Cole Crops Integrated Pest Management (E3231)
Michigan’s economically important cole crops include cabbage, cauliflower and broccoli. Cole crop integrated pest management (IPM) starts with crop rotation, cultivar selection and proper transplant production.
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Cole crops belong to the mustard family, or Brassicaceae. Michigan’s economically important cole crops include cabbage, cauliflower and broccoli. Cole crops grown on smaller acreages include Brussels sprouts, turnips, rutabaga, kale, mustard greens and many other types of greens. Cabbage, cauliflower, broccoli and Brussels sprouts are all the same species, Brassica oleraceae, and have been selected for different economic traits. For example, broccoli and cauliflower are flower mutants, while cabbage and Brussels sprouts are stem mutants. In Michigan, cole crops are grown for fresh and processing markets, while some mustards are planted as cover crops.
Cole crop integrated pest management (IPM) starts with crop rotation, cultivar selection and proper transplant production. Cole crops can be planted early in the spring and late in the fall, which makes them ideal to incorporate easily in crop rotation. The use of tunnels, cold frames and row covers protect the crops from spring and fall frost. Drip irrigation along with plastic mulch can improve moisture delivery and retention, suppress weeds and decrease disease pressure. The system can be reused for tender crops that follow early-planted cole crops in the same season, such as tomatoes, peppers, cucumbers, summer squash and zucchini among others.
When selecting cole crops cultivars, consider not only the market and days-to-maturity, but also resistance to insects, diseases and disorders. Cole crop production must start with healthy transplants in the greenhouse that are frequently scouted for disease and insect pests. Be sure to practice sanitation. Avoid excessive watering or heavy nitrogen fertilization at this stage, which can contribute to poor root system development or disease development.
Crop rotation, cultivation, mulching and use of cover crops are practices that contribute to weed management.
Maintain a record of weed species or types observed in the field in the previous season. It is important to have an idea of the population of broad leaf and grass weeds in your field to select the best herbicide combinations. (Consult Michigan State University Extension bulletin E-433 for current labeled herbicides.) When selecting herbicides, use different Weed Science Society of America (WSSA) classifications listed on the herbicide label to minimize the risk of resistance development. Use at least two herbicides preemergence of the weeds either before or after transplanting. Always read the labels for potential limitations. Most herbicides require moisture for activation.
Herbicide injury. The main two causes of herbicide injury in crops are improper timing or rate, and unintended exposure of the crop to herbicides. High rates of clomazone (WSSA11) can cause marginal leaf chlorosis in some cabbage cultivars. High rates of trifularin combined with wet conditions can cause injury in seedlings that range from stunting to hypocotyl (stem) enlargement.
Glufosinate (WSSA 10) drift causes chlorosis and desiccation at the contact point. Herbicides in WSSA group 2 (Sulfonylureas), whether applied as foliar or soil incorporated, can result in foliar discoloration (chlorosis or purpling of the new growth). The extent of the damage depends on the rate the herbicide was applied and environmental conditions. Triazine and benzoic acid (WSSA 5 and 3 respectively) can injure crops due to carryover. For example, Triazines can cause leaf margins or interveinal chlorosis. Always consult the label for rotational restrictions, appropriate rate and timing.
Problematic weeds. Controlling weeds in the mustard family, including wild mustards, wild radish, shepherds purse, marsh yellow cress and mouse ear cress, is very important for disease management as they can be alternate hosts for diseases.
NUTRIENT-RELATED PHYSIOLOGICAL DISORDERS
Nutritional deficiencies can occur in cole crops due to inadequate nutrient uptake related to soil pH, soil moisture, weather conditions and lack of appropriate fertilization. Soil and plant tissue analysis is recommended to help pinpoint the specific nutrient deficiency.
Nitrogen deficiency is associated with yellowing of older leaves. In contrast, excess nitrogen can impact the quality of cabbage heads due to the increase in foliage development.
Potassium deficiency shows as yellowing along leaf margins or scattered, chlorotic spots that turn necrotic. This deficiency may impact crop quantity and quality if the symptoms are observed in early stages of the crop.
Boron deficiency may occur on acidic (pH less than 6.0) or alkaline soil (pH greater than 7.0). It is associated with lack of appropriate soil moisture and is most common in sandy or organic soils with low organic matter. Low soil moisture accentuates boron deficiency. Excess rainfall may leach boron from the root zone of sandy or muck soils. Symptoms in seedlings can be observed as brittle, yellow and distorted young tissue. Boron deficiency may play a role in broccoli and cauliflower hollow stem.
Calcium deficiency may occur on acidic soils with intermittent or low moisture and when weather conditions are favorable for rapid crop development. Slow calcium translocation can be driven by high concentrations of other cations (e.g., Na+, K+, NH4+, Mg2+), high salinity, low temperatures and high humidity. Symptoms of calcium deficiency include tip burn in cabbage heads, broccoli brown bud and cauliflower curd breakdown and leaf tip burn (see photo).
Manganese or magnesium deficiencies. Interveinal chlorosis could be a symptom of manganese or magnesium deficiencies. Manganese deficiency causes an olive-green to yellowish discoloration between the veins, while magnesium deficiency results in a mottled yellowing between the veins. Manganese deficiency is more likely to occur when the soil pH is above 6.5 in mineral soils and above 6.0 in muck soils. Magnesium deficiency may occur in acidic sandy soils (pH less than 5.8) and is more prevalent in older leaves.
Molybdenum deficiency is common in cauliflower and causes the symptoms known as whiptail, a narrowing of the leaf blades when severe. When less severe, leaf blade expansion is moderately restricted.
Sulfur deficiency results in a general yellowing of the leaves and is most likely to occur on low organic matter sandy soils.
Other less common micronutrient deficiencies are iron and zinc. For additional information, consult Rimmer S. R, et. al (see resource section). For specific nutrient management recommendations, consult MSU Extension bulletins E2934 and E486.
A combination of IPM tactics is needed to manage the key insects that damage cole crops in Michigan (Table 1). These tactics include cultural practices, cultivar selection, biological and chemical control. When using pesticides, monitoring pests and using treatment thresholds can reduce the number of applications needed to achieve effective control. When treatment thresholds are reached, alternate insecticides with different modes of action to prevent insecticide resistance from developing. The Insecticide Resistance Action Committee (IRAC) group number on the insecticide label represents modes of action. Alternation of IRAC groups minimizes the risk of insecticide resistance. Products labeled for organic insect management in cole crops includes Spinosad, botanicals and oils. Consult the “Production guide for organic cole crops” NYS IPM publication No 1342.
Larvae are yellow-white and approximately 0.25 inches long. Cool, wet springs favor this pest. Select plant varieties tolerant to this pest when available, rotate away from cole crops and thoroughly incorporate crop residue. Time planting to avoid adult peak flight – at least one week before or after the peak. To check for peak flight, visit the MSU Enviro-weather website at http://www.enviroweather.msu edu a.nd navigate to the vegetable tab. If cole crops are planted during peak flight period, apply a soil insecticide. (Check MSU Extension bulletin E312 for registered insecticide options.)
Many genera and species, Family Chrysomelidae
Adult beetles are approximately 2 to 3 millimeters long and have strong hind legs for jumping. Early crop planting and soil applied insecticides are recommended on fields with known infestations. Rotating crops and destroying crop residue can help decrease flea beetle numbers in the field. Row covers can be used to exclude flea beetles.
Cabbage, turnip and green peach aphids attack cole crops and can cause severe infestations in warm weather. Crop residue removal and conservation of natural enemies can help to manage the aphid populations in the field. When threshold is reached (see Table 1), thorough coverage with an insecticide, including the underside of leaves, is critical. Select an insecticide spray program that protects natural enemies; avoid using broad-spectrum insecticides and use those that target aphids specifically, such as pymetrozine or flonicamid.
Thrips rasp cole crop tissues, which results in scratch-like patterns (see photo) on cabbage and cauliflower. Thrips numbers quickly increase in hot and dry weather. Select varieties that can tolerate thrips and avoid planting cole crops close to alfalfa, clover or small grains, which can be a source of this pest. Insecticide applications are more effective at the beginning of cupping or curd formation.
For additional information on caterpillar pests, including diamond back moth, imported cabbage worm and cabbage looper, consult MSU Extension bulletin E3165.
Manage diseases with multiple tactics that apply the principles of plant disease control (Table 2). The sum of the tactics is the IPM strategy. The FRAC group (Fungicide Resistance Action Committee group) number on the fungicide label represents the fungicide mode of action. Spray programs that rotate fungicides with different modes of action are needed to prevent fungicide resistance development.
Caused by the soil and seedborne pathogen Rhizoctonia solani
In young plants, characteristic wire stem symptoms are constriction and girdling of the stems that can result in wilting and damping off in the greenhouse and field (see photo). In mature plants, this pathogen causes black, sunken lesions called crater rot in radishes, rutabagas and turnips. It causes a soft head rot in cabbage. Conditions that favor this disease are planting on soils with low macronutrient levels (N, P and K), high copper levels and seasons with high rainfall that result in high soil moisture levels and temperatures greater than 68 degrees Farenheit.
Caused by the seedborne pathogen Xanthomonas campestris pv. campestris
Distinctive symptoms are V-shaped, brown lesions with yellow margins (see photo) in the leaves including head leaves. Black rot is favored by warm (68–77 F), humid conditions. This bacterial pathogen spreads from plant to plant through splashing rain or irrigation water.
Caused by the soilborne pathogen Plasmodiophora brassicae
Typical symptoms include clublike, deformed roots (see photo) and stunted plants. Low soil pH of less than 6.5 and wet conditions favor this disease. Swimming spores move in water to infect plant roots. Resting spores can survive many years in the soil.
Alternaria leaf spot
Caused by several Alternaria spp. (A. brassicae, A. brassicicola and A. japonica)
The symptoms caused by these seed-borne pathogens are brown-to-gray, round lesions with concentric circles and yellow margins (see photo). Sunken, dark-brown lesions can occur in broccoli curds and cabbage heads. These fungal pathogens are spread by wind, rain or runoff water, and favored by high relative humidity and temperatures ranging from 52-88 F.
Caused by Hyaloperonospora parasitica
Downy mildew occurs under cool (about 59 F) and humid conditions. This pathogen is windborne and can be dispersed short distances by water splashed from irrigation or rainfall. Typical symptoms are discolored, greenishyellow to light-brown lesions with sporulation on the leaf undersides, while gray-to-brown discoloration can occur in curds and heads in the field or postharvest. The pathogen can overwinter in cruciferous weeds or, especially in years following a mild winter, in cole crop volunteers.