Radish

Description

Radish, Raphanus sativus, is an herbaceous annual or biennial plant in the family Brassicaceae, grown for its edible taproot. The radish plant has a short hairy stem and a rosette (ground level horizontal and circular leaves) of oblong shaped leaves which measure 5–30 cm (2–12 in) in length. The top leaves of the plant are smaller and lance-like. The taproot of the plant is cylindrical or tapering and commonly red or white in color. The radish plant produces multiple purple or pink flowers on racemes which produce 2–12 seeds. The reddish brown seeds are oval, and slightly flattened. Radish is generally grown as an annual plant, surviving only one growing season and can reach 20–100 cm (8–39 in) in height depending on the variety. Radish may also be referred to by the name of the cultivar and names may include Chinese radish, Japanese radish or oriental radish. The origin of the radish plant has not been determined but they are found growing native from the Mediterranean to the Caspian Sea.


Uses

The radish root can be eaten fresh in salads or cooked with other ingredients such as meat. The leaves of the plant are also edible and can bu used as a salad green.


Propagation


Basic requirements
Radishes are fast growing cool-season vegetables that grow very well in cool moist climates. the optimum temperature for the growth of radishes is between 10 and 18°C (50–65°F) and they grow best in a well-draining sandy loams which are rich in organic matter with a pH between 5.8 and 6.8.. Radish should be grown in full sun to part shade.

Propagation
Radish is propagated directly from seed into a prepared bed. Seeds should be planted in late winter to early spring for the first spring crop and plantings can be staggered to provide a continuous harvest. A small amount of nitrogen may be applied to the soil prior to planting (up to 60 kg per hectare). Seeds should be planted at a depth of 1 cm (0.5 in), allowing 2.5 cm (1 in) between individual plants and a further 30 cm (12 in) between rows. Commercial producers may drill seeds using planting rates of 30–40 kg per hectare.

General care and maintenance
Radishes will benefit from the addition of small amounts of nitrogen fertilizer at regular intervals spaced over the growing season. Seedlings should be kept uniformly moist as they develop, but not wet. Weeds should be carefully removed from around the plants.

Harvesting
Radishes usually reach full maturity between 30 to 50 days after sowing. They should be harvested promptly as over-mature radishes become woody and develop a bitter taste. The plants may be topped by cutting back the leaves to a height of 7–10 cm (2.8–3.9 in) prior to harvesting, or whole plants can be pulled from the soil. Roots should be washed prior to storing to increase longevity.


References

Anderson, C. R. Radishes. University of Arkansas Division of Agriculture. Available at: http://www.uaex.edu/publications/pdf/.... [Accessed 03 April 15]. Free to access.

CABI Crop Protection Compendium. (2012). Raphanus sativus (radish) datasheet. Available at: http://www.cabi.org/cpc/datasheet/46796. [Accessed 03 April 15]. Paid subscription required.

Daniels, C. (2013). growing radishes in the home garden. Washington State University Extension. Available at: http://ext100.wsu.edu/benton-franklin.... [Accessed 03 April 15]. Paid subscription required.




Common Pests and Diseases

Scab
Streptomyces scabies

Symptoms
Brown-yellow circular lesions on roots; sunken, cracked lesions which may be irregular in shape and coalesce
Cause
Bacteria
Comments
Disease also occurs in potato, turnip and rutababga
Management
Management of scab can be very difficult; rotate crops to non-host for four years; maintain a high level of soil moisture; avoid increasing soil pH through soil amendments

Alternaria blight
Alternaria spp.

Symptoms
Yellow, dark brown or black circular spots on leaves with concentric rings on leaves, petioles, stems and/or flowers; center of lesions may dry and drop out, giving the leaf spots a shot-hole appearance; spots coalesce to form large necrotic patches; leaf drop may occur
Cause
Fungus
Comments
Disease emergence favors warm, wet weather
Management
Plant only certified, disease-free seed; treat seeds with hot water prior to planting; rotate crops to non-brassica species; irrigate plants in morning to allow sufficient time to dry out during the day; apply appropriate fungicide

Black root
Aphanomyces raphari

Symptoms
Small black-blue areas on roots which expand and girdle taproot; roots become constricted at site of lesions; black discoloration extends into root
Cause
Fungus
Comments
Fungus can survive in soil for prolonged periods
Management
Plant resistant radish varieties; rotate crops with non-brassica species

Downy mildew
Peronospora parasitica

Symptoms
Small angular lesions on upper surface of leaves which enlarge into orange or yellow necrotic patches; white fluffy growth on undersides of leaves
Cause
Fungus
Comments
Disease emergence favors cool damp weather
Management
Remove all crop debris after harvest; rotate with non-brassicas; it is possible to control downy mildew with the application of an appropriate fungicide

Fusarium wilt (Yellows)
Fusarium oxysporum

Symptoms
Leaves turning yellow on one side of plant; leaves fall from plant leaving a defoliated stem
Cause
Fungus
Comments
Fungus can survive in soil for many years and can be spread to new areas via infected transplants, soil or on infested equipment. It may also be spread via infected water or by wind
Management
Disease can be effectively controlled by planting resistant radish varieties; once the pathogen has established, very little can be done to control it; spread can be prevented by sanitizing all equipment regularly; do not plant susceptible crops in previously infested soils

Wirestem (Damping-off)
Rhizoctonia solani

Symptoms
Death of seedlings after germination; brown-red or black rot girdling stem; seedling may remain upright but stem is constricted and twisted (wirestem)
Cause
Fungus
Comments
Disease emergence favors cool, wet soils
Management
Plant pathogen-free seed or transplants that have been produced in sterilized soil; apply fungicide to seed to kill off any fungi; shallow plant seeds or delay planting until soil warms

White rust
Albugo candida

Symptoms
White pustules on cotyledons, leaves, stems and/or flowers which coalesce to form large areas of infection; leaves may roll and thicken
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

Clubroot
Plasmodiophora brassicae

Symptoms
Slow growing, stunted plants; yellowish leaves which wilt during day and rejuvenate in part at night; swollen, distorted roots; extensive gall formation
Cause
Fungus
Comments
Can be difficult to distinguish from nematode damage; fungus can survive in soil for periods in excess of 10 years; can be spread by movement of contaminated soil and irrigation water to uninfected areas
Management
Once the pathogen is present in the soil it can survive for many years, elimination of the pathogen is economically unfeasible; rotating crops generally does not provide effective control; plant only certified seed and avoid field grown transplants unless produced in a fumigated bed; applying lime to the soil can reduce fungus sporulation