Seed Production
- by Todd C. Wehner
- Department of Horticultural Science
- North Carolina State University
- Raleigh, NC 27695-7609
Hybrid Cultivar Types
Early cucumber cultivars were populations or inbred lines produced commercially by open pollination of bulk-increased or hand-pollinated breeder seeds. In the 1960s, large-scale production of hybrid seed began as the result of development of gynoecious inbreds, and the availability of growth regulators to control sex expression. Hybrids have now taken over most of the commercial production in North America.
Most cucumber cultivars are hybrids produced by crossing one gynoecious inbred as the female parent with one monoecious inbred as the male parent. Greenhouse cucumber hybrids are usually produced using two gynoecious inbreds to produce a hybrid with very high gynoecious sex expression. Hybrids for home garden use are often produced using two monoecious inbreds to produce a hybrid with monoecious sex expression. Monoecious hybrids have the same total yield as gynoecious hybrids, but they have lower early yield, so require more harvests. For the home gardener, it is often preferable to have fewer fruit produced over a longer period of time. Three-way hybrids of pickling types were produced for a while during the 1980s, and involved crossing a gynoecious with a hermaphroditic inbred, and then crossing the hybrid with a monoecious inbred.
Inbred Production
Inbred seeds are produced in the inbred parent by pollination of their pistillate flowers using pollen transferred from staminate flowers on the same plant (self-pollination), or an adjacent plant (sib-pollination). The pollinator (pollen vector) for cucumber is usually honey bees, but bumble bees and other insects will also move pollen from flower to flower. The inbred parent is planted in multiple rows in an isolation block or insect-proof cage. Isolation blocks are fields that are located at least one mile from the nearest cucumber field. Insect-proof cages will have one hive of bees added during flowering time for the cucumbers to be pollinated.
If the inbred line being increased is monoecious, there will be staminate and pistillate flowers produced on each plant. These will be sufficient for seed production. If the inbred line being produced is gynoecious, the plants will have to be treated with a growth regulator soon after seedling emergence to make them phenotypically monoecious. This is usually done by spraying the plants with a growth regulator (hormone). Common growth regulators used to make gynoecious cucumber inbreds monoecious are gibberellic acid 3 (GA3), silver nitrate, and amino ethoxyvinyl glycine (AVG). Plants should be treated at the cotyledon or first true leaf stage, and one a week for up to four weeks (10-leaf stage). Rates used are 250 ppm of silver nitrate, or 250 to 500 ppm of AVG. Some inbreds, such as WI 2757, are highly gynoecious and are difficult to convert to monoecious sex expression. Those will require higher rates and more applications of growth regulators.
Hybrid Production
Hybrid seeds are produced in the seed parent by pollination of their pistillate flowers using pollen transferred from staminate flowers in the male parent. The pollinator (pollen vector) for cucumber is usually honey bees, but bumble bees and other insects will also move pollen from flower to flower. The two inbred parents are planted in an isolation block or insect-proof cage in alternating rows, with 1 or 2 rows of the female parent for each 1 row of the male parent.
For monoecious hybrid seed production, the rows of female inbred should be treated with ethrel 3 to 6 times (once/week) starting at the full cotyledon stage. Ethrel will cause the plants in the rows of female parent to convert to gynoecious sex expression. The first few fruit harvested from the plants in the rows of female inbred will contain hybrid seed until the plants change back to monoecious sex expression. The longer the plants are treated with ethrel, the longer they will stay gynoecious.
At flowering, the sex expression of the plants in the rows of female inbred should be checked to make sure they are gynoecious. Off-type (rogue) plants can be removed from the isolation block or cage. If the number of off-type plants is large, the seed increase should not be used from that hybrid production block.
About six weeks after fruit set, or 13 weeks after planting (depending on growth temperature), the cucumber fruit will turn from green to white, yellow or orange. The fruit will become softer and a few will have started to rot. At that time, fruit will be harvested from the rows containing the female parent. Rows containing the male parent will not be harvested, since they will be the male parent inbred, not the hybrid. Fruit will be put into a bulk seed extractor that crushes the fruit between rollers, and separates the fruit pieces from the seeds. Yield will be about 200 seeds per fruit, with an average of 427 lb/A (700 lb/A from a good field).
In order to reduce the possibility of disease for the surrounding crops, and to prevent theft of the seeds of the cucumber inbreds, the field should be disked as soon as possible after the seeds have been harvested.
Commercial Seed Production
Most commercial cucumber seed production is located in semi-arid areas of the west such as California. Arid conditions favor the production of high quality, disease-free seeds.
Sanitation is important at all stages of production. Workers should wash their hands with antibacterial soap or rinse them with 70% isopropyl alcohol before handling plants or fruit and between seed lots. All equipment should be cleaned and all soil and plant material removed before use in production areas. Clean and disinfect harvesting tools and equipment with alcohol or 0.5% NaOCl or Ca(OCl)2 between seed lots. Sanitation, harvest, and control procedures for production of foundation and stock (parent) seed should be at least as stringent as that for commercial seed.
The process of growing cucumber seed crops is similar to that for growing market crops except that site selection is more critical. Choose a field that has not had any cucurbits (watermelon, cantaloupe, honeydew, cucumber, summer or winter squash, pumpkin, or gourd) in it for at least 2, but preferably 4 years. A field that has a history of foliar diseases such as anthracnose should be avoided. Fields for open-pollinated cucumber seed production should be isolated by at least 1 mile from other cucumber fields to prevent contamination by outcrossing.
Selection of parental seed from elite or foundation seed is the first critical element of seed production. Use seed that was produced in dry climates and has been tested to be free of the pathogens causing gummy stem blight, anthracnose, and angular leafspot. Direct-seeded plantings are the standard method, and reduce the risk of seedling contamination in greenhouses.
Drip or furrow irrigation should be used in the production field instead of overhead irrigation to reduce leaf wetting and disease spread. Roguing of off-type and diseased plants within the field should be done throughout the growing season. There are four useful stages for roguing. The first is before flowering when vegetative characters are checked. The second stage is at early flowering when morphology of undeveloped fruit is checked. The third stage is when the developing fruit are checked for trueness to type, and the final roguing is confirming the external morphological characters of the fruit to be harvested. Roguing for off-types is not effective after pollination in a field for open-pollinated seed production. It is only effective when fruit have been self or cross-pollinated and the male has no off-types. Inspectors should be trained to recognize common mixups.
All fruit should be inspected by trained technicians for symptoms that are suspected to be fruit blotch. All fruit suspected of having fruit blotch must be discarded. No fruit should be harvested from vines that have anthracnose or gummy stem blight symptoms. When seeds of fruit are mature the fruit are picked up into seed harvesters that crush the fruit and separate the seeds and pulp from larger fruit chunks. The seed slurry is transferred to bins where it is allowed to ferment for 24 to 48 hours. During this time the sugars and gelatinous material surrounding the seeds are degraded.
After fermentation, seeds are separated from pulp and juice by washing in a rotary washer or flume system. Seeds are dried using forced air warmed by propane heaters. Seeds are placed on flat drying beds or in large rotary dryers. Dry seeds are run through a mill containing sizing screens that separates large seeds from trash and small seeds.
Seeds should be stored in hermetically sealed containers at low moisture content. Under favorable storage conditions, seeds should last 5 years. To be salable, germination of the seed lot must be at least 80%.