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Carrot

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Description

Carrot, Daucus carota, is an edible, biennial herb in the family Apiaceae grown for its edible root. The carrot plant produces a rosette of 8–12 leaves above ground and a fleshy conical taproot below ground. The plant produces small (2 mm) flowers which are white, red or purple in color. The root can grow to between 5 and 50 cm (2.0–20 in) long and reach 5 cm (2.0 in) in diameter. The foliage of the plant can reach a height of 150 cm (59.1 in) when in flower. The carrot plant can be annual or biennial and may also be referred to as wild carrot. The plant is believed to have originated in Europe or the Western Mediterranean.


Uses

Carrot roots are eaten as a vegetable and can be consumed fresh or cooked. Carrot juice is consumed as a beverage. The leaves of the plant can be used as feed for animals.


Propagation

Basic requirements Carrots are cool-season crops which can be planted in early Spring and left in the ground all summer for harvest in the fall. Carrots grow best in a well-draining, loose, sandy soil which is free of large rocks and has a pH between 5.5 and 7.0. They require full sun for optimum development but will tolerate some very light shade. The optimum temperature for their growth is between 16 and 24°C (61–74°F). Carrot seedlings are very sensitive to extremes of temperature, very hot weather may kill the plant whereas temperatures below 10°C (50°F) tend to cause the development of longer, paler roots. The plants also require plenty moisture and organic matter. Carrots do very well in raised beds and can also be grown in containers. Sowing seeds Carrot is usually direct seeded and should be planted 3–5 weeks before the least frost date in your area. Soil should be prepared prior to planting by removing rocks and breaking up any hard lumps of soil down to a depth of at least 30 cm (12 in). It is also beneficial to work some compost into the soil prior to planting. Avoid using fresh manure as it can cause forking of the roots. Sow seeds 6 mm (0.25 in) deep, leaving 5 cm (2 in) between seeds and approximately 30 cm (1 ft) apart. When seedlings reach 2.5 cm (1 in) in height, thin them to a final spacing of 7.5 cm (3 in) between plants by snipping with scissors - this avoids damaging plant roots. General care and maintenance Carrots benefit from a plentiful moisture supply and should be provided with 2.5 cm (1 in) of water each week. Mulching around the plants helps to conserve moisture and reduce weeds. Any weeds growing around the plants should be carefully removed. The plants should be fertilized 5–6 weeks after the seeds are sown. Harvesting Carrots are generally ready to harvest after around 2–3 months when the roots have reached 1.3 cm (0.5 in) in diameter. Allowing the carrots to stay in the ground for at least one frost makes them sweeter but care should be taken to harvest before the ground freezes or to cover the plants to prevent freezing. Carrots are harvested by gently digging around the plant to expose the top of the root and gently, but firmly pulling the root from the soil by grasping the top of the carrot just above the root. Carrot tops should be twisted off and the roots washed prior to refrigeration in airtight bags. Carrots may also be stored in moist sand to keep them fresh prior to use.


References

CABI Crop Protection Compendium. (2008). Daucus carota datasheet. Available at: http://www.cabi.org/cpc/datasheet/18018. [Accessed 10 November 14]. Paid subscription required. Davis, R. M. & Raid, R. M. (2002). Compendium of Umbelliferous Crop Diseases. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopapspress/Pages/42872.aspx. Available for purchase from APS Press. Delahaut, K. A. (1998). Growing Beets, Radishes and Other Root Crops in Wisconsin. A Guide for Fresh Market Growers. University of Wisconsin Extension. Available at: http://learningstore.uwex.edu/assets/pdfs/A3686.PDF. [Accessed 10 November 14]. Free to access. Ophardt, M. (2013). Vegetables. Growing Carrots in Home Gardens. Washing State University Extension. Available at: http://mtvernon.wsu.edu/path_team/Growing-Carrots-in-Home-Gardens-WSU-FS118E-Ophardt-2013.pdf. [Accessed 10 November 14]. Free to access.


Common Pests and Diseases

Diseases

Category : Fungal

Alternaria leaf blight Alternaria dauci

Symptoms
Green-brown water-soaked lesions on leaves which enlarge and turn dark brown or black; lesions may coalesce causing leaves to yellow and die; lesions may be present on petioles
Cause
Fungus
Comments
Disease emergence favored by wet foliage and warm weather; rain and fog enhance the development of the disease; fungus survives in soil on crop debris but is killed when the debris decomposes
Management
Disease can be difficult to control in wet, warm conditions; apply appropriate fungicides when first symptoms appear or as a protective measure in humid areas; treat seeds with fungicide or hot water prior to planting; apply gibberellic acid to carrot foliage to promote upright growth and promote air circulation through canopy

Black rot Alternaria radicina

Symptoms
Damping-off of seedlings; root and crown necrosis; blighted foliage; lower portion of petioles black and necrotic; black ring around petiole attachment black, sunken lesions on taproot
Cause
Fungi
Comments
Disease is spread through infected seed and can survive in soil for up to 8 years
Management
Black rot is difficult to control and can survive in the soil for long periods of time: practice long crop rotations ; plow crop residue into soil immediately after harvest; plant resistant varieties; plant only pathogen-free seed; treat seeds with hot water prior to planting

Cercospora leaf blight Cercospora carotae

Symptoms
Small, necrotic flecks on leaves which develop a chlorotic halo and expand into tan brown necrotic spots; lesions coalesce and cause leaves to wither, curl and die
Cause
Fungus
Comments
Disease can be introduced through infested seed and spread by wind or water splash; symptoms usually occur on younger foliage first
Management
Plant only pathogen-free seed; rotate crops; plow crop debris into soil ofter harvest; apply appropriate fungicide sprays

Cottony rot (Sclerotinia rot) Sclerotinia sclerotiorum

Symptoms
Small, water-soaked, soft lesions on crown and roots; white fluffy fungal growth all over affected tissues; soft and decaying tissue developing
Cause
Fungus
Comments
Fungus can survive in soil for up to 10 years; disease emergence is favored by soils that are held close to saturation for periods in excess of 2 weeks
Management
Cultural practices play an important role in the control of cottony rot as there are no resistand carrot varieties: in carrot fields, the use of drip irrigation 5-8 cm below the soil surface can provide good control; deep plowing of soil and trimming back carrot foliage to promote air circulation can also be useful; fungicides may be warranted in periods of extended cool, damp weather

Downy mildew Peronospora umbellifarum

Symptoms
Yellow spots on upper surface of leaves; white fluffy growth on underside of leaves; lesions become darker as the mature
Cause
Fungus
Comments
Disease affects young, tender leaves; disease emergence and spread is favored by prolonged leaf wetness
Management
Plant pathogen-free seed; do not overcrowd plants; rotate crops with non-umbelliferous varieties

Powdery mildew Erisyphe heraclei

Symptoms
Powdery growth on leaves, petioles flowers stalks and bracts; leaves becoming chlorotic; severe infections can cause flowers to become distorted
Cause
Fungus
Comments
Fungus can spread long distances in air; disease emergence is favored by high humidity and moderate temperatures; infection is most severe in shaded areas
Management
Plant tolerant varieties; avoid excess fertilization; protective fungicide applications provide adequate protection; sulfur application can be used in infection occurs early in season

Category : Bacterial

Bacterial leaf blight Xanthomonas campestris

Symptoms
Small, angular, yellow spots on leaves which expand into irregularly shaped, brown, water-soaked lesions with a yellow halo; centers of lesions dry out, become brittle; leaves may become curled or distorted; flower stalks may develop elongated lesions that exude a bacterial ooze; infected umbels may be blighted
Cause
Bacterium
Comments
Bacteria can be spread by splashing irrigation water or rain or on contaminated equipment
Management
Plant pathogen-free seed; avoid using sprinkler irrigation; apply appropriate bactericides if available

Soft rot Erwinia carotovora
Erwinia chrysanthemi
Pseudomonas marginalis

Symptoms
Sunken dull orange lesions on taproot which causes tissue to collapse and become soft
Cause
Bacteria
Comments
Bacteria thrive in oxygen depleted plant tissue; disease emergence requires long periods of water saturated soil; bacteria enter plants through wounds
Management
Control relies on the avoidance of conditions conducive to bacterial infection: plant carrots in well-draining soils; allow plants to dry before irrigating again; avoid wounding plants during harvest to prevent pst harvest development of disease; disinfect all equipment regularly

Category : Oomycete

Cavity spot Pythium spp.

Symptoms
Sunken, elliptical, gray lesions across the root; outer layer of root ruptures and develops dark, elongated lesions; small vertical cracks may form on the cavities
Cause
Fungi
Comments
Fungi can persist in soil for several years and disease outbreaks are associated with wet soils; flooded soil increases the number of cavities formed
Management
Some cultural practices can control the disease: avoid planting in fields/areas known to previously had carrot spot; do not over-fertilize plants; application(s) of appropriate fungicide can provide adequate control

Category : Fungal, Oomycete

Damping-off Pythium spp.
Rhizoctonia solani

Symptoms
Soft, rotting seeds which fail to germinate; rapid death of seedling prior to emergence from soil; collpase of seedlings after they have emerged from the soil caused by water-soaked reddish lesions girdling the stem at the soil line
Cause
Fungi
Comments
Damping-off diseases favor conditions which slow seed germination; fungi can be spread in water, contaminated soil or on equipment
Management
Avoid planting carrots in poorly draining, cool, wet soil; planting in raised beds will help with soil drainage; plant high quality seed that germinates quickly; treat seeds with fungicide prior to planting to eliminate fungal pathogens

Pests

Category : Insects

Aphids (Willow-carrot aphid) Cavariella aegopodii

Symptoms
Small soft bodied insects on underside of leaves and/or stems of plant; usually green or yellow in color; if aphid infestation is heavy it may cause leaves to yellow and/or distorted, necrotic spots on leaves and/or stunted shoots; aphids secrete a sticky, sugary substance called honeydew which encourages the growth of sooty mold on the plants
Cause
Insect
Comments
Distinguishing features include the presence of cornicles (tubular structures) which project backwards from the body of the aphid; will generally not move very quickly when disturbed; willow-carrot aphid will also attack parnip, parsley and celery
Management
If aphid population is limited to just a few leaves or shoots then the infestation can be pruned out to provide control; check transplants for aphids before planting; use tolerant varieties if available; reflective mulches such as silver colored plastic can deter aphids from feeding on plants; sturdy plants can be sprayed with a strong jet of water to knock aphids from leaves; 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

Carrot rust fly Psila rosae

Symptoms
Surface scarring of taproot caused by tunnels; tunnels are filled with a rust colored mush; adult insect is a small, dark colored fly; larvae are white maggots approximately 1 cm (0.3 in) long
Cause
Insect
Comments
Carrot rust fly also attacks parsnip, celery and other Umbelliferous crops which will also need to be protected if carrot rust fly is a problem
Management
Use of row covers will help to protect plants from damage but they must be installed before adult fly lays eggs on plants; harvest carrots in blocks; do not leave any carrots in the ground over winter to reduce overwintering sites

Carrot weevil Listronotus oregonensis

Symptoms
Irregular dark grooves in zig-zag pattern on roots; leaves of plant may yellow; adult insect is a dark colored beetle; larvae are white to pinkish white C-shaped grubs with a yellow-brown head
Cause
Insect
Comments
Adult weevils overwinter in crop debris remaining in the ground; larvae feed for approximately 2 weeks before pupating in the soil; insect undergoes several generations each year
Management
Remove all debris from Umbelliferous crops (e.g. parsley, dill, celerey etc.) to reduce sites where weevil can survive and persist; try to rotate Umbelliferous crops to different areas of the home garden each year to reduce survival of larvae in soil

Flea beetle Systena blanda

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
Insect
Comments
Younger plants are more susceptible to flea beetle damage than older ones; older plants can tolerate infestation; 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

Category : Nematodes

Root-knot nematodes (Stubby root nematodes, Needle nematodes) Meloidogyne spp.
Paratrichodorus spp.
Longidorus africanus

Symptoms
Forked, distorted or stunted taproots; reduced stand; reduced yield
Cause
Nematodes
Comments
Root-knot nematodes are most damaging to carrot; nematodes are microscopic and not visible to the naked eye
Management
Leaving land to fallow when not planting can be effective at reducing nematode numbers; solarizing soil for 4-6 week period to a depth of 6 inches can temporarily reduce nematode populations; new carrot varieties are currently being developed that are resistant to nematodes
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