Banga (1976) discussed the history and development of the four present-day cultivated radish types, which are:
1. Raphanus sativus L. var. radicula, cultivated for its swollen hypocotyl as a field and protected crop, mainly in temperate areas of the world, although they are also cultivated in other areas.
2. Raphanus sativus L. var. niger, the larger rooted type, cultivated mainly in Asia but still locally important in Germany.
3. Raphanus sativus L. var. mougri, which has a relatively insignificant root but is cultivated as a vegetable in South-east Asia for its edible foliage and rela- tively long seed pods that are eaten raw, cooked or pickled.
4. Raphanus sativus L. var. oleifera, the type cultivated as a fodder crop, especially in northern Europe.
All these types described above readily cross-pollinate with each other and also with the four wild Raphanus species, R. raphanistrum, R. maritimus, R. lan- dra and R. rostatus. The occasional purple-rooted off-types found in seed stocks are either a result of cross-pollination between cultivated and wild spe- cies or an admixture of seed derived from wild types that were not rogued or weeded out during seed production.
Cultivar description of radish
● There are some hybrid cultivars of which the Japanese radish (‘Mooli’) is prominent.
● Diploid or tetraploid.
● Seedling: anthocyanin absent or present.
● Cotyledon: size.
● Leaf: attitude, pose, length. Leaf blade features.
● Radish at market maturity: shape, colour, bicolour.
● Radish hypocotyls at market maturity: thickness, width, shape, colour, bicolour.
● Flower; petal colour at start of anthesis.
● Time of market maturity.
● Resistance to specific pests and pathogens.
(A detailed test guideline (064) is obtainable from UPOV; see Appendix 2.) Cultivar descriptions for the ‘black’ radish, Raphanus sativus L. var. nigerfollow a similar outline to the above, but there are some variations. (A detailed test guideline (063) is obtainable from UPOV; see Appendix 2.)
A classification of forcing radish was made by Watts and George (1958), later work by George and Evans (1981) studied the morphology of 55 cultivars of radish recommended for protected cropping and classified them into 25 classes.
Soil pH and nutrition
Radish crops tolerate slightly acid conditions and a soil pH between 5.5 and 6.8 is suitable. The general N:P:K fertilizer application during site preparation is 1:3:4, although a lower amount of potassium is applied where there are satis- factory nutrient residues from previous crops. Excessive amounts of nitrogen will delay the start of anthesis and also predispose the seed crop to lodging.
Seed production systems
Both root-to-seed and seed-to-seed systems are used. The root-to-seed system is used for the biennial types, especially in Europe, where the roots are lifted in the late autumn, the tops taken off and the radishes are stored, usually in clamps, during the winter. It is also the method used for stock seed production of the annual types but in this case the material is replanted immediately after selection. In some areas of the world, especially in Asia, up to half of each steckling’s root is removed before replanting. Work by Jandial et al. (1997) has suggested that for seed production of the Japanese white cultivars this results in a higher seed yield.
The seed-to-seed system is used for final multiplication stages where inspections of the mature root are not considered necessary and is normally used only for spring sown seed crops unless the cultivar has a vernalization requirement.
Sowing rates and spacing
Sowing rates of up to 6 kg/ha are used, the seeds are drilled in rows from 50 to 90 cm apart to give a final plant distance within the rows or approximately 5–15 cm.
For hybrid seed production the ratio of female-to-male parent rows is usually 1:1 (Takahashi, 1987).
Flowering
According to Banga (1976) there has been selection for adaptation to different day-lengths and types of seasons. The cultivars developed for early spring fresh market production have an annual habit, and material without a vernalization or specific day-length requirement has been developed from both Raphanus sativus var. radicula and Raphanus sativus var. niger. However, these types flower earlier when grown in long days.
Selection of types suitable for crop production in summer, autumn or win- ter has resulted in cultivars that are biennials with a vernalization requirement.
The removal of early bolting plants from late spring and summer cultivars has probably resulted in the decreased sensitivity to day-length (Banga, 1976).
The flowers are cross-pollinated by bees and other insects.
Isolation
The recommended isolation distance is 1000 m, although the seed regulations of some authorities only require a minimum distance of 200 m between com- mercial seed stocks of similar cultivars.
Roguing stages (seed-to-seed system)
1. At market maturity stage of radish. Root: relative size, shape, colour, pro- portions of each colour on bicoloured cultivars, solidity.
2. At stem elongation remove wild radish types. Check that the remaining plants are true to type for foliage and stem characters.
3. At flower bud and very early at start of anthesis. As described for stage 2.
4. Flower colour.
Basic seed production
The root-to-seed system is used for basic seed production. When selecting plants according to their external morphology, care must be taken to ensure that those with pithy roots are rejected. Traditionally, selected roots were examined for internal solidity by removing a small wedge of tissue with a knife, but Watts (1960) described an immersion technique that is very suitable for screening roots for solidity at or after normal market maturity and can be used to increase the length of time that they remain solid. After selection the plant’s leaves are twisted off (leaving the growing point undamaged) and the radish roots put in a bucket of water. Those roots with a degree of pithiness float and are discarded. The solid roots sink and are retained. The selected solid roots are then planted up and grown on for seed production.
Harvesting
When the seeds are nearing maturity the seed pods turn from green to brown, lose their fleshy appearance and become parchment-like. The pods do not shatter very readily and it is therefore better to harvest under very dry conditions if combined. The dry pods are then relatively brittle and seed extraction is easier. Combines with a roller attachment on some types of bean threshers are most suitable. The rollers are adjusted so as to crack (but not crush) the pods and unthreshed pods are returned to the threshing cylinder.
A standard swather is used if the seed crop is cut before threshing, and the material is left to dry further before passing it through a thresher or combine.
The cylinder speed should be reduced to 500–600 rpm.
Seed yield and 1000 grain weight
A seed yield of approximately 1000 kg/ha is satisfactory, although yields of 1500–2000 kg/ha are achieved by some seed producers.
The 1000 grain weight of radish is approximately 10 gm.
Pathogens
The main seed-borne pathogens of Raphanus sativus with common names of the diseases they cause are listed in Table 9.2.
Table 9.2. The main seed-borne pathogens of Raphanus sativus; these pathogens may also be transmitted to the crop by other vectors.
Pathogens Common names
Alternaria alternata
Alternaria brassicae Grey leaf spot
Alternaria brassicicola Black leaf spot Alternaria raphani syn. A. matthiolae Leaf spot
Colletotrichum higginsianum Anthracnose, leaf spot
Gibberella avenacea Root and stem rot
Leptosphaeria maculans Black leg
Rhizoctonia solani Damping-off, canker
Xanthomonas campestris pv. raphani Bacterial spot Radish yellow edge virus
Tobacco streak virus Turnip mosaic virus