Pre-harvest and postharvest determinants and/or Seed quality attributes operations influencing seed quality attributes Germination and vigour Seed-ripening conditions on the mother plant
Postharvest ripening
Drying
Processing (avoidance of mechanical damage) Storage including package and container environment Adverse and/or untimely seed treatments
Genetic purity Accurate labelling at all stages and times (trueness to type) Purity of original stock and/or basic seed
Control of ground keepers (volunteer crops) Isolation (in time and distance)
Roguing efficiency, at appropriate stages Implementation of cultivar descriptions
Mechanical purity Processed seed free from seeds of other crop species, weeds or parasitic plant species
Efficient winnowing, seed cleaning and correct use of appropriate specialist seed cleaning machines, depending on contaminants present in the seed
Seed health Stock seed free of seed-borne pests and pathogens Adequate control of volunteer plants in the field that are alternative hosts to pests and pathogens
Roguing out infected mother plants as soon as identified Timely and efficient seed treatments
Moisture content Harvesting conditions, timely harvesting and drying
Further Reading
Douglas, J.E. (1980) Successful Seed Programmes. Westview Press, Boulder, Colorado.
Gregg, B.R. (1983) Seed marketing in the tropics. Seed Science and Technology 11, 129–148.
Gregg, B.R., Delouche, J.C. and Bunch, H.D. (1980) Inter-relationship of the essential activities of a stable, efficient seed industry. Seed Science and Technology 8, 207–227.
Haldemann, C. (2008) ISTA and biotech/GM crops. In: Seed Testing International. ISTA News Bulletin No. 136, October 2008, 3–5.
Kelly, A.F. (1994) Seed Planning and Policy for Agricultural Production. Wiley, Chichester, UK.
116 ©R.A.T. George 2009. Vegetable Seed Production, 3rd Edition (R.A.T. George)
The main genera in this family that are cultivated as vegetables are:
Beta vulgaris L. ssp. esculenta: beetroot, red beet
Beta vulgaris L. ssp. cycla: spinach beet, leaf beet, chard or Swisschard Spinacea oleracea L.: spinach
There are also other species that are of local importance, especially as edible leafy vegetables. Chenopodium quinoa Willd., commonly known as quinoa, is grown in South America, particularly in the Andes, as a staple grain crop and to some extent for its edible leaves.
Beetroot: Beta vulgaris L. ssp. esculenta
The cultivated beetroot with its swollen root is derived from B. vulgaris ssp.
maritima (L.) Thell. Sugarbeet, which is an important industrial crop, and man- gold (also known as mangel-wurzel), grown as a root crop for stock feed, are also forms of B. vulgaris; they are both closely related to beetroot to the extent that they freely inter-pollinate.
Beetroot, which is generally cultivated for the production of its swollen roots, is a popular crop in the Middle East, Europe and North America. Processors have shown an increased interest in this crop in recent years. It is not widely grown in the humid tropics, but is increasing in popularity in the cooler tropics.
Criteria for development and selection of the modern cultivars have been based on root shape and colour, appearance of the root’s transverse section, shape and colour. Root morphology includes globe-, cylindrical- and long- rooted types. Plant breeders have selected lines with resistance to early vernal- ization in the development of cultivars suitable for early sowing.
The ‘seed’ of beetroot is formed by an aggregation of flowers fused by their swollen perianths to form a multigerm fruit, which is a cluster of two to
7 Chenopodiaceae
five seeds. Plant breeders working with sugarbeet have developed monogerm types in which the flowers remain unattached to each other and the fruits therefore bear only a single seed. This character has important agronomic advantages, allowing the seeds to be sown singly for precision drilling and optimum spacing of single plants, obviating the need for singling. The mono- germ character has also been transferred to beetroot, but not all cultivars are available with this character; the majority still offered by the seed trade being multigerm.
Cultivar description of beetroot
There are open-pollinated and hybrid cultivars. The following is a basis for cultivar description:
● Germity: (i.e. monogerm or multigerm).
● Seedling: anthocyanin coloration of hypocotyl – absent or present.
● Leaf: attitudes and morphology of petiole and blade, degree of anthocyanin.
● Root: position in soil, shape, relative length and width, colour:
Transverse section, colour, absence or prominence of rings.
Shape of longitudinal section.
● Bolting tendency (from an early sowing), suitability for early sowing.
● Resistance to specific pathogens and pests.
(A detailed test guideline (060) is obtainable from UPOV; see Appendix 2.)
Soil pH and nutrition
Beetroot are slightly tolerant to acidic soil conditions, i.e. soils with a pH between 6.0 and 6.8 are suitable.
The general ratio of fertilizers applied during the seedbed preparation is N:P:K 2:1:2, although some growers prefer to give a lower proportion of nitro- gen than this and apply 1:1:2 during preparations, with further nitrogen given as top dressings in the first season. A spring dressing of nitrogen is given to the transplanted stecklings at the rate of 50 kg/ha. Supplementary nitrogen is also applied as a top dressing before flowering in areas with a relatively high leach- ing rate.
Soils with a low boron status should either be avoided or a supplementary dressing of boron applied during preparations. Ideally, a boronated fertilizer is used for the main base dressing or, if not available, boron is applied as sodium tetraborate (borax) at the rate of 1.5–2 kg/ha. The main symptoms of boron deficiency in beet are black cankerous areas on the root exterior and also between the concentric rings. This interior symptom is clearly seen when affected roots are cut transversely.
Manganese deficiency occurs in some specialist production areas, e.g. in Washington state, USA, where the problem is counteracted by dressings of up to 100 kg/ha of manganese sulfate during seedbed preparation.
The main beetroot seed production areas grow the crop on the flat, but in the Middle East, where furrow irrigation systems are used, the crop is grown on ridges.
Seed production methods
There are two basic methods of seed production for beetroot; these are ‘seed to seed’ and ‘root to seed’. The ‘seed to seed’ system is normally only used for the final stage of seed multiplication while the ‘root to seed’ system is used for the production of basic seed and also preferred in some areas for the final multiplication. This latter system allows for inspection and roguing of roots.
Root to seed
This system is in two stages. The first stage is very similar to the production of beet for market. Seed is sown during July–August in areas such as California, where growing conditions are satisfactory into the autumn. The seedlings are singled as soon as possible following emergence to allow the root to develop its characteristic shape, but this operation is not normally necessary if monogerm or rubbed seed has been precision drilled.
The roots are lifted in the autumn, selected for root characters and the desirable material stored. The lifting operation is completed before the onset of damaging frosts, and thus the timing depends on the local climate. Whatever lifting procedure is adopted, every care must be taken to avoid mechanical damage to the roots. The lifting and subsequent root handling is easier if the plants are ‘topped’ first by passing over the crop with a cutter bar or mower.
The crown of the root must not be damaged during topping or lifting.
ROOT STORAGE The two basic systems for storage of roots are either the use of suitable buildings or storage in the field in clamps or pits. Most beetroot seed producers have developed suitable shed systems, but field storage is still used in some areas.
Storage in buildings: the main advantage of storage in buildings is that the air temperature can usually be controlled when necessary, thus avoiding over- heating and frost damage. The optimum temperature for beetroot storage is 4–5 °C. The optimum relative humidity (RH) is between 80 and 90%, although very few seed producers have sufficient facilities to control the RH. A stacked tray or crate system is very suitable and can be coordinated with field opera- tions at lifting and planting times. Where possible, the boxes of selected roots are air dried under cover before being stacked in the store.
Field storage: there are several versions of field storage, which include clamps and pits. The selected roots are arranged in pyramids or ridges on well-drained sites. In both these versions the roots are stacked 60–200 cm above ground level. The piles of roots are covered with straw, which is held in position by a covering of loose soil. Polythene sheets can also be used to exclude the rain and to give some frost protection, but care must be taken
to avoid condensation. Straw funnels or chimneys in the ridges reduce the risk of condensation.
The roots are replanted in the spring, as early as local conditions allow, in rows 100 cm apart, with approximately 30 cm between roots within the rows.
The roots must be set upright with their crowns at finished soil level. Some growers plant into a relatively loose soil, and pass a roller over the field after planting in order to firm the soil around the roots. This reduces drying out and is claimed to assist the early establishment of new fibrous roots.
In some areas, with a very well-organized and specialized vegetable seed industry, the roots are produced and stored by the seed companies and sup- plied to growers who plant them to produce the seed on a contract basis.
Seed to seed
In this system, a later sowing date is adopted compared with the root-to-seed system described earlier and the plants remain in the field all winter; therefore, this method is not suitable in areas with severe frosts. The actual sowing date depends on local climatic conditions, but August–September is the main time in the northern hemisphere. The sowing rate is approximately 12 kg/ha, which produces sufficient stecklings to plant 4 ha. The seed is sown in a four-row bed system with 25–30 cm between the rows, with a bed width of 110 cm. Sowing rates are adjusted according to seed type to give an optimum plant density in the beds of 200 plants/m2.
The stecklings are transplanted in the early spring, but in some of the Mediterranean areas spring transplanting is difficult because the soil is very wet, and late autumn transplanting is adopted.
The optimum size of transplant is approximately 2.5–2.75 cm (weighing from 40 to 45 g). Although the trimming of transplants facilitates the oper ation, especially the long-rooted cultivars, the swollen tap root must not be cut.
The planting distances are 60 cm between the rows and 45–60 cm within the rows. In drier areas the transplants are frequently irrigated until established.
Flowering and pollination
The beetroot is a quantitative long-day biennial with a cold requirement for flower initiation. The detailed investigations into this requirement were first made by Chroboczek (1934). More recent work has demonstrated that expo- sure of the ripening seeds at low temperatures can reduce the subsequent low temperature requirement. This early vernalization sometimes contributes to the incidence of bolting in the first year.
The inflorescence emerges from the growing point relatively early in the spring. Some seed producers ‘top’ the flowering shoot when it is approxi- mately 40–50 cm high. It is claimed that this increases seed yield by reducing the duration of flowering and concentrates the seed maturity period, which in turn reduces seed losses from shattering.
Beetroot flowers are predominantly wind-pollinated, although there is also some insect pollination by Diptera species.
Isolation
It is generally accepted that the pollen of B. vulgaris is wind-borne over rela- tively long distances, and sufficient isolation should therefore be ensured. Most authorities stipulate isolation distances of at least 500 m between cultivars of the same type (e.g. red globe) and at least 1000 m between different types of cultivar (e.g. between red globe and cylindrical types).
Beetroot is cross-compatible with the other subspecies of B. vulgaris (i.e.
sugarbeet, mangolds, spinach beet and Swiss chard) and adequate isolation of the different seed crops has to be ensured. This is usually accomplished by a zoning scheme aimed at confining seed production of each of the different subspecies in separate geographical areas. Zoning schemes can either be vol- untary agreements between seed producers, plant breeders and seed organiza- tions, or they may be a legal requirement. The minimum isolation requirements between the different types of B. vulgaris vary from one authority or scheme to another, and may be 1 km, although the recommended distance in some states of the USA is at least three times this distance.
When seed of high genetic quality is required, or pollen contamination is suspected, the discard strip technique, described by Dark (1971), can be used.
Although this technique was developed for sugarbeet seed production, the same principles apply with other wind-pollinated B. vulgaris types.
Roguing stages
The roguing of plants for beetroot seed production is considerably more thor- ough when the root-to-seed system is used. The seed-to-seed system does not allow the mature root characters to be observed.
Seed to seed
The main roguing is done at lifting and replanting, although plants which bolt prematurely can be removed before lifting:
1. Lifting: discard plants showing any incorrect leaf shape and/or colour, pre- mature bolting, incorrect root shape or seed-borne pathogens.
2. Replanting: characters as described for lifting.
Root to seed
1. Before cutting tops for lifting: remove plants showing any incorrect leaf colour and/or morphology, early bolters and plants showing symptoms of seed-borne pathogens.
2. Lifted roots: discard roots which are not true to type. Shape, size, crown, base and surface corkiness should be taken into consideration.
3. Replanting: if roguing has been done in accordance with stage 2, no further roguing is required, although roots showing storage diseases should be discarded.
4. Bolting plants (before ‘topping’): remove plants showing incorrect leaf shape, colour, vigour and seed-borne pathogens.
Improvement of basic stock
The production of basic stock seed is always done by the root-to-seed method.
All the crop is rogued at all stages described for root-to-seed production, and in addition interior root characters are observed before replanting by cutting out a thin wedge of root flesh. This enables the degree of colour differentiation between the vascular and parenchyma tissues to be observed. Alternatively a cork borer is used to remove a sample of root flesh and the core examined.
Unsatisfactory roots are discarded before planting the remainder. Some seed producers replace the wedge or core while others prefer to apply a fungicide dust to the exposed surfaces before planting.
In addition to this inspection of interior root characters, the discard strip technique can be used for the production of stock seed.
Harvesting
The harvesting of beet seed commences when the ‘fruits’ at the bases of inflo- rescence side shoots mature. By this stage the fruits have turned from green to brown. An additional check is to cut a few samples of ripe fruits transversely;
unripe fruit are milky when cut and the ripe fruit are mealy. Seeds ripen succes- sively from the bases of the side shoots to the terminal point. Care is needed to determine the optimum time for cutting because the immature seeds shrivel if cut too early, and if cut too late seeds are lost as a result of shattering.
Ripening beetroot stems tend to be prostrate rather than vertical. The method of cutting depends on the scale of operation. The large-scale producers in the USA use a swather, but for small-scale production, basic seed or larger- scale production where hand labour costs are relatively low the crop is cut with knives or hooks.
The cut stalks are left in windrows to dry and carefully turned once or twice. In areas where autumn rain is a problem, the cut stalks are tied in bun- dles and dried on ‘four poles’ placed in shocks (stooks) or alternatively trans- ferred to the protection of open sheds. Large heaps of cut material are placed on tarpaulins or polythene sheets to avoid loss of seed from shattering. The cut material can take from 3 to 14 days to dry according to air temperature and rainfall.
Threshing
After drying, the materials are threshed by a stationary thresher or combine.
The dry straw of beetroot seed is extremely brittle; it is therefore important to use a relatively low cylinder speed and air blast. Concave openings must be wide in order to avoid producing too many small pieces of straw as it is difficult to separate and remove these afterwards. There is relatively little chaff in beet- root material.
The final separation of beetroot ‘seeds’ from the small pieces of plant debris is done on a gravity separator.
Seed yield and 1000 grain weight
A satisfactory yield of beetroot seed in most areas of the world is approximately 1000 kg/ha, although up to twice this amount is usually achieved in the USA.
The 1000 grain weight of multigerm beetroot ‘fruits’ is approximately 17 g, rubbed and graded ‘seed’ has the lower 1000 grain weight of approxi- mately 10 g, but as the size of the ‘fruits’ varies between seed lots these figures are only given as a guide.
Pathogens
The main seed-borne pathogens of B. vulgaris with common names of the diseases they cause are listed in Table 7.1.
Swiss Chard, Chard, Leaf Beet, Spinach Beet: Beta vulgaris L. ssp.
cycla (syn. Beta vulgaris L. var. vulgaris L.)
This crop, which is another subspecies of B. vulgaris, has been developed and selected for its broad leaves and wide petioles. It is a biennial and is cross- compatible with the other types of B. vulgaris (beetroot, sugarbeet and mangolds).
The fresh market crop is grown commercially in Europe (especially France and Italy) and North America. It is also a popular garden vegetable in these Table 7.1. The main seed-borne pathogens of Beta species; these pathogens may also be transmitted to the crop by other vectors.
Pathogens Common names
Alternaria alternate Seedling rot, leaf spot
Cercospora beticola Leaf spot (of warmer climates) Colletotrichum dematium f. spinaciae
Erysiphe betae Powdery mildew
Fusarium spp.
Perenospora farinosa f. sp. betae Downy mildew
Pleospora betae Blackleg, damping-off, leaf spot
Ramularia beticola Leaf spot
Curtobacterium flaccumfaciens pv. betae Silvering of red beet
Pseudomonas syringae pv. aptata Bacterial blight, leaf spot, black streak, black spot
Arabis mosaic virus
Tomato black ring virus Beet ringspot virus Raspberry ringspot virus
Lychnis ringspot virus
Ditylenchus dipsaci Eelworm canker
areas, the Mediterranean, Asia and the higher elevations of the tropics and subtropics where it is grown in the dry season. The adoption of this crop for home gardens schemes is encouraged due to its nutrition value as a leafy vegetable.
Cultivar description of spinach beet
● Season of production: suitability for specific seasons, resistance to early bolting and low temperatures and adverse weather.
● Leaf blade: length, width, intensity of green colour, reflexing of margin, glossiness, blistering, anthocyanin absent or present.
● Petiole: length, width, colour, red, green, yellow green or bicolour.
● Relative width and length.
● Resistance to specific pathogens and pests.
(A detailed test guideline (106) is obtainable from UPOV; see Appendix 2.)
Agronomy
The soil and nutrient requirements and seed production methods are similar to beetroot. However, because the leaves are relatively large, a lower plant popu- lation is used per unit area. Seed is drilled at the rate of 6 kg/ha in rows 90 cm apart. Seedlings are singled and thinned to a final stand of 30–45 cm within the rows.
As with the production of beetroot seed, both seed-to-seed and transplant- ing systems are used. The transplanting system allows a better opportunity for plant inspection and is the only method used for the production of basic seed.
Unlike beetroot, the transplants are not stored. Commercial seed is produced in areas of the USA where the winter climate is sufficiently mild for plants to survive in the field and be transplanted in the spring. In Europe, especially, southern France and Italy, the selected plants from a summer sowing are trans- planted in the autumn and overwintered in unheated plastic structures. The plastic protection is removed in the spring. In cooler climatic areas the plastic is left on the structures.
Isolation
The minimum isolation distance between any similar leaf colour cultivar is 1 km.
This is doubled for cultivars with different leaf colours. The recommendation or stipulation for Swiss chard isolation from other types of B. vulgaris is fre- quently at least 5 km in the USA, but depends on whether, or not, sugarbeet seed is produced in the area. The importance of sugarbeet seed production tends to dictate the isolation distances between the different types of B. vulgaris in the major seed producing areas of the world.