaves. As the disease spreads, more leaves turn yellow. The yellowing often occurs more intensely on one side of the broccoli although sometimes the entire broccoli turns yellow. The disease progresses from the lower leaves to the upper leaves. The leaves become brown and brittle. The veins may also turn black and resemble black rot.

Thick walled spores can remain dormant in the soil for years until the right environmental conditions and susceptible broccoli are planted. When a susceptible broccoli is planted, the spores germinate, and the fungus penetrates the plants through the roots or wounds. The fungus then progresses through the water conducting tissue to infect the rest of plant. The spores are also spread through infested soil on farm machinery and tools, and transported through animal and human foot traffic.

Damping off (Pythium spp.) and Wirestem (Rhizoctonia solani)

Soilborne fungi, Rhizoctonia solani, or several of the Pythium spp cause damping off or seedling diseases. Pythium can attack seeds and cause them to rot before they germinate. Pythium can also attack seedlings before they emerge above the soil line or after the emerge above soil line. Pythium often causes lesions on the stems that cause the seedling to collapse, and then becomes dark and shriveled. Rhizotina invades the cortical cells of seedlings which may girdle the stem. Some broccoli crops continue to grow slowly after Rhizoctonia invasions but the stem is typically obtains a small, spindly, woody characteristic that is referred to as wirestem.

Either disease may occur anywhere in field but typically occurs under wet conditions. Pythium is more of a problem when seeds are planted in cold, damp soils. Rhizotinia, however, can be a problem in warmer, damp soils. Fields high in green organic matter, poor drainage, or compacted soils provide conditions that make seedlings more susceptible to these diseases.

The best controls are good sanitation practices, good preparation of seedling beds, seed treatment, and not planting seeds in cold soil. In warmer soils, seeds grow more vigorously, and have less time to be susceptible to dampening off diseases.

Weeds compete with broccoli crops for light, nutrients, and water. Weeds that exist in and around the field can also harbor disease pathogens and pest insects that can invade the broccoli crop after planting. Many annual weeds produce copious amounts of seeds that often remain viable in the soil for years. Early in the growing season, cultivation may control weed seedling. As the growing season progresses, however, cultivation may damage the broccoli roots. Application of herbicides may be the only effective control method (13, 14).

Mr. Patrick O’Rourke
MN Pesticide Survey & Impact (PSI) Group
Dept. of Entomology
219 Hodson Hall
1980 Folwell Avenue
St. Paul, MN 55108
Phone: 612-624-9292
Email: orour010@tc.umn.edu

Dr. William D. Hutchison
University of Minnesota
Pesticide Survey & Impact Group
Department of Entomology
219 Hodson Hall
1980 Folwell Avenue
St Paul, MN 55108
Phone: 612-624-1767
Email: hutch002@maroon.tc.umn.edu

Mr. Eric Burkness
University of Minnesota
Department of Entomology
219 Hodson Hall
1980 Folwell Avenue
St Paul, MN 55108
Phone: 612-624-3670

Dr. James Percich
University of Minnesota
Department of Plant Pathology
316 Stakman Hall
1519 Gortner Ave
St Paul, MN 55108
Phone: 612-625-6240

Dr. Roger Becker
University of Minnesota
Department of Agronomy/Plant Genetics
Agronomy and Plant Genetics
A 203A Hayes Hall
1509 Gortner Ave
St Paul, MN 55108
Phone: 612-625-5753

Acknowledgements

The Minnesota broccoli crop profile was compiled by and edited by Patrick O’Rourke, Kathleen Bennett and W. D. Hutchison, Minnesota Pesticide Survey and Impact Group, Department of Entomology, University of Minnesota.

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3. Cole Crop Production Requirements. 2000. University of Minnesota Extension Service http://www.vegedge.umn.edu/vegpest/colecrop/product.htm
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11. Hutchison, W.D., Bolin, P.C., and Hines, R.L. 2001. Diamondback Moth. University of Minnesota Extension Service. http://vegedge.umn.edu/vegpest/colecrop/diamond.htm
12. Hutchison, W.D., Bolin, P.C., and Hines, R.L. 2001. Imported Cabbbageworm. University of Minnesota Extension Service. http://vegedge.umn.edu/vegpest/colecrop/cabbworm.htm
13. Integrated Pest Management for Cole Crops and Lettuce. 1987. University of California Statewide IPM Project Division of Agriculture and Natural Resources Publication 3307.
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15. Metcalf, R.R. and Metcalf, R.A. 1993. Destructive and Useful Insects Their Habits and Control. 5^th Ed. Insects Injurious to Cucurbits. McGraw-Hill, Inc. New York.
16. Plfeger, F.L. and Gould, S.L. 1999. Diseases of Cole Crops. University of Minnesota Extension Service. http://www.extension.umn.edu/distribution.horticulture/DG1169.html
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