Soil pH and nutrition
Cucumber plants require a soil with a pH of 6.5 or slightly above. The crop responds to soils with a relatively high organic matter content, therefore the site should receive a dressing of up to 80 t/ha of decomposed organic manure during the early stages of preparation.
A suitable base N:P:K nutrient ratio application of 1:2:2 should be applied during the final stages of seedbed preparation, but the nutrient value of any bulky organic manures already applied should be taken into account. The amount of nitrogen is increased in soils with a high phosphorus and potassium status. A higher proportion of nitrogen is also necessary where frequent irriga- tion is required in order to allow for leaching; in this situation the N:P:K ratio should be nearer to 2:1:1, with approximately half the nitrogen applied as a top dressing about a month after seedling emergence. Care should be taken to avoid foliar scorch from this operation.
Plant establishment
Seeds of open-pollinated cultivars are normally sown direct into the field at sta- tions 10–12 cm apart, with up to 2 m between the rows. In arid areas where irrigation channels are necessary, the seeds are sown on flat ridges which are up to 30 cm high. The seedlings are thinned to a single plant per station as soon as they have emerged. Similar systems are used for hybrid seed produc- tion except that a closer row spacing of 50 cm is usually adopted, and there are six to eight rows of the female parent between two male parent (or pollinator) lines; this pattern is repeated across the field.
The plants are ‘stopped’ by pinching out the initial leader between three and five leaves; two main laterals are subsequently secured. About four to five fertilized fruits are obtained per plant in the female rows. A similar stopping system is usually adopted for open-pollinated cultivars.
Sex expression in cucumber flowers
Cucumber plants produce both male and female flowers. The monoecious cul- tivars normally produce these in approximately equal proportions, although there is a tendency to produce only male flowers initially.
Sex expression in cucumber is generally influenced by environment. Under long days and high light intensities male (staminate) flowers predominate, whereas under short days and low light intensities female (pistillate) flowers predominate.
Plant breeders have produced cultivars which under normal conditions bear predominantly or completely female flowers. These are usually referred to as gynaecious cultivars. The advantages of these, many of which are F1 hybrids, are that they are earlier, higher yielding and all fruits are parthenocarpic and therefore seedless. In addition, some of the F1 hybrids have resistance to spe- cific pathogens.
There is an obvious technical problem for the seed producer who has to manipulate the plant in order to produce a higher proportion of male flowers to ensure an adequate supply of pollen. Different breeders and seed producers have developed their own techniques for producing staminate flowers on cucumbers based on experiences with individual cultivars or lines in specific environments or locations. These are based on the use of gibberellic acid or silver nitrate. Three alternative methods have been developed, these are:
1. Three applications of GA3 at 1000 ppm, sprayed on at fortnightly inter- vals, commencing when the plants have two leaves.
2. As above, but using GA4/7 at 50 ppm.
3. A single application of silver nitrate solution (600 mg/l) before the first flow- ers open.
Hybrid seed is collected only from the female parent plants; the presence of any pistillate flowers on the male plants is not a problem but it is important that staminate flowers are completely suppressed on the female parent. This is normally achieved by two applications of ethrel (250 ppm), the first when the plants show their first true leaf and the second at the fifth true leaf stage.
A visual check must also be made and any male flowers on the female parents removed by hand. The development of suitable male sterile lines or the applica- tion of a satisfactory gametocide would be a useful development in the produc- tion of hybrid cucumber seed.
A further safeguard which avoids admixture of seeds from the male parents is to rotavate the male parent rows before harvesting commences. The advice and instructions of the maintenance breeder should be followed throughout the programme.
Pollination
Cucumbers are self-compatible, but are predominantly cross-pollinated.
Pollination is mainly achieved by bees when the plants are grown as a field crop. When the seed is produced in greenhouses, attention should be given to ensure that adequate pollinating insects are visiting the crop unless hand polli- nation is being undertaken. Cucumis sativus is not cross-compatible with Cucumis melo, gourds, marrows, squash, pumpkins or watermelons.
Isolation
Cucumber seed production in fields or plots should be at least 1000 m from all other cucumber crops including those for market production. It is especially important that the different types of cucumbers have sufficient isolation during production of commercial seed. For basic seed production the isolation dis- tance should be at least 1500 m. These recommended distances can be reduced when seed is produced in insect-proof structures.
Roguing stages and main characters to be observed
Seed crops intended for commercial category seeds are usually only examined and rogued at stages 3 and 4.
1. Before first flowers open: desirable characters; growth habit, vigour and foliage typical of the cultivar.
2. Early flowering: plant habit, foliage characters as checked in stage 1.
● Observable characters of undeveloped fruit, especially colour of spines.
● Check for any specific seed-borne pathogens present.
3. Fruit setting: as for stage 2. Also:
● Satisfactory level of productivity.
● Time of main production.
● Fruit characters, including size, shape and colour.
4. Ripe fruit: colour of ripe fruit in accordance with cultivar description, e.g.
fruits green, yellow, white or orange.
Harvesting
Ideally the fruit should remain on the plant until it is fully mature. This is indi- cated externally by the development of the ripe rind colour characteristic of the cultivar; additionally the fruit stalk adjacent to the fruit withers when the seed is mature.
In order to confirm the external signs that the seeds are actually mature, several fruits should be cut open longitudinally and the seeds examined. Mature seeds separate easily from the interior flesh.
The mature fruit are hand-picked and put in a fruit crusher and seed extrac- tor (as described for watermelon). Large-scale specialist producers use mecha- nized harvesting machines incorporating crushers and seed extractors. If the seeds are extracted by hand, the ripe fruits are cut in halves longitudinally and the seeds are scraped into a container.
The seed and juice mixture can be fermented for about a day before screen- ing and washing the seed in suitable-sized sieves. Water troughs with riffles, as described for tomato seed washing, are used in large-scale operations.
The seed is dried as described for watermelon. After drying, the seeds are screened to remove any remaining fruit debris. Aspirated screens will remove light and immature seeds.
Seed yield and 1000 grain weight
The average seed yield under field conditions is 400 kg/ha, although yields of up to 700 kg are often reported. The yield from a single fruit depends on the cultivar and the amount of successful pollination, but estimates can be based on approximately 500 seeds per fruit. The seed yield for F1 hybrids produced in fields with a male/female population ratio of 1:4 is 300–350 kg/ha.
The 1000 grain weight of the smaller fruited cultivars is 25 g. The seeds of the longer fruited greenhouse cultivars have a 1000 grain weight of 33 g.
Pathogens
The main seed-borne pathogens of Cucumis spp. with common names of the diseases they cause are listed in Table 10.2.