Tomato Diseases and Pests Second Edition
Seed Production and Quality Assurance Seed Production Tomato is predominantly a self-pollinating crop, with seed formation occurring via pollination of the pistil by pollen from the same flower. Until World War II, tomato varieties consisted of pure-line, open-pollinated selections. The first hybrids were developed for fresh-market production and home garden use in the 1950s, but it took several decades before the advantages of hybrid seed became widely recognized. Beginning in the early 1970s, hybrids gained acceptance by the tomato industry. These new hybrids—with increased vigor, yield, and quality— gradually replaced most open-pollinated tomato varieties. A hybrid results from the deliberate crossing of two different parent varieties, usually inbreds. Hybrids can provide increased yield, stronger resistance to disease, greater adaptability, uniformity in maturity, and other fruit-quality traits (e.g., higher- soluble solids, better color). The technology of producing hybrid tomato seed involves specialized skills and requires a thorough understanding of crop management and hybridization practices to ensure high quality yet reasonable seed costs. Production of hybrid varie-
Fig. 1. An emasculated tomato flower, with the sepals, ovary, style, and stigma of the female plant. (Courtesy H. Bolkan)
Fig. 2. Transferring pollen collected from a male parent to the stigma of a female flower that was previously emasculated. (Cour tesy H. Bolkan)
ties is labor intensive, and consequently, most hybrid seed is produced in regions where labor costs are low.
Hybrid Seed Production Hybrid tomato seed production requires the planting of inbred breeding lines that have originated from single plants chosen for their special traits. Like hybrid seeds of other vegetable crops, hybrid tomato seeds are produced by emasculating the flowers on female-designated plants (Fig. 1) followed by application to the stigma of pollen from male-designated plants (Fig. 2). The male parent is typically planted 7–10 days earlier than the female parent to ensure sufficient flowers are available for pollen collection before and during the flowering of the female. In hybrid seed production, the purity of the parents is extremely important. Therefore, every effort is made to avoid seed mixture during planting and transplanting. In addition, any off-type plants are removed by trained personnel who walk through the male and female plant rows in the field. This process is repeated two to three times prior to pollen extraction and pollination. For small-scale hybridization, pollen may be collected from the male plants manually using forceps or a mechanical hand- held vibrator (Fig. 3). For commercial hybrid seed production, a large amount of pollen is needed and thus extracted from dry flowers. The pollen-extraction process varies according to each hybrid seed producer. The most common practice is to collect the staminate cones from male plants and then dry them in a desiccator overnight (Fig. 4). Pollen is extracted by agitating the staminate cones over a fine net (Fig. 5) or sieve and collecting the pollen in a container placed below the net (Fig. 6). Alternatively, flowers are collected from the male plant, placed on screens in a thin layer, and then placed in an oven at 30°C for 24 h or until completely dry. The fresh flowers can be dried under direct sunlight as long as the temperature does not exceed 30°C. After the flowers have dried, they are manually or mechanically crushed and passed through a sieve. For manual extraction after passing through the sieve, the crushed flowers are placed in a plastic bag. The bag is closed, leaving some air in it, and then shaken by hand. The shaking action causes the pollen and some small parts of the crushed flowers to stick to the inside surfaces of the plastic bag. Pollen is then collected by scraping the inside
Fig. 3. A mechanical hand-held vibrator used for collecting pollen from male parent flowers. (Courtesy H. Bolkan)
of the bag into a petri dish or similar vessel. The collected pollen and crushed flower parts are then passed through a testing sieve. When most of the pollen passes through the sieve, a bright-yellow color appears in the container under the sieve. To maintain viability, the collected pollen is stored in a cool place in an airtight container with a desiccant. Pollen stored in a refrigerator maintains its viability up to 1 month.
Emasculation and Hybridization Hybridization begins in the morning by selecting flowers from female plants for emasculation. Flowers must be in the late bud stage of development (i.e., are closed and have not turned yellow) (Fig. 7). The flower bud is opened with a pair of fine forceps or fingernails, and the staminate cone is removed (Fig. 8). Care must be taken to ensure that the receptive stigma is not damaged during the cone removal. Since the female plant will have many flowers at different stages of development, two sepals are removed from emasculated flowers (Fig. 9) to mark the crossed flower and to document that the developing fruit is a hybrid. Open flowers and flowers that are past the accepted stage of emasculation are removed. Generally, transfer of the male pollen is accomplished by dabbing the pollen with a fine brush or finger and then dabbing the brush or finger on the stigma of the emasculated flower. An alternative way to transfer pollen to the stigma of the emasculated flower is to use a small plastic tube. The plastic tube containing the male pollen is worn like a ring by the pollinator, and
Fig. 6. Pollen extraction by agitating the staminate cones over a nylon net. (Courtesy H. Bolkan)
Fig. 4. Staminate cones collected from male-designated plants. (Courtesy H. Bolkan)
Fig. 7. A tomato flower at the correct stage for emasculation: closed and not yet yellow. (Cour tesy H. Bouzar)
Fig. 5. Staminate cones placed over a fine nylon net for pollen extraction. (Courtesy H. Bolkan)
Fig. 8. The emasculation process, involving removal of the staminate cone. (Courtesy H. Bolkan)
pollination is accomplished by holding the emasculated flower and dipping the stigma into the plastic tube containing the pollen (Fig. 10). In either method, care must be taken not to break the pistil. The number of hybrid fruit that will be produced varies from three or four fruit per cluster and from 20 to 25 fruit per plant, depending on the vigor of the plant.
Harvesting Fruit is harvested manually and transported to the seed- extraction area in crates or bags. Only fruit with marked sepals and red color are harvested. Fruit without marked sepals are discarded prior to harvest to prevent accidental mixture of fruit from open flowers with hybridized fruit. Similarly, fruit with symptoms of blossom-end rot are removed, because seeds from these fruit have poor quality. Fruit with pink color are sometimes harvested for seed extraction, but generally, seed from fully ripened fruit gives the highest seed quality.
Seed Extraction Seed can be hand or machine extracted. Hand extraction consists of cutting the fruit in half and either squeezing out the seeds or removing them with a spoon (Fig. 11). Mechanical seed extraction involves placing the fruit into a crusher to separate the gelatinous seed matrix from the remaining fruit tissue and then pressing it through a sieve or screen (Fig. 12). Separation of the gel from the seed is accomplished naturally by fermentation or accelerated with the use of chemicals. When natural fermentation is used, the seed is kept in the tomato ex-
tract until microorganisms degrade the gelatinous material surrounding it. Depending on air temperature, fermentation takes 24–48 h. Chemical extraction involves the use of hydrochloric acid. The acid is added to the fermenting extract, which allows rapid degradation of the gelatinous material surrounding the seed. When acid is used, the fermentation time is shortened to a few hours. Fermentation longer than 4 h may adversely affect seed quality. Regardless of the method used, the seed is immediately washed to remove digested material and any remaining chemicals. Clean water is added to the container with the seed extract. Viable seeds sink, and digested material along with nonviable seed float. The digested material can be decanted, leaving good- quality seed in the bottom of the container. The washing process uses clean, fresh water and is repeated three or four times or until the seed is clean and free of unwanted tomato tissue. Following washing, the seed is air dried to a moisture content of 6–8% on a fine nylon net (Fig. 13). During the drying process, the seed is hand stirred two or three times daily to separate clumps and to ensure uniform drying. As an alternative to air drying, seed can be placed in fine nylon-net bags and dried in an air tunnel or a seed dryer. After the seed has dried to the desired moisture content, it is placed in cloth bags or plastic containers and delivered according to the specifications of the seed company. Each seed container is labeled with the name or code of the hybrid, the year produced, and, most importantly, the lot number, which identifies the production location and the grower. The seed is then stored in a cool, dry place. For a large quantity of seed, as is produced in commercial operations, a special room with
Fig. 9. A pollinated tomato flower with removed sepals, allowing easy recognition of hybridized fruit. (Courtesy H. Bolkan) Fig. 11. Hand harvesting of seed from cut tomato fruit. (Courtesy H. Bolkan)
Fig. 10. Pollinating a previously emasculated flower by dipping the stigma into a container of pollen collected from a male parent. (Courtesy H. Bolkan)
Fig. 12. Mechanical harvesting of seed, in which fruit are passed through a crusher. Fruit and seed with gelatinous material are then passed through a screen. (Courtesy H. Bolkan)
assays and develop and implement seed assay techniques. Most seed certification programs for tomato hybrid seed production involve two interrelated components: field certification and laboratory assays.
Fig. 13. Drying seeds on a fine nylon mesh and exposing them to air or direct sunlight. (Courtesy H. Bouzar)
controlled humidity (less than 30%) and temperature (less than 20°C) is used. Under these conditions, tomato seed can be stored for at least 3–5 years without losing viability.
Open-Pollinated Seed Production With the exception of emasculation, the steps involved in open-pollinated seed production are the same as those described for hybrid seed production.
Certification and Quality Assurance In tomato seed production, maximum attention is paid to producing hybrid seed that is free from seedborne pathogens and that produces high levels of germination, vigor, and genetic purity. The strategies outlined in the following sections are representative of the seed-quality assurance measures practiced by most European and U.S. seed companies. Most private seed companies employ in-house seed technologists, pathologists, and agronomists, who conduct field inspections and laboratory
Locations for hybrid seed production are selected based on the absence of seedborne pathogens in the region. Because little outcrossing occurs in tomatoes, minimal isolation of production fields is required. Fields for hybrid seed production are inspected by trained personnel or by plant pathologists to identify self-fruit and seedborne diseases. Field certification is based on visual field inspections of the growing crop to ensure that no self-fruit are found on female plants and to detect the presence of diseases that have the potential to become seedborne. If self-fruit are found, the grower is asked to immediately reinspect each plant in the field and to remove any self-fruit. Production fields with no evidence of disease symptoms of seedborne pathogens are given the highest field rating. Fields showing symptoms of diseases caused by pathogens that can become seedborne are “red flagged.” Each production field is scouted at the critical periods of emasculation/pollination and at the start of fruit harvest.
Laboratory Assays Following the final cleaning, seed sizing, and density gradient separation, each commercial production lot is tested in the laboratory for germination, vigor, and physical purity. A controlled environment is used to ensure reliability of the germination and vigor tests. In some cases, seed lots may be evaluated for vigor under greenhouse or artificially imposed stress conditions. This allows the technologist to more accurately assess and categorize the quality of each seed lot. Genetic purity testing may be done in the lab using protein or DNA separation techniques for identification of hybrid seed as well as any inbreds. Alternatively, seed lots may be grown in the plant nursery or field (purity grow-out test) until typical characteristics of the hybrid can be distinguished. (Prepared by H. Bolkan and J. Watterson)
Containerized Production of Tomato Transplants Containerized tomato transplants are an alternative to field- produced transplants and direct seeding for crop establishment. They are generally used for fresh-market tomato establishment, but in some production areas, much of the processing crop is produced with containerized transplants. Using transplants provides several benefits, including reduction in seed costs, planting to stand, optimal spacing, and earliness of harvest. The ideal tomato transplant is stocky, green, and pest free and has a well-developed root system. Once transplanted, the plant should tolerate environmental challenges and continue growth to optimum yield. The conditions and procedures for production of high-quality containerized tomato transplants are outlined in the following sections.
frequent source of insects and diseases, which can easily find their way into the transplant production facility. In addition, the site should be level and well drained and have ready access to an abundant supply of high-quality water. The plant-growing structure (PGS) should be located a sufficient distance from surrounding trees and buildings to prevent falling under shadows during the day. Other important considerations for the PGS location include (1) the directional orientation (i.e., an east-facing orientation supplies more overall light, but a north–south orientation supplies more uniform light), (2) the availability of labor, (3) the proximity to shipping routes, (4) the ease of access to utilities, (5) local zoning and tax laws, and (6) the availability of room for expansion.
Plant-Growing Structures Location If possible, the transplant production facility should be located away from tomato production areas. Tomato fields are a
Most tomato transplant PGSs are a modified Quonset-style, with metal trusses and metal or wood sidewall support structures covered with polyethylene film. This type of structure has a life expectancy of 3 years. The movable sidewalls (often called curtains) can be moved up (opened) to allow cross ventilation or down (closed) to conserve heat and to provide 9