SEED PRODUCTION AND CERTIFICATION

Transcription

1 FPO 400%

2 FPO 110% SEED PRODUCTION AND CERTIFICATION Stephen L. Love, Phillip Nolte, Dennis L. Corsini, James C. Whitmore, Lorie L. Ewing, and Jonathan L. Whitworth chapter 4 H igh quality seed is essential for the production of a profitable potato crop. Seed production is a specialized segment of the potato industry and involves complex operations and conformity with numerous rules and regulations. This chapter explains the process of producing high quality seed potatoes and completion of the certification process. PRODUCING HIGH-QUALITY SEED POTATOES Recent technological advances in plant propagation have enabled commercial potato growers to rapidly increase basic pathogen-tested seed stocks through the use of tissue culture. This system helps minimize the transmittal of tuber-borne diseases, particularly potato viruses (PVX and PVY), and leafroll (PLRV); bacterial ringrot; and the blackleg/bacterial soft rot complex. The high cost of greenhouse space and labor-intensive practices previously limited the amount of basic stock increases. Advances in micro-propagation techniques now allow commercial 49

3 50 potato growers to produce large numbers of plants in less space and with fewer employees. Limited Generation Seed The limited generation seed potato program used at the University of Idaho is typical of most North American programs and is based upon a 7-year generation system. The Pre-Nuclear stage most commonly involves the growth of greenhouse tubers for field planting. Potato producers can grow these tubers, commonly referred to as mini-tubers, two ways: (1) tissue culture multiplication and (2) mother plant multiplication. 1. Tissue culture multiplication, the most common method, uses plantlets derived from mother stocks, which are cut and propagated through multiple cycles under aseptic laboratory conditions. New plantlets are created from cuttings of plantlets from the previous cycle (Fig. 4.1). After the final cycle of propagation, growers transplant the plantlets into a greenhouse for mini-tuber production. 2. Mother plant multiplication, a less common method, uses plants grown in the greenhouse from pathogen-tested tubers. Small pieces of these mother plants, known as stem cuttings, are removed and transplanted into a sterile or pasteurized potting soil. After the plants become rooted, they are grown in greenhouses for mini-tuber production (Fig. 4.2). This Figure 4.1. New plantlets come from cuttings of previous potato plantlet cycles. method requires less sophisticated equipment, but it is slower and more labor intensive. Regardless of the method employed, the propagation source materials must be tested for the presence of common pathogens, including bacterial ring rot; Erwinia spp. (soft rot/blackleg); potato viruses S, Y, M, and A; potato latent virus; potato leafroll virus; and potato spindletuber viroid. The occurrence of new pathogen threats may require additions to this list. In addition to mini-tubers, transplants from tissue culture or stem cuttings can be used to initiate propagation. Greenhouse-grown tubers, tissue culture plantlets, and rooted stem cuttings are all classified in Idaho as Pre-Nuclear stocks. Growers can plant these stocks directly in the field for production as Nuclear class seed. Regardless of the method of multiplication, the first generation in the field is classified as Nuclear. Growers use mini-tubers most frequently because they are easier to handle and have lower susceptibility to environmental stress. Transplanting either tissue culture transplants or stem cuttings directly to the field can minimize the time needed to obtain field production and marketable increase, but is a high-risk procedure because of potential losses caused by environmental stresses. One other method of producing Nuclear seed stocks is listed in certification manuals but is rarely used hill selection. This multiplication technique involves hand harvesting individual hills from within a seed field. Tubers from each hill are tested for viruses and, if found to be clean, are considered to be Nuclear generation seed. From that point on, hill-selected tubers are handled in the same limited-generation manner as Nuclear seed originating from greenhouse-grown stocks. Completion of the first year of field multiplication provides seed stocks for further increases. Nuclear seed produced one year is replanted the next season to produce Generation 1 seed (G1). In subsequent years, sequential field plantings are classified as G2,G3,G4,G5, and G6.Growers can plant seed back and increase quantity for up to 7 years (Nuclear to G6). Most seed in Idaho is sold for commercial production as G3 or G4.

4 FPO 80% Figure 4.2. Mini-tuber production occurs in the controlled environment of a greenhouse. One exception to the limited generation seed program is written into the rules of certification. This involves the production of unnamed breeding selections and new varieties. Breeder seed may be obtained from the breeder of an advanced selection (unreleased variety) and multiplied for as many generations as needed under the Experimental Class. Once the selection is released and named as a new variety, the Experimental Class seed must be sold for commercial production within 2 years and limited-generation seed used for further propagation. Sources of Pre-Nuclear Seed Stocks Pre-Nuclear stocks (plantlets or mini-tubers) for greenhouse or field planting are available from several sources. For example, tissue culture plantlets are available from the University of Idaho for individuals wishing to produce their own mini-tubers. Plantlets are also available through several private labs. The University of Idaho and other private and public labs also produce mini-tubers for sale. Production of Pre-Nuclear Mini-Tubers Maintain Clean, Disinfected Greenhouses Before greenhouses can be used for Pre-Nuclear seed production, they must be thoroughly cleaned and disinfected. If pots are used, they must be washed with detergent and disinfected. One method of disinfection is to wash the pots in a concentrated solution of laundry detergent (½ cup in 5 gallons of water), rinse in clean flowing water, and finally dip briefly into a 2 percent solution of sodium hypochlorite (Chlorox or similar product) or a prescribed solution of chlorine dioxide (Oxidate ) or quaternary amine (Sanitol ). If an open bed system is used, growers should wash and disinfect the bed. This can be done by thoroughly steam cleaning the beds until no organic residue remains, then spraying to complete wetness with a disinfectant solution, such as those described for cleaning pots. Only pathogen-free or pasteurized potting mix should be used for production of greenhouse minitubers. The environment in which Pre-Nuclear mini-tubers is grown must be sanitary, and growers should have precautions in place to assure the growing conditions are sanitary, including: Maintain an insect-free environment in the greenhouse. Scout regularly for insects, and use control strategies as needed. A discussion of methods for controlling insects is included later in the chapter. Observe strict sanitation practices at all times. Restrict entry to authorized personnel only, and use footbaths (a shallow pan or tray 51

5 TISSUE CULTURE PROPAGATION Nearly all seed potato production in North America begins with tissue culture-based propagation. This technique was adopted because of the near-perfect isolation it allows. Potato growers can place plants in sterile culture, test for known pathogens, and keep plants free of diseases indefinitely. Tissue culture is the process of growing plants in an artificial medium in a closed container. The most common types of media used to grow potatoes are agar-based. Agar is a powdered seaweed extract that when put into water and heated, partially solidifies into a gel-like substance. When combined with appropriate amounts of sugars, nutrients, and hormones the agar provides an ideal growth environment for potato plantlets. Successfully operating a tissue culture lab requires extensive knowledge, experience, and equipment. Operators must prepare culture media accurately and consistently to ensure rapid plant growth. The media provide a fertile place for growth of most microorganisms, so completely sterile conditions must be maintained. This is accomplished through the use of laminar flow hoods (provide micro-filtered air) and heat or chemically sterilized instruments and surfaces. It may also be necessary, when establishing a new variety in culture, to use procedures for eliminating viruses from the original sprouts or plants. Propagation of potatoes uses a process of serial cuttings. One or more pathogen-tested plantlets serve as mother stock and are the starting point for multiplication. Plantlets are cut into five to 10 pieces, each one with at least one node and leaf. Small buds at the base of the leaf are capable of growing into a new plantlet and do so in about 3 weeks. At that time the process is repeated. In 3 months, this cycle can be completed up to four times, and one plantlet can be multiplied to produce over 2,500 plantlets. Because of the rapid nature of tissue culture propagation, one tissue culture lab can supply a large quantity of initial stocks to the seed industry. In fact, one well-equipped lab can supply a sufficient number of Pre-Nuclear plantlets to seed growers, who then continue the increase process in greenhouses and in the field, to eventually produce tens, if not hundreds of thousands of acres of commercial potatoes. 52

6 in which growers add ¼ to ½ inch of a disinfectant solution) (Fig. 4.3). Maintain the footbaths for all visitors who briefly step into the pan or tray upon entry. An effective alternative is inexpensive disposable boots that may be present at each entry point and routinely used by anyone entering the facility. To prepare a disinfectant solution for a footbath, put ¼ cup of laundry soap and 1 quart Chlorox into 5 gallons of water. Change the solution weekly. Personnel entering or working in greenhouses should never do so after spending time in cellars or fields without first changing clothes, and then washing and disinfecting exposed skin surfaces. FPO 350% Courtesy of Potato Grower Magazine Inspections and Frequency of Crops In Idaho, greenhouse mini-tuber crops must be inspected two times by Idaho Crop Improvement Association (ICIA) personnel. Inspectors pick leaf samples for detection of viruses and pathogenic bacteria. See the section later in this chapter, Certification of Greenhouse Pre-Nuclear Stocks. Logistically, growers are able to produce a greenhouse crop of mini-tubers every 3 to 4 months. However, most greenhouses are typically used to produce one or two crops per year because of cost constraints associated with winter production. Heating, Cooling, and Lighting Costs of operating a greenhouse increase during the winter months because of heating requirements. Greenhouse cooling is necessary during the summer months, but this is less expensive. The most common equipment for cooling uses an intake and exhaust fan system with evaporative cooling pads. Greenhouse growers must cover all intake and exhaust openings with an aphid-proof mesh hardware cloth. Doors must remain closed; a double-door entry system is recommended to minimize insect entry. Supplemental heat is generally needed during the early spring, fall, and winter months. Gas heaters are the most commonly used heating equipment. Figure 4.3. Sanitation is critical during production of early generation seed. Use of signs supports one important aspect of sanitation, which is prevention of pathogen movement into greenhouse and storage facilities. A supplemental lighting system may be required for mini-tuber production during both the winter and summer months. Ultraviolet light deficiencies, common in glass covered greenhouses, may cause leaf injury or defoliation during periods of alternating clear and cloudy skies. Use of fluorescent, metal halide, or incandescent lighting that produces UV rays, will reduce this problem. After planting, lights should be turned on for a minimum of 16 hours per day during the early growth phase, but turned off during maturation. If producers grow plants during the winter, they should use high intensity (metal halide) lights that extend the day length to 14 to 16 hours. Potting Mixes Many potting mixes are available for use in mini-tuber production. A standard mix consists of one part sterile washed sand (heated for 24 hours at 240 F), one part sterile milled peat moss, one part perlite or vermiculite, and a 53

7 54 complete fertilizer. Another option is to buy a premixed, sterilized, commercial potting mix. Crops grown during the hot summer months need a higher proportion of peat to increase water-holding capacity, while crops grown during the cooler months need more perlite or sand to improve aeration. Before planting, it is necessary to moisten the potting mix and fill each pot or open bed about one-third full with the mix. Optimum Growing Conditions Commercially purchased plantlets are produced under aseptic conditions and are typically received in petri dishes (Fig. 4.4). During the transplanting operation, growers must maintain aseptic conditions. Each person transplanting should use disposable gloves, which are frequently discarded, and/or should wash hands and transplanting equipment with soap and water before and after completion of each petri dish. All plants in any petri dishes that have been contaminated with fungal or microbial growth should be discarded. If possible, transplanting should be done late in the afternoon or on a cloudy day to reduce transplant shock. Before planting, it is necessary to moisten pots or open beds. If growers use an overhead mist system, they should start and set it to run 15 seconds every hour if the weather is sunny; less often on cloudy days. Adjustments Figure 4.4. Commercially acquired plantlets are typically grown in petri dishes. should be made as needed to ensure adequate moisture for the plants, while at the same time avoiding problems with damping-off. A drip irrigation system also works well, especially during cooler months, because it reduces foliar wetting and the potential for foliar diseases. Labor costs can be reduced by using time-clocks to automate the watering process. Greenhouse temperature should be maintained between 50 F minimum and 80 F maximum to promote tuberization and tuber growth. Fertilizers that contain both macro and micronutrients are needed to maintain good growth of the transplants. A slow-release fertilizer, such as Osmocote, can be applied at soil mixing or just after planting, or a readily soluble fertilizer can be applied through the irrigation system. Many types of fertilizer mixes are available for use in greenhouses. A typical formulation may be (N-P-K) (nitrogen + phosphorus + potassium) + micros and calcium. The need for fertilization is minimal until stolon initiation. Then fertilizer requirements increase because of demands on the plant associated with tuber development. Fertilizer applications should be scheduled based on petiole nitrate-nitrogen (N) levels. Normal levels of petiole nitrate-n in the greenhouse are lower than what is normal in the field. Optimal levels are near 12,000 ppm until stolon formation. During early tuber development, the petiole nitrate-n levels can be raised to about 18,000 ppm but then should be allowed to drop slowly until tubers are about 1/2 ounce in size. At this point, no additional fertilizer should be applied. The above described fertilization schedule aids in vine maturation, skin set, and tuber storage. If early applications of nitrogen are limited, plant height can be restricted to about 14 inches. Many pot or bed systems are available to successfully produce mini-tubers. Pots of various sizes have been evaluated for 3 years at the University of Idaho s Tetonia Research and Extension Center. Six-inch-diameter pots gave the optimal number of tubers, while also limiting tuber size.

8 FPO 120% Figure 4.5. Eight-inch diameter pots are commonly used to grow two potato plants. The same results may also be achieved by using 8- to 10-inch pots with two to three plantlets per pot (Fig. 4.5). As plantlets grow, additional soil in the pot should be added to the pots. This allows for more subsurface stolon development and an increase in tuber numbers. If larger tubers and higher yield per unit area are desired, potato producers can grow transplants in raised beds. Insect Control Control of insects, especially aphids, is critical during the production of Pre-Nuclear seed in the greenhouse. A preventative insect control program is recommended. This may involve applying a systemic insecticide during soil mixing or just after planting. In addition to, or in lieu of, using a preplant insecticide, control measures may be needed during the production cycle. Plants should be checked regularly for insects, with particular emphasis on aphids. Many foliar insecticides are available for use in the greenhouse, including Avid, Dursban, Endeavor, Marathon, Maverik. Success, and endosulfan products. Growers are advised to always read the label for recommended rates and proper handling of chemicals. Harvest and Storage of Pre-Nuclear Tubers To prepare for harvest, greenhouse-grown plants are often artificially killed, although some producers harvest the tubers from under green vines to speed the process. The timing of the vine killing procedure is based on tuber size and is done after two Crop Improvement Association inspections. If killed before harvest, growers should remove the tops and leave the crop in the pots until the tubers are mature. Instruments used to remove vines should be dipped into a disinfecting solution between each plant or test unit. The harvesting process is simplified by dumping pots onto an expanded metal screen, sifting through the potting soil, and separating the tubers (Fig. 4.6). The potting soil should be discarded and never reused. Tubers can be stored in any kind of open mesh bags at 39 F and a relative humidity of 95 percent until the following spring (Fig. 4.7). If 55

9 Figure 4.6. Pre-Nuclear tubers are separated from the potting soil by sifting contents of a growing pot through a metal screen. Figure 4.7. Pre-Nuclear mini-tubers should be stored in mesh bags in a facility dedicated for this purpose and where temperature and humidity can be controlled. 56 mini-tubers are green-dug, they should be cured for 2 weeks at 55 to 60 F before cooling to the final storage temperature. Seed from a late fall crop may express dormancy beyond planting time and may need to be warmed to 55 to 60 F for several weeks before planting. PRODUCTION OF NUCLEAR SEED Production from Mini-Tubers Mini-tubers are the best source of propagation material for the production of Nuclear (first field generation) seed potatoes. Mini-tubers have longer dormancy than field-grown tubers and will tend to emerge slowly. Minitubers should be planted as early as possible to take advantage of the full growing season. Ground prepared for the production of Nuclear seed must have been in rotation with another crop for at least 1 year (longer rotations are better), wherein volunteers from the last potato crop have been completely controlled. Fields can be prepared for potatoes in the usual manner. Fertilizer applications are based upon soil tests. If growers are to apply additional N during the growing season, they should reduce preplant N accordingly. See the fertility recommendations in Chapter 8, Nutrient Management. A systemic insecticide applied at planting is recommended to provide early control of aphids. Establish Units of Production Uniting (establishing discrete, identifiable groups of plants) is important in Nuclear seed production. This allows arrangement of the field to minimize physical contact between plants during the various field operations. Also, these discrete units help to isolate disease and to prevent its spread within the field. Growers should maintain units from Pre-Nuclear production, or arbitrarily assign units at planting if mini-tubers are purchased. The size of the unit will depend on the individual grower and the degree of risk the grower is willing to assume. If disease is detected in any plant(s) in the unit, the whole unit should be removed. Although uniting may result in more expense during certification, in cases where disease is present, a lack of uniting may result in the loss of an entire crop. In Nuclear fields planted from mini-tubers the likelihood of disease is relatively low and harvesting strategy is an important consideration in planning the size of the unit. Generally, 1- to 4-row units are recommended, with at least one blank row between units. The length of the unit down the row depends on the grower s preference, but 100 feet is adequate. Picked leaf

10 samples for PVX testing can be based upon row and unit numbers down the row. Adequate walkways must be left for roguing, certification inspections, and leaf sampling. Leaving every fifth row blank (four planted rows, one blank) provides adequate access to the field and keeps contact to a minimum. The grower may want to leave more than one blank row so the unplanted rows can be cultivated more easily. A solid set irrigation system can be used to minimize plant contact with irrigation equipment and personnel. If dormancy has been properly broken, tubers planted 4 inches deep will emerge in 3 to 5 weeks, depending on the size of the mini-tuber. A 12- inch plant spacing keeps weed growth low and tuber size moderate. Mini-tuber sizes from ¼ to 1 ounce produce acceptable results (Table 4.1). table 4.1. Tuber size vs. emergence and yield, UI s Tetonia Research and Extension Center. Tuber size Days to emergence Yield/plant 3/4 to 1 oz 20 days 22.0 oz 1/2 to 3/4 oz 22 days 14.0 oz 1/4 to 1/2 oz 31 days 9.0 oz less than 1/4 oz 37 days 1.4 oz Mini-tubers may be planted by hand or with mechanical planters. In either case, it is critical to use strict, sanitary practices. Periodic disinfecting of equipment is essential. Personnel handling tubers must wash regularly and wear clean clothing each day. Production from Pre-Nuclear Transplants Producing Nuclear seed directly from Pre- Nuclear transplants, although risky, can be successful if done properly. Plantlet or cuttings should first be rooted and grown for 3 to 6 weeks in a greenhouse (Figs. 4.8a, b, and c). Caution: Planting tissue-cultured plantlets or rootless cuttings directly to the field is not always successful. If growers attempt a transplanting operation, it must be done after frost danger is past. Also, hot, dry, and windy weather immediately after transplanting will cause high plant mortality. Pre-Nuclear transplants (greenhouse plants) must be hardened before planting them in the field. Plants rooted in a 2 x 2 inch cell pack should be removed from the cell pack and loaded into a holding tray, allowed to dry to a slight wilt, and then watered until completely rehydrated. This drying and rehydration procedure should be repeated for 2 to 3 days before transplanting. During handling, growers need to take care to minimize root damage. Handlers should wash hands between each tray. The plants should be well hydrated for planting, and while being transplanted, plants should not be allowed to desiccate. The most successful transplanting method involves 6- to 7-inch tall plants that are planted about 5 inches deep, leaving just the top leaves out (Fig. 4.9). If totally buried, the plant will die; if planted too shallow, it will easily desiccate, and the wind may break the top. Irrigation pipes, if properly spaced, can be left in the field during planting to assist in making applications of water immediately after transplanting each day. This aids in cooling the soil, firming the soil around the roots, and lessening transplant shock. No herbicides or systemic insecticides should be applied during planting because transplants are susceptible to chemical injury. Light, frequent irrigations can be used to keep the root zone damp and the soil surface cool until plants become established. This normally takes 2 weeks. It is important to use the same unit field arrangement that was discussed previously for mini-tubers. Row Covers Row covers can be used to provide added protection from both wind and freezing temperatures for transplants (Fig. 4.10). Row covers are commercially available in many forms and include both supported and floating types. Growers may cover one or several rows and anchor the edges with soil. Floating row covers are loosely placed over the plantlets, with only the edges covered to keep them in place. This type requires little labor to install. If weeds become a problem under the covers, growers may need to remove them later in the season. 57

11 a FPO 300% After transplants are established potato producers may cultivate the field and create hills. At this time applications of fertilizer and insecticides can be made using side dressing or chemigation. Hand weeding may be necessary with transplants because immature plants are susceptible to herbicide injury. Weed control should be done with cultivation or light amounts of herbicides that are safe for postemergent use. Once the plants are completely established, and 8 to 10 inches tall (Fig. 4.11), further mechanical operations should be avoided to minimize disease spread. The crop should be managed as any other seed potato crop in terms of irrigation timing and frequency. 58 c b FPO 305% FPO 320% Figure 4.8. Steps in transplanting include removing plantlets from petri dishes (a), planting in individual trays (b), and transplanting when plant height is 6 to 7 inches (c). Vine Killing and Maturing the Nuclear Seed Crop Vine killing is essential for early generation seed potato production. Growers who use vinekilling techniques will reduce late-season virus spread into seed fields by aphid vectors. Early vine kill also ensures plenty of opportunity for tubers to mature (set skin) before harvest (see Chapter 2, Potato Growth and Development, Growth Stage V). Vines on early generation seed crops are difficult to kill because of their vigor and lack of natural senescence. Despite the difficulty, potato growers must kill Nuclear seed field vines earlier than surrounding fields so they do not become an oasis for late-season aphid vectors. Since most vine-killing chemicals act slowly and vines are vigorous, killing vines usually requires repeated chemical applications or some type of mechanical vine treatment before the chemical is applied. Growers need to be cautious in using mechanical treatment, however, because the use of machines increases the potential for virus movement into the tubers. A vine-killing product, such as sulfuric acid, should be selected that kills vines rapidly. If satisfactory vine killing can be achieved with the application of the vine-killing agent alone, mechanical treatments should not be used.

12 Because it is physiologically young, Nuclear Class seed matures much slower after vine killing than later field generation seed. Tubers commonly require 20 to 30 days to mature. The preharvest maturing process is essential to maintain quality during storage. FPO 100% Harvesting and Storage Early generation seed should be harvested before any danger of frost injury occurs. During harvest, the units established at planting may or may not be maintained, depending on the potential for problems. Bulk harvesting of Nuclear fields is not always the appropriate course of action. Often, the best strategy is to harvest in units that are as small as possible and then bulk one or more of these units during the next year s planting. The amount of bulking would depend upon the outcome of winter testing. The process of uniting ensures that virus or other problems are restricted to a small, identifiable portion of the total seed lot. During Nuclear harvest, all tubers from a block of plants (unit) should be kept together in sacks or bins (Fig. 4.12). Each of these units should be given an identification code that is maintained through storage. Nuclear seed should be stored off the cellar floor in any kind of mesh bags or slatted boxes to ensure good air-flow. Immediately after harvest, the storage temperature should be held at 50 to 55 F and 95 percent relative humidity for 2 weeks, which will promote wound healing. After 2 weeks, the storage temperature should be lowered as quickly as possible (without causing condensation) to about 38 F and maintained there until spring. Fluctuations in storage temperature should be avoided. PRODUCING LATER GENERATIONS The principles of producing later generation seed are similar to those for producing a Nuclear seed crop. However, distinct differences result from the nature of the later generations and the quantity of seed that must be handled. Figure 4.9. Workers prepare fields for planting by creating hills and applying a side dressing of fertilizer and insecticides. FPO 265% Figure Floating row covers effectively protect potato transplants from wind, frost, and insects. FPO 200% Figure Further mechanical cultivation is not advised after plants have become fully established. 59

13 Courtesy of Paul C. Peck Photography FPO 720% 60 Figure Potato growers keep Nuclear tubers from blocks or units of plants in sacks or bins.

14 Identifying a source of quality seed for multiplication is paramount for success. Planting seed with disease problems can result in failure to meet certification with that particular lot and can also jeopardize an entire seed operation. If later generation seed is produced from Nuclear seed grown on the same farm, potential problems are known and difficulties avoided. If the seed source is purchased from another grower, potential problems are unknown. In such a case, at a minimum the purchaser should carefully inspect all certification records. A document called a plant health certificate (Fig. 4.13) is used by the certification agencies in the U.S. and Canada and can be requested from the responsible agency in the state where the seed was grown. It lists such information as summer and winter test results, certification pedigree of the seed, certification lot numbers, seed class, and farm history. If possible, the buyer should also inspect the field and storage facility that was the source of seed to be purchased. Any disease problems present in early generations of seed will be magnified in later generations. Field preparation and agronomic production practices for later generation seed is similar to that used for any type of commercial production. Information concerning these practices is found throughout Potato Production Systems. Note that some practices are specific for seed production. G1 to G6 Seed Tuber Crops As with Nuclear seed, it is wise to place into units all G1 through G6 seed lots to avoid loss of large amounts of seed to disease problems. At planting time, units are separated by one blank row or by an empty 6-foot border along the row. Borders also should be used for divisions between varieties. Control Disease Spread Control of disease spread is the most important component of potato seed production. Several practices help with this process. The first is cleanliness. All surfaces with which seed potatoes come in contact should be cleaned and disinfected. At the beginning and end of each operation all harvesters, truck beds, storages, cutters, planters, and handling equipment should be thoroughly cleaned and decontaminated. Decontamination can be done with a steam cleaner and a sterilizing agent. Handling equipment should be disinfected between each unit, lot, or variety of seed. The practice of frequently cleaning equipment will help control the spread of bacterial and fungal diseases including ring rot, soft rot, blackleg, dry rot, late blight, and silver scurf. It may also help control spread of some virus diseases. For the most part, control of viruses, including leafroll, PVY, PVX, and PVA, involves isolation and control of the vectors that spread these diseases. Isolation of seed fields from other potatoes using a distance of at least 1/4 mile will prevent most infections with PVY, PVX, and PVA. Isolation for control of leafroll may require distances of more than a mile and even then may not be completely effective. The use of a green border, such as spring-planted winter wheat, has been shown to be effective at slowing spread of viruses into seed fields. Proper Use of Insecticides The second part of a good virus prevention program involves effective use of insecticides. This is especially important for control of the green peach aphid, the vector for the leafroll virus, which colonizes potato seed fields. Use of an effective preventative systemic insecticide is the cornerstone of a control program (see Chapter 12, Insect Pests and Their Management). This will minimize early infestations and provide control well into the season. Once green peach aphid colonies are detected within a production area, supplemental use of systemic insecticides with weekly applications of general foliar insecticides or aphicides is advised. Intergenerational isolation, both in the field and in storage, is often overlooked as a disease control strategy but can be as important as insect control or isolation from neighbors. This seed production strategy is especially important for controlling non-persistent viruses, such as PVY and PVA, and for controlling diseases that spread in storage, such a silver scurf and blackleg. 61

15 62 Figure North American Certified Seed Potato Health Certificate.

16 UNDERSTANDING THE BENEFITS OF EARLY GENERATION SEED A perception exists that early generation seed (e.g., G2) is better than later generation (e.g., G4) seed by reason of being younger. One frequently cited benefit of early generation seed is higher yield potential. To test this theory, some carefully controlled trials were conducted at the University of Idaho s Kimberly Research and Extension Center to determine the impact of generation on yield potential. Results of one trial did not show a consistent increase in yield with early generation seed. In seed lot A, yield and size tended to increase slightly with earlier generations, while in seed lot B the opposite occurred, showing that generation is not necessarily related to productivity. Other trials comparing seed of different generations have shown similar results. Effect of seed generation on performance of two seed lots at Kimberly, Idaho. (Source: Gale Kleinkopf, UI Kimberly R&E Center) Seed lot Generation Total yield Large U.S. No. 1 (cwt/acre) (cwt/acre) A B So, why then is limited generation seed of value? The purpose of limited generation seed programs is to limit field exposure to disease, especially viruses. It also helps minimize the accumulation of pathogens by constantly flushing out the seed with the longest exposure to the environment. Overall, the quality of seed is markedly better since limited generation seed programs began. Some diseases, such as bacterial ring rot, have almost been eliminated. However, that does not mean that the generational designation of any given seed lot has a direct relationship with productivity. Too many other factors influence seed behavior. Generation designation is not a good reflection of important factors, such as general seed crop health or storage conditions. Therefore, it is important to look at all factors that go into producing good quality seed when making a seed purchase. In addition to generation, growers should evaluate certification records, reputation and history of the seed grower, condition of seed handling equipment and storage facilities, and the physical condition of the seed. Even though the limited generation program has improved the quality of seed, it has not simplified the selection of seed lots. 63

17 Courtesy of Potato Grower Magazine FPO 250% Figure Roguing crews remove diseased plants from seed potato fields before inspection for certification. any building with a common air system may result in failure to break the disease cycle and will result in contamination of early generation seed with fungal and bacterial pathogens. Roguing, the term used to describe elimination of diseased plants in a field, is also an important disease control strategy, especially with respect to viruses. If there is a need to hire such services from outside providers, growers should employ only experienced roguing crews (Fig. 4.14). If there is a need to rogue later generation seed, it should be done at least twice. The first time should be timed to eliminate diseased plants as early as disease symptoms are recognized. Fields should be rogued a second time to eliminate diseased plants missed the first time or that express symptoms later than the first roguing. Growers should rogue early generation seed throughout the season and as often as is economically feasible. 64 Courtesy of the Idaho Crop Improvement Association FPO 303% Figure A tag attached to a container of seed potatoes assures the buyer that the contents meet quality standards. Colors represent different grades. Unit and Storage Isolation Production fields of early generation seed must be physically separated from later generation seed to prevent back spread of viruses. Nuclear seed should be isolated from all other potato fields by the greatest distance possible (2 or more miles is best). G1 seed fields should be separated from later generation fields. This is especially critical for controlling the spread of PVY and PVA in varieties that are susceptible to those viruses. Although not feasible in every operation, where possible, potato growers should store each generation of seed in a completely separate facility. Storage of several generations of seed in CERTIFICATION OF SEED POTATOES The use of appropriate production practices is only half of the process of growing high-quality seed potatoes. The second part involves a quality assurance procedure known as certification. In Idaho, potato seed certification is the joint responsibility of the Idaho Crop Improvement Association (ICIA) and the Federal/State Inspection Service. This section briefly explains the certification process. No effort is made to detail all certification procedures and tolerances. This information is available from the ICIA Rules of Certification. Definition of Certified Seed Certification does not constitute a warranty nor a guarantee that the seed potatoes are disease free. Rather, certification means that the seed potatoes have met the standards of a grower-supported state certification agency. This means the seed was produced, inspected, graded, and handled according to the regulations of the agency. Culminating the certification process is the attachment of a tag that assures the buyer that the seed meets published quality standards (Fig. 4.15).

18 Application for Certification Before the crop is planted for seed, certification staff review an application, submitted by the grower, that covers cropping and disease history. Seed stocks planted by the seed grower must be approved by the certifying agency. An application must be made for every lot of seed. Usually, a lot makes up one generation of a single variety. Each application must be accompanied by a payment of fees, including a membership fee, an acreage fee, and a variety fee. Additional fees may be required for laboratory testing for virus diseases. FPO 350% Courtesy of Potato Grower Magazine Certification of Greenhouse Pre-Nuclear Stocks The source of material used for Pre-Nuclear seed production must be tested before propagation for common diseases including Erwinia spp., ring rot, PLRV, PVY, PVX, PVM, PVA, PVS, potato latent virus, and PSTV. This is usually done by personnel at the laboratory from which plantlets are purchased (Fig. 4.16). Responsibility for notification of readiness for inspection of a greenhouse falls to the grower. In Idaho, ICIA inspectors routinely collect leaf samples for laboratory testing before vine killing. Leaf samples from 2 percent of the plants are required for testing. After harvest, inspectors gather a sample of tubers, equivalent to 1 percent of the crop, for winter testing (discussed below). ICIA inspection standards have a zero tolerance in Pre-Nuclear stocks for potato viruses A, X, Y, and LR, for bacterial ring rot, and for Erwinia carotovora. In Idaho, ICIA inspectors visit every seed potato farm during the growing season and inspect each seed field twice for diseases and varietal purity (Fig. 4.17). ICIA inspectors determine percentages of disease and varietal mixture by inspecting a specific number of plants for a given acreage and use this information to determine if the potatoes meet certification tolerances. Potato fields not meeting these tolerances are rejected from certification (Table 4.2). Storage facilities are inspected by the ICIA before harvest, and the size, location, and cleanliness of each facility are noted. Movement of seed pota- Figure ELISA testing is used to quantify PVX and to confirm field detection of other virus diseases. FPO 290% Figure Inspectors evaluate seed potato fields twice a season for evidence of disease and varietal mixture. Courtesy of the Idaho Crop Improvement Association 65

19 table 4.2. Idaho Crop Improvement Association (ICIA) field inspection tolerances for factors affecting seed potatoes. First field inspection generation Disease Nuclear a G1 a G2 G3 G4 G5 and G6 Well-defined mosaic Leafroll Blackleg b c Ring rot d Root-knot PVX Varietal mix Second field inspection generation Disease Nuclear a G1 a G2 G3 G4 G5 and G6 Well-defined mosaic Leafroll Blackleg b c Ring rot d Root-knot Varietal mix a With the exception of bacterial ring rot and root-knot nematode, 0% means the indicated disease or varietal mixture observed was required to be rogued in the indicated seed class. b Tolerances based on visible symptoms. c Visible blackleg will not be used as a rejection factor in the G5, G6, or certified classes. d In the case of bacterial ring rot and root-knot nematode, 0% means none of these diseases are allowed in seed of any class. toes by the grower from the original storage facility requires prior approval from the ICIA. Idaho Certification Standards for G1 to G6 Seed During harvest, random samples of seed tubers are collected and submitted to ICIA for winter testing. Each sample consists of 400 tubers, with the number of samples dependent on the generation number and field size of the seed lot. Growers must submit one sample per 10 acres for G1 seed, one sample per 50 acres for G2 seed, one sample per 100 acres for G3 seed, and one sample per lot for G4, G5, and G6 seed. Information on the number of samples required for Nuclear seed is available from ICIA. The tuber samples are planted in Oceanside, Calif., in late November for the winter growout test (Fig. 4.18). Inspectors check for disease infections that may have occurred since the last field inspection and other problems, such as herbicide carryover. Potato leafroll virus and potato virus Y and A (both listed as mosaic ) are viruses that may spread via aphids after the last field inspection (Fig. 4.19). Tolerances for the winter inspection are in Table 4.3. A seed lot that passes all inspections described earlier is then eligible for shipping point inspection and tagging as certified seed. The shipping point inspection is performed by inspectors from the Federal/State Inspection Service, with the purpose of verifying that the seed lots meet table 4.3. Inspection tolerances for post-harvest winter test of seed destined for recertification. Leafroll 0.8% Mosaic 2.0% Chemical injury 5.0%

20 Courtesy of the Idaho Crop Improvement Association Figure Inspectors examine the winter test grow-out plots in California. Similar tests are also done by state and provincial certification agencies in Hawaii and Florida. United States standards for grades of seed potatoes. The inspector also looks for the presence of zero-tolerance diseases, such as bacterial ringrot (Fig. 4.20), corky ringspot, and root-knot nematode. If any of these diseases are found, the seed lot will be rejected for certification. In addition, diseases such as common scab, late blight, Fusarium dry rot, Rhizoctonia canker, and soft rot must be below tolerances established by the ICIA (Table 4.2). Depending on the grade (blue, green, or yellow tag) (Fig. 4.16), levels of other defects, such as hollow heart and growth cracks, may need to be considered. At the same time, the inspector verifies the seed lot identity, tags the seed lot, and seals the transport vehicle. Differences in State Certification Programs Seed purchased from different states and countries are subject to different certification rules. This creates situations wherein the generation designation may not be what the buyer is used to, nor are allowable disease levels the same. Designations for the number of generations that seed has been grown in the field are shown in Table 4.4. FPO 300% Figure Inspectors of the winter grow-out test look for signs of viruses, other foliar diseases, and herbicide carryover. FPO 300% Courtesy of the Idaho Crop Improvement Association Courtesy of the Idaho Crop Improvement Association 67 Note: Alaska, Colorado, Maine, New York, Utah, and Wisconsin do not designate the first harvest year from field conditions as Nuclear. Figure Federal/state inspectors complete a shipping point inspection of seed tubers.

21 It is advisable to check Table 4.4 when purchasing seed to be sure of obtaining the desired generation of seed. Generation designations may change from year to year. For the latest information growers should consult the current seed directory from each state or province. For a seed grower purchasing out-of-state seed, knowledge of the generation system and the disease tolerances allowed in the state of origin is especially important. Seed brought into Idaho is eligible for recertification only if it meets the standards established for seed grown in Idaho. Establishing proof that a lot of seed meets tolerances is the responsibility of the importing grower. Growers of high quality seed will always have a place in the potato industry because demand for good seed is continual. The process of producing and certifying seed potatoes is possibly the most complex among all certified commodities. A thorough understanding of production and certification principles can make the process feasible and profitable. 68 table 4.4. Limited-generation certified seed potatoes terms used for seed potatoes from one to eight generations removed from laboratory-tested stocks. a generation from tissue culture State 1 b Alaska G1 G2 G3 G4 G5 G6 G7 G8 California N G1 G2 G3 G4 G5 Colorado G1 G2 G3 G4 G5 G6 Idaho N G1 G2 G3 G4 G5 G6 Maine N1 N2 N3 N4 G1 G2 G3 G4 Michigan N G1 G2 G3 G4 G5 Minnesota N G1 G2 G3 G4 G5 C Montana N G1 G2 G3 G4 Nebraska/Wyoming N G1 G2 G3 G4 G5 New York N1 c N2 G1 G2 G3 G4 G5 G6 North Dakota N G1 G2 G3 G4 G5 C Oregon N G1 G2 G3 G4 G5 Utah N(G1) G2 G3 G4 G5 G6 Washington N G1 G2 G3 G4 G5 Wisconsin E1 E2 G1 G2 G3 G4 C Canada PE(G1) E1(G2) E2(G3) E3(G4) E4(G5) F(G6) C(G7) Source: Certification Section of the Potato Association of America. a This table expresses equivalent terms used by various certification agencies for seed potatoes harvested from a series of successive field plantings. For specific criteria relating to disease tolerances and other requirements, the reader is referred to the certification regulations of the state in question. b The first field planting uses laboratory-tested stocks, which may be tissue cultured plantlets, greenhouse-produced mini-tubers, stem cuttings, or line selections. Contact agencies for details as to types of stocks planted in their programs. c If lots originate at Cornell-Uihlein farm, the first three generations are designated with a U to denote source. C = certified; E = elite; F = foundation; N = nuclear; U = Uihlein; PE = pre-elite; G = generation

22 Additional Reading Bohl W. H., P. Nolte, and M. K. Thornton, Potato seed management: Seed certification and selection. Idaho Ag Exp. Sta. Current Information Series No Idaho Crop Improvement Association Idaho rules of certification, ICIA. Meridian, ID. Slack, S. A Seed certification and seed improvement programs. In: R. C. Rowe (ed.), Potato Health Management pp American Phytopathological Society, St. Paul, MN. Whitmore, J. C., R. Clarke, and L. Ewing Guidelines for Russet Burbank nuclear seed potato production in Idaho, Idaho Ag Exp. Sta. Bull. No Wurr, D. C. E Seed tuber production and management. In: P. M. Harris (ed.) The Potato Crop. pp, Chapman and Hall, London. 69