Rapeseed (colza)

Description

Rapeseed, Brassica napus, is an herbaceous annual or biennial member of the family Brassicaceae primarily grown for the oil which can be extracted from its seeds. The rapeseed plant has several erect, branched stems originating from a single base, the stems are purple in color towards the base. The leaves of the plant are bluish-green and mostly smooth. The basal leaves are stalked whereas the highest leaves grow straight off of the stem. The plant produces pale to bright yellow flowers which are 11–15 mm (0.4–0.6 in) in diameter, and after pollination the plant develops pods containing a single row of seeds. Each pod can contain 20–40 dark brown to black seeds. The rapeseed plant can reach 1.0–2.5 m (3.3–8.2 ft) and is grown as an annual, harvested after one growing season. Rapeseed may also be referred to as colza, oilseed rape, canola, swede rape or fodder rape and is believed to originate from the Mediterranean.


Uses

The seeds of the plant are used to extract rapeseed oil which can be used as a cooking oil or in the production of margarine. Some recently developed cultivars have a high erucic acid content which is extracted for the production of industrial oil. Rapeseed is also grown as fodder for livestock.


Propagation


Basic requirements
Rapeseed is adapted to grow in cool, moist climates, requiring a temperature range of 2–10°C (35.6–56°F), although temperatures closer to 10°C (56°F) promote the most rapid growth. As a result, rapeseed is grown as a cool season crop in sub-tropical regions and a winter crop in more temperate areas. Rapeseed can be grown on a variety of soil types but medium textured, well-draining soils work best. Rapeseed should be grown in soil with a pH between 5.5 and 8.3. Rapeseed should not be grown in soil in which other brassicas have been grown within the past 3–4 years.

Propagation
Rapeseed in almost always propagated from seed which is sown in prepared fields by drilling in rows. Seeds are sown shallowly as the seeds are very small and seed should be sown at a depth of 2–3 cm (0.8–1.2 in). Spring crops of rapeseed should be drilled in rows 18–23 cm (7–9 in) apart, whereas winter crops require more space and should be drilled in rows approximately 40 cm (16 in) apart.

General care and maintenance
It is important to control weed growth in rapeseed fields as they can have a significant impact on the growth and productivity of the crop. Good preparation of the seedbed helps to limit weed growth. This is achieved by tilling the soil in the fall prior to planting, followed by shallow cultivation just prior to seeding. Rapeseed has a high nutrient requirement and a soil test should be carried out prior to planting in order to prevent nutrient deficiencies from occurring. fertilizers are most successfully applied to the side of the seed furrows to prevent damage to the plants. Nitrogen fertilizer should be applied at a basic rate of 50–60 kg per hectare for spring planted rapeseed and 70 kg per hectare for winter crops. Fertilizer is commonly applied at time of sowing. in addition to nitrogen, rapeseed may also require the addition of phosphorus, potassium, sulfur and magnesium to the soil. The rate of application of these nutrients should be determined by a soil test.

Harvesting
Rapeseed is a fast-ripening plant and is ready for harvest when the seeds have turned from green to black in color. Rapeseed is mechanically harvested by combine or by swathing. in some countries, such as China, the crop is cut by hand.


References

CABI Crop Protection Compendium. (2011). Brassica napus var. napus (rape) datasheet. Available at: http://www.cabi.org/cpc/datasheet/10098. [Accessed 06 April 15]. Paid subscription required.

Herbek, J. (2012). Canola. University of Kentucky Cooperative Extension Service. Available at: http://www.uky.edu/Ag/CCD/introsheets.... [Accessed 06 April 15]. Free to access.

Kandel, H. & Knodel, J. J. (2011). Canola production field guide. North Dakota State Extension Service. Available at: http://www.ag.ndsu.edu/extensionentom.... [Accessed 06 April 15]. Free to access.

Rimmer, S. R., Shattuck, V. I. Buchwaldt, L. (Eds) (2007). Compendium of Brassica Diseases. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopap.... Available for purchase from APS Press.


Common Pests and Diseases

Black rot
Xanthomonas campestris

Symptoms
Irregularly shaped dull yellow areas along leaf margins which expand to leaf midrib and create a characterstic "V-shaped" lesion; lesions may coalesce along the leaf margin to give plant a scorched appearance
Cause
Bacterium
Comments
Pathogen is spread via infected seed or by splashing water and insect movement; disease emergence favored by warm and humid conditions
Management
Primary method of controlling black rot is through the use of good sanitation practices; rotate crops to non-cruciferous crops every 2 years; plant resistant varieties; control cruciferous weed species which may act as a reservoir for bacteria; plant pathogen-free seed

Blackleg
Leptosphaeria maculans

Symptoms
Circular gray lesions on the leaf surfaces with black specks forming in the center; cankers form on the stems later in the season and can cause plants to lodge and produce very little seed
Cause
Fungus
Comments
Disease emergence favors wet weather; disease stubble from previously infested crop is the main source of inoculum
Management
Use of resistant varieties is very important in the control of the disease; pathogen varies in virulence throughout the world and rapeseed varieties therefore differ in susceptibility based on geographic location; rapeseed should be planted on a 3 year rotation to promote plant vigor and reduce disease susceptibility; crop stubb;e should be plowed into soil

Downy mildew
Peronospora parasitica

Symptoms
Irregular yellow patches on leaves which turn light brown in color; fluffy gray growth on the undersides of the leaves
Cause
Fungus
Comments
Disease emergence favors cool temperatures; disease spreads quickly in wet conditions
Management
Remove all crop debris after harvest; rotate with non-brassicas; application of appropriate fungicides may be required if symptoms of disease are present

Scleroninia stem rot (White mold)
Sclerotinia sclerotoirum

Symptoms
Bleached stems; white fungal growth in or on the stems near the soil line; black fungal structures develop in the white fungal mass
Cause
Fungus
Comments
Disease emergence favors moderate to cool temperatures and high humidity
Management
Rotate crop to non-hosts (e.g. cereals) for at least 3 years; control weeds; avoid dense growth by planting in adequately spaced rows; apply appropriate foliar fungicides

White rust
Albugo candida

Symptoms
White masses of fungal spores on on the underside of leaves; green blisters on the leaves which turn white
Cause
Fungus
Comments
Fungus can survive for long periods of time in dry conditions; disease spread by wind
Management
Rotate crops; plant only disease-free seed; apply appropriate fungicide if disease becomes a problem

Alternaria leaf spot
Alternaria spp.

Symptoms
Small dark spots on leaves which turn brown to gray; lesions may be round or angular and may possess a purple-black margin; lesions may form concentric rings, become brittle and crack in center; dark brown elongated lesions may develop on stems and petioles
Cause
Fungus
Comments
Disease emergence favors warm, wet conditions
Management
Plant only pathogen-free seed; rotate crops; applications of appropriate fungicides control disease when present

Cutworms (Black cutworm)
Agrotis ipsilon

Symptoms
Stems of young transplants or seedlings may be severed at soil line; if infection occurs later, irregular holes are eaten into the surface of fruits; larvae causing the damage are usually active at night and hide during the day in the soil at the base of the plants or in plant debris of toppled plant; larvae are 2.5–5.0 cm (1–2 in) in length; larvae may exhibit a variety of patterns and coloration but will usually curl up into a C-shape when disturbed
Cause
Insect
Comments
Cutworms have a wide host range and attack vegetables including asparagus, bean, cabbage and other crucifers, carrot, celery, corn, lettuce, pea, pepper, potato and tomato
Management
Remove all plant residue from soil after harvest or at least two weeks before planting, this is especially important if the previous crop was another host such as alfalfa, beans or a leguminous cover crop; plastic or foil collars fitted around plant stems to cover the bottom 3 inches above the soil line and extending a couple of inches into the soil can prevent larvae severing plants; hand-pick larvae after dark; spread diatomaceous earth around the base of the plants (this creates a sharp barrier that will cut the insects if they try and crawl over it); apply appropriate insecticides to infested areas of garden or field if not growing organically

Cabbage aphid
Brevicoryne brassicae

Symptoms
Large populations can cause stunted growth or even plant death; insects may be visible on the plant leaves and are small, grey-green in color and soft bodied and are covered with a white waxy coating; prefer to feed deep down in cabbage head and may be obscured by the leaves.
Cause
Insect
Comments
Cabbage aphids feed only on cruciferous plants but may survive on related weed species.
Management
Insecticides are generally only required to treat aphids if the infestation is very high - plants generally tolerate low and medium level infestation; insecticidal soaps or oils such as neem or canola oil are usually the best method of control; always check the labels of the products for specific usage guidelines prior to use.

Flea beetles
Phyllotreta cruciferae

Symptoms
Small holes or pits in leaves that give the foliage a characteristic “shothole” appearance; young plants and seedlings are particularly susceptible; plant growth may be reduced; if damage is severe the plant may be killed; the pest responsible for the damage is a small (1.5–3.0 mm) dark colored beetle which jumps when disturbed; the beetles are often shiny in appearance
Cause
Insects
Comments
Flea beetles may overwinter on nearby weed species, in plant debris or in the soil; insects may go through a second or third generation in one year
Management
In areas where flea beetles are a problem, floating row covers may have to be used prior to the emergence of the beetles to provide a physical barrier to protect young plants; plant seeds early to allow establishment before the beetles become a problem - mature plants are less susceptible to damage; trap crops may provide a measure of control - cruciferous plants are best; application of a thick layer of mulch may help prevent beetles reaching surface; application on diamotecoeus earth or oils such as neem oil are effective control methods for organic growers; application of insecticides containing carbaryl, spinosad, bifenthrin and permethrin can provide adequate control of beetles for up to a week but will need reapplied