Leek

Description

Leek, Allium ampeloprasum, also known as Allium porrum, is a biennial vegetable in the family Liliaceae, grown for its edible bulb and leaves. The plant is a slightly developed bulb attached to a cylindrical stem formed by the overlapping thick, flat leaves. The plant can produce clusters of white, pink or purple flowers and blue-black seeds in the second year. The plant can reach 0.6–0.9 m (2–3 ft) and can be grown as an annual, harvested after one growing season or as a biennial with two growing seasons. Although modern leek does not grow wild, it was likely domesticated from wild ancestors in the Mediterranean region.


Uses

Leeks are consumed as a vegetable after cooking and are incorporated into many dishes.


Propagation


Basic requirements
Leeks grow very well in cool climates and can be successfully grown in most soils as long as they are rich and well draining. Leek will grow optimally in a well-draining loam with a pH between 5.5 and 7.0. Leek will grow optimally at temperatures between 18-21˚C (65-70˚F) with 8 hours of bright sunlight.

Propagation
In areas with short growing seasons, leeks should be planted from transplants started indoors. Seeds should be planted at a depth of 0.6-1.3 cm (0.25-0.5 in) leaving 7.5-10 cm (3-4 in) between plants and allowing 20-40 cm (8-16 in) between rows. The soil should be moist to a depth of (18 in) and have reached a minimum temperature of 7˚C (45˚F) for successful germination. Transplants should be planted 5-8 cm (2-6 in) apart in rows spaced 30-90 cm (12-36 in) apart. In order to produce large stalks either plant the leek in a depression 7-10 cm (3-4 in) deep and gradually fill to the leaves. Alternatively, the leeks may be planted at ground level, with soil being added around the stalk throughout the season.

General care and maintenance
Leeks require regular watering for optimum development and should be provided with water once a week by soaking the soil to a depth of around 18 inches. Blanching leeks encourages the production of long white stalks. Blanching is achieved by gradually mounding the soil around the stalk to leaves. Blanching should not be carried out until the plants have reached an appropriate size - roughly that of a pencil. Leeks will benefit from the addition of nitrogen fertilizer throughout the growing season. fertilizer should be applied as a side dressing. Keep leek beds weed free by carefully cultivating around the plants taking care not to damage the leek roots.

Harvesting
Leeks develop slowly and take about 100 and 120 days to reach maturity. Leeks are ready for harvest when the stalk has reach 3.5 cm (1 in) in diameter. Harvest by carefully loosening the plant with a garden fork and pulling from the soil.



References

Anderson, C.R. Leeks. University of Arkansas Division of Agriculture. Available at: http://www.uaex.edu/publications/pdf/.... [Accessed 07 February 15] Free to access.

Drost, D. (2010). Leeks in the garden. Utah State Cooperative Extension. Available at: http://extension.usu.edu/files/public.... [Accessed 07 February 15] Free to access.

MacKenzie, J. (2008). Leeks. University of Minnesota Cooperative Extension. Available at: http://www.extension.umn.edu/garden/y.... [Accessed 07 February 15] Free to access.


Common Pests and Diseases

Damping-off
Fusarium spp.

Symptoms
Rotting seeds that are covered in mold; discolored root tips which may be pink, tan, yellow, red or black; slowly growing seedlings which wilt and die
Cause
Fungi
Comments
Fungus survives in soil and disease emergence is favored by moist to wet soil
Management
Plant only disease-free seed; treat seed with fungicide; rotate crops with cereals or grasses to reduce levels of pathogen in soil; steam treatment or fumigation of soil can help reduce levels of Fusarium in the soil

Downy mildew
Peronospora parasitica

Symptoms
Pale spots or elongated patches on leaves; gray-purple fuzzy growth on leaf surface; leaves turning pale then yellow; leaf tips collapsing
Cause
Fungi
Comments
Disease favors cool, humid weather
Management
Avoid planting infected sets; rotate crops to non-allium species for 3-4 years; plant in well-draining areas and do not overcrowd plants; destroy all infected crop debris; apply appropriate foliar fungicides taking care to apply thoroughly to waxy leaves

Purple blotch
Alternaria porri

Symptoms
Small water-soaked lesions lesions on leaves or stalk with white centers; which enlarge to become zonate and brown to purple in color with red or purple margin surrounded by yellow zone; large lesions may coalesce and girdle leaf, killing any tissue between the lesions and the leaf tip; severely infected foliage may die
Cause
Fungi
Comments
Disease emergence favored by wet foliage, with sporulation occurring during the night during periods of high humidity
Management
Cultural controls include long rotations with non-hosts and the reduction of leaf wetness by planting in well-draining soil and timing irrigation to allow plants to dry adequately during the day; some fungicides are effective at controlling the disease but should be rotated for optimal control

White rot
Sclerotinia cepivorum

Symptoms
Older leaves yellowing; stunted growth; death of all leaves; fluffy white growth on base of bulb which spreads up bulb to storage leaves
Cause
Fungus
Comments
Fungus can survive in soil for 20 years and is one of the most damaging diseases of Allium crops worldwide, causing major crop losses
Management
Fungicide treatment may not be effective at controlling white rot under conditions which are favorable to the fungi's development and control may have to rely on cultural methods: avoid transferring soil or plant material between sites; treat seeds with hot water prior to planting; use a long term rotation with non-allium crops; apply appropriate fungicides if available

Botrytis leaf blight
Botrytis squamosa

Symptoms
Small white lesions with light green halos which may expand slightly as they age; in prolonged periods of moisture fungus may develop rapidly and cause leaf blighting
Cause
Fungus
Comments
Disease emergence favors high humidity and warm temperatures; fungus survives on piles of crop debris or in soil; older leaves more susceptible to blighting than younger leaves
Management
Plant leeks in single rows allowing at least 30 cm between plants to promote good air circulation and quick drying of foliage after rain; time irrigation to allow plants time to dry out sufficiently; apply appropriate fungicide sprays when plants have at least five true leaves and early symptoms of disease

Pink root
Phoma terrestris

Symptoms
Light pink roots which darken and turn purple; roots become transparent and water soaked; plant may look like it has a nutrient deficiency; infected seedling may die; stunted plants with undersized, shriveled bulbs
Cause
Fungus
Comments
Fungus colonizes plant through root tips; fungus can survive in soil down to a depth of 45 cm (17.7 in)
Management
Disease is most severe when alliums are planted continuously or in a 1-year rotation, a rotation of 3-6 years is preferred; plant more resistant varieties; solarization and/or fumigation can help reduce the levels of pathogen in the soil

Onion maggot
Delia antiqua

Symptoms
Stunted or wilting seedlings; plant will commonly break at soil line if an attempt is made to pull it up; if infestation occurs when plants are bulbing, bulbs will be deformed and susceptible to storage rots after harvest; adult insect is a greyish fly which lays white, elongate eggs around the base of the plant; the larvae that emerge from the eggs are tiny and white and bore into the onion plant; mature larvae are about 1 cm (0.4 in) long with feeding hooks
Cause
Insect
Comments
Females can lay several hundred eggs during their 2-4 week lifespan; insect overwinters as pupae in the soil
Management
Management of onion maggots is heavily reliant on good sanitation; all bulbs should be removed at the end of the season as maggots will die without a food source; commercial onion growers must often rely on the application of appropriate granular insecticides and, in some cases, insecticide sprays are also required; home gardeners should try to remove any volunteer wild onion and chive plants as these can act as an infection source; floating row covers may provide protection by preventing females from laying eggs around the plants

Thrips
Thrips tabaci

Symptoms
Leaves turning silver to gray in color; leaves twisted and dying
Cause
Insect
Comments
Insect favors hot, arid conditions; builds up immunity to insecticides quickly

Thrips (Onion thrips, Western flower thrips)
Thrips tabaci
Frankliniella occidentalis

Symptoms
Discolored, distorted tissue; scarring of leaves; severly infected plants may have a silvery appearance
Cause
Insect
Comments
Thrips are most damaging when they feed at the early bulbing stage of development; both onion thrips and western flower thrips have an extensive host range and can be introduced from other plants
Management
Natural enemies include some species of predatory mite, pirate bugs and lacewings; avoid planting onion in close proximity to grain fields as thrips populations build up on these plant in the spring; overhead irrigation of plants may help reduce thrips numbers; apply appropriate insecticides at first sign of thrips damage

Leafminers
Lyriomyza spp.

Symptoms
Thin, white, winding trails on leaves; heavy mining can result in white blotches on leaves and leaves dropping from the plant prematurely; early infestation can cause fruit yield to be reduced; adult leafminer is a small black and yellow fly which lays its eggs in the leaf; larave hatch and feed on leaf interior
Cause
Insects
Comments
Mature larvae drop from leaves into soil to pupate; entire lifecycle can take as little as 2 weeks in warm weather; insect may go through 7 to 10 generations per year
Management
Check transplants for signs of leafminer damage prior to planting; remove plants from soil immediately after harvest; only use insecticides when leafminer damage has been identified as unnecessary spraying will also reduce populations of their natural enemies