If you are interested in breeding potatoes from seed, you might find this interesting.Potato Breedingfor Durable Resistance to Pests and Disease
Dr. Raoul Robinson
Until a few hundred years ago saving seed of varieties particularly well-adapted to a farmer's local conditions was an integral part of farming. Restoring Our Seed is advancing the ancient art of seed-saving by introducing selective seed-saving for durable resistance.
ROS Potato Seed
Bryan Connolly crossed Blossom (the maternal plant) that produces abundant flowers, has dazzling magenta-red skin and pink flesh, rich, earthy flavor, is resistant to leaf-hoppers and late blight, with:
Caribe: Earliest, high-yielding lustrous purple skin covers smooth snow-white flesh.
Island Sunshine: Delightful creamy, golden-fleshed potato with high resistance to late blight.
Prince Hairy: Bred by Cornell for prickly leaves that repel potato beetle, but poor flavor.
Green Mountain: Dry texture, delicious baking qualities, late potato, susceptible to virus.
Purple Peruvian: Purple flesh, smooth earthy flavor, some tolerance to frost, scab, late blight.
Growing Potato from Seed
Use a small seed drill, or sow by hand. Let blight and beetle do their worst. The most susceptible seedlings will die. Mark the most resistant with a brightly painted stick. These may look susceptible in the early breeding cycles but that's ok. They will be more resistant than all the others. Pull out all the others. Let the bees cross-pollinate all the selected plants. If Colorado beetles are likely to devour the plants before they set fruit, either hand-pick off the egg custers and beetles or protect with Bt or natural pyrethrum. The object is to harvest the potato fruits which are the result of pollination.
When 50-100 potato fruits are soft like a ripe tomato, harvest them. Put the fruits in a kitchen blender, cover with water, and blend just enough to break up the fruits and liberate the seeds. Leave this mixture in a plastic bowl to ferment a day. The seeds sink. With several rinsings, you will have clean seeds. Drain seed and spread on a paper towel to dry. These will be the parents of the second breeding cycle. Come spring, sow again. Repeat for several years, discarding plants with poor flavor or low resistance.
You will be amazed how quickly resistance to beetle and blight accumulate a multi-gene durable resistance. It will not break down to new races of blight fungus in the way single-gene resistances do.
'TPS-true potato seed-is harvested from the berries that grow among the foliage of potato plants. An average plant produces dozens of berries, each of which contains hundreds of tiny seeds. Similar in appearance to tomato seed, TPS is usually sown in seedbeds three or four weeks prior to the potato planting season. The plants in the beds produce small tubers, sometimes called tuberlets, which farmers plant in the field much as they would conventional seed tubers.
This practice sidesteps much of the drudgery involved in handling heavy seed tubes, provides farmers with vigorous disease-free seed, and eliminates the need to store part of the previous year's crop for following year's planting. Many of the production problems that potato farmers experience result from the deterioration of the seed tubers they save for planting, storing them for eight to nine months in inadequate storage facilities.
TPS tuberlets, which are normally no larger than 1 inch in diameter, rival the best tuber seed produced by commercial seed companies.
Any potato that looks really good may be kept and propagated vegetatively as a potential new variety.
Do not be put off by the low yields of seedlings. Remember that potato plants grown from true seeds produce fewer and smaller tubers than those grown from seed tubers.
A word of warning. Some of your original parent varieties may be carrying single genes for vertical resistance to blight. This resistance is useless because it will fail quite quickly, now that both mating types of the blight fungus are present in North America. So, if you see a plant that appears to be completely free of blight, check it carefully. If it shows dead spots that are just visible to the naked eye, it has a functioning vertical resistance. Weed it out to prevent it producing undesirable pollen that is carrying the single genes. There are no single-gene resistances to Colorado beetle or to any other North American pests and diseases of potatoes. So all your resistances will be durable multi-gene complex resistances.
Get seed tubers of about twenty good potato varieties (or varieties you want to cross and improve). Plant a row of each variety, with about ten plants in each row. Some varieties may not flower, or may not set fruit, but if about half of them have fruits, that will do. Aim to harvest 50-100 fruits. Each fruit will contain up to 300 true seeds. Every true seed will be genetically different from every other seed.
When the fruits are ripe and soft like a ripe tomato, harvest them. Keep the fruits of each variety separate. Put the fruits in a kitchen blender, cover with water, and blend just enough to break up the fruits and liberate the seeds.
Leave this mixture in a plastic bowl to ferment for twenty- four hours. The seeds will sink and the fruit debris will float. With several washings, you will have clean seeds.
Drain them through a coffee filter and spread them on a paper towel to dry. Store them in an air-tight jar with silica gel in your refrigerator. They will keep for several years if necessary.
Come spring, prepare a plot up to half an acre to a fine, weed-free tilth by repeated rotovating. Using a small hand seed drill, sow the seeds from each of the original varieties in separate rows. (This is necessary in the first breeding cycle only, to ensure a suitably wide genetic base. In all subsequent breeding cycles, you will work with a single screening population and a single gene-pool). Let the blight and the beetle do their worst. The most susceptible seedlings will die and disappear. Mark the most resistant with a prominently painted stick. These may look quite susceptible in the early breeding cycles but that's O.K. They will be more resistant than all the others. In the early breeding cycles they will be very susceptible. It may be necessary to protect them with a fungicide and insecticide to ensure their survival. Pull out all the others because they might produce undesirable pollen. Leave the bees to cross-pollinate all the selected plants. Harvest the fruits, mix them, and prepare the seeds. These will become the parents of the second breeding cycle. Aim once again for 50-100 fruits. Store the seeds. Come spring, sow them. Repeat the whole process each year for several years, discarding any plants that have poor tubers.
You will be amazed at how quickly resistance to both beetle and blight will accumulate. This happens because of a genetic phenomenon called transgressive segregation. It produces the so-called horizontal resistance, which is quantitative and is controlled by many genes of small effect (polygenes). This kind of resistance is durable. It does not break down to new races of the blight fungus in the way that single-gene (so-called vertical) resistances do. Because horizontal resistance is durable, a good variety need never be replaced, except with a better variety. This means that breeding for horizontal resistance is cumulative and progressive until an eventual ceiling of perfection is reached.
Any plant that looks really good may be kept and propagated vegetatively as a potential new variety. Remember that potato plants grown from true seeds produce fewer and smaller tubers than those grown from seed tubers. Do not be put off by these low yields of seedlings.
A word of warning. Some of your original parent varieties may be carrying single genes for vertical resistance to blight. This resistance is useless because it will fail quite quickly, now that both mating types of the blight fungus are present in North America. So, if you see a plant that appears to be completely free of blight, check it carefully. If it shows necrotic flecks that are just visible to the naked eye, it has a functioning vertical resistance. Weed it out to prevent it producing undesirable pollen that is carrying detrimental genes. There are no single-gene resistances to Colorado beetle or to any other North American pests and diseases of potatoes. So all your resistances will be durable resistances.
Amateur Potato Breeder's Manual Specific, practical advice for the amateur breeder of potatoes, using the author's horizontal resistance techniques.
http://www.sharebooks.ca/ebooks_by_author2.php?author=Robinson%2C+Raoul+A.
The Breeding Cycle
A plant breeding cycle would be called a ‘generation’ in people or animals. A plant breeding cycle begins with pollination, and the subsequent production of true seed. It ends when the next pollination occurs among selected plants that become the parents of the next breeding cycle. Most breeding programs require some 10-15 breeding cycles. In the temperate regions, there will normally be one breeding cycle each year but, with potatoes, it may be possible to complete two breeding cycles each year (see 180-day breeding cycle).
Transgressive Segregation
This term means that progeny may have a higher level of an inherited, quantitative variable than either of their parents. Breeding for horizontal resistance depends very heavily on this phenomenon.
Suppose there is a population of wild potatoes in which every individual plant possesses only 10% of the polygenes contributing to horizontal resistance. The population as a whole will be highly susceptible, and so will every individual within that population. But suppose also the every individual possesses a different combination polygenes in its 10% share. This means that all the polygenes are present in the population, but no individual
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possesses enough of them to be resistant. With random cross-pollination, some of the progeny will have more than 10% of those polygenes (and others will have less). And, given adequate selection pressures, this percentage will increase with each successive generation. After enough generations of cross-pollination, there will be progenies that have most, possibly all, of the polygenes. These individuals will be highly resistant. And their resistance will be durable.
This breeding process of artificially increasing a variable character by transgressive segregation is called recurrent mass selection.
Recurrent Mass Selection
The main feature of recurrent mass selection is that it deals with quantitative variables, such as horizontal resistances. And it raises the level of many desired variables simultaneously, including all the horizontal resistances to all the locally important parasites.
The method of recurrent mass selection is to grow a large screening population of seedlings which all genetically different. The best 10-20 individuals in that population are selected, and they become the parents of the next screening generation. Each generation (or breeding cycle) exhibits transgressive segregation,
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and the desired variables increase accordingly. This increase is usually 10-20% in the early breeding cycles, and the rate of increase gradually declines until no further progress is possible. This ‘ceiling’ is normally reached after 10-15 breeding cycles and, with horizontal resistances, it should provide a complete control of all locally important parasites without any use of crop protection chemicals.
Selection Pressures
The term ‘selection pressure’ is used in the sense of bringing pressure to bear, and it applies to variable populations. For example, if a heterogeneous population of potato seedlings is exposed to a parasite such as blight, the most susceptible individuals will be killed, and the least susceptible individuals will survive, and they will reproduce the most. As a consequence, the next generation will be more resistant.
The selection pressure is exerted by the blight fungus, and this can happen in both a wild population, and the screening population of a breeding program.
Screening in the Tropics
There are no winters in the tropics and breeding can continue throughout the year, particularly if irrigation is available
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during the dry season. When I was breeding potatoes in Nairobi, Kenya, in the 1960s, I sowed 1000 pre-germinated seeds every working day. At about six weeks of age, these were exposed to blight and bacterial wilt (a tuber-borne disease), and the few survivors were transplanted in the field. The selected parents of the next breeding cycle were grafted on to tomatoes. There were two breeding cycles of recurrent mass selection each year, with some 150,000 seedlings per cycle. This work produced two new cultivars called Kenya Akiba and Kenya Baraka, which have now been cultivated for about 65 vegetative generations without any renewal of seed stocks, and without any spraying against blight (there are two crops each year in Kenya, and Colorado beetle is absent from this part of the world). According to the Food & Agricultural Organisation of the United Nations, potato production in Kenya has increased 35 times since these cultivars were released to farmers in 1972. Two older cultivars, Rosslyn Eburu and Dutch Robijn, also have fairly good horizontal resistance to blight and they are still being cultivated.
These Kenya cultivars are short-day plants which cannot be cultivated in the temperate regions, and which should not be used a parents in a temperate region breeding program. However, they would be useful in a tropical breeding program. At present, tropical potato cultivation is confined to the cool, relatively dry, high
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altitude areas. But there is no reason why attempts should not be made to breed cultivars for the warmer and wetter areas. Note that day-neutral cultivars from temperate countries can be grown in the tropics, and they can be used as parents in a breeding program. Unless stated otherwise, all further discussion concerns potatoes in the temperate regions, but it is generally relevant to the tropics also.
After blight, the most important tropical disease of potatoes is bacterial wilt caused by Pseudomonas solanacearum. This is a tuber-borne disease and horizontal resistance to it is important if potatoes are to become an important crop in the country concerned. Colorado beetle has not reached the tropics, and the most serious tropical insect pest is the potato tuber moth.
Choosing the Parents
The breeding should begin with 10-20 parents, and each parent should be a popular modern cultivar. In choosing the parents, the aim is to have as wide a genetic base as possible, while maintaining high standards of yield and quality. There is no need to choose parents on the basis of their resistance to parasites, because a reasonably wide genetic base will accumulate all the horizontal resistance we need by transgressive segregation. If it does not, the genetic base can always be widened later.
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Because of possible problems in flower production, or with pollen sterility, it is advisable to use a few more parents than are needed, so that some can be discarded if they prove difficult to cross. Alternatively, new parents can be added to the breeding population at a later date.
Neo-tuberosum
N.W. Simmonds (see Recommended Reading) did an interesting experiment in Scotland. He wanted to prove that Solanum tuberosum was derived from the South American species Solanum andigena. He also wanted to show that horizontal resistance to blight could be accumulated in the very susceptible S. andigena. He proved both these points by recurrent mass selection, and he produced a new kind of potato which he called neo-tuberosum. This new potato is closely similar to cultivated potatoes, and it has a useful level of horizontal resistance to blight. It has been used in commercial potato breeding, and these new clones would be useful parents for amateur breeders, particularly if they were trying to widen the genetic base.
Grafting
Recurrent mass selection requires very large numbers of seedlings, almost without limit. Grafting potato scions on to
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tomato rootstocks is a technique for producing very large numbers of true seeds of potato.
Selected potato scions can be grafted on to tomato rootstocks to induce flowering. Being unable to form tubers, the potato scion grows upwards as a vine, with a bunch of flowers every few inches. These flowers can be used for controlled cross-pollination. One good graft can produce up to fifty fruits, with up to 300 seeds per fruit, totalling some 15,000 true seeds.
There are several steps in this grafting.
Grow one tomato plant in each pot so that it is about six weeks old when the grafting is to take place. Each pot should be fairly large (about 12 inch diameter) to avoid any need for re-potting later.
With a new razor blade, cut off the tomato stem with a horizontal cut, about 1½ inches above the first two leaves (photo in preparation). Then make a vertical cut down the centre of the stem to a depth of about one inch (photo in preparation).
Cut a vegetative shoot off the potato parent, several inches long (photo in preparation). Place it in water and take it immediately to the grafting house. Cut off a terminal shoot about one inch long (photo in preparation) for use as a potato scion. Slice the sides of the scion to make a wedge (photo in preparation).
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Insert the wedge into the vertical cut in the tomato stem (photo in preparation).
Wrap the graft union in a ribbon of very thin, transparent, plastic film (photo in preparation). This ribbon serves two purposes. It binds the graft-union surfaces together, and it prevents the union from drying out. It should be fairly tight, but not too tight.
Place the entire pot inside a mist propagator for about five days, or until the potato scion shows signs of active growth (photo in preparation). Alternatively, put three sticks in the pot, and cover the entire pot with a plastic bag that is tied to the top of the pot, in order to maintain a high humidity (photo in preparation).
The two tomato axillary buds may start growing. If they do, cut them off. The tomato leaves should be removed later, when they show signs of senescence. The ribbon of plastic film may be removed soon after the scion starts growing and the graft union is well calloused over.
Place the pot in a greenhouse with a supporting string hanging from the roof. The potato vine may grow to a height of several feet. A few, small aerial tubers may form, and they should be removed.
There should be two grafts of each potato parent, mainly as an insurance in case one graft is lost for any reason. With 10-20
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parents, there will be 20-40 grafts. With practice, cross-pollination among this many plants should not require more than 1-2 person-hours of work each day.
Tomato rootstocks are useful because we know that they graft easily and well. However, they have the disadvantage that they can get blight (Phytophthora infestans) and, if they do, the entire graft is in grave danger of being lost. There is also a risk of blight attacking the potato scion, particularly in the early breeding cycles. There are two ways of avoiding blight on both the stock and the scion. One is to spray them with a copper or dithiocarbamate fungicide. An alternative is to water the plants from the base of the pots, and to keep both the scion and the above-ground stock dry at all times. This is easily achieved in a greenhouse. Blight spores need free water to germinate and, if all the aerial parts of the grafted plant never get wet, they will not get blight. For complete safety, both methods may be used.
Some useful research, that can easily be undertaken by amateur breeders, would involve a search for alternative rootstocks that are immune to blight. Possible species include the common weed called ‘thorn-apple’ (Datura stramonium); wild species of Solanum, such as S. nigrum and S. dulcamara in Europe, or S. rostratum, known as prickly potato, or the buffalo burr, in North
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America; eggplant (Solanum melongena); and tobacco (Nicotiana tabacum).
Flower Production
The potato scion will start producing flowers quite soon after grafting. Each inflorescence usually has 5-10 flowers, and they normally open one or two at a time each day (photo in preparation).
In a few potato cultivars, flowering does not occur, or the young buds are shed before opening. Some may produce pollen that is sterile. Should this happen, the parent in question should be discarded. This problem is discussed further in the next section.
Cross Pollination
Cross pollination is done in order to produce genetic variability and transgressive segregation. There are two steps in cross-pollination.
Emasculation involves the removal of the anthers so that the flower cannot self-pollinate. Emasculation is done the day before the flower opens, when the anthers are still immature and infertile. This stage is easily recognised because the petals are fully formed but still adhering to each other, so that the flower is closed. The petals must be gently separated with a needle (photo in
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preparation) and then each of the five anthers must be bent away from the pistil until it breaks off and falls to the ground (photo in preparation). The next day, an emasculated flower is wide open and is easily recognised by the absence of yellow anthers (photo in preparation). Each day, all the flowers that are ready should be emasculated, except those of the male parent (see next).
Cross-pollination involves the placing of pollen on the now receptive stigma of each emasculated flower. It is a good idea to use only one male parent, and a different male parent, each day in rotation, so that each parent is represented equally. The male parent flowers are wide open and are easily recognised by the presence of yellow anthers. One anther is picked with a pair of fine forceps (photo in preparation), and it is touched to the stigma of each of several emasculated flowers (photo in preparation). Make sure that the side with free pollen is used. A small dot of yellow pollen should be visible on the stigma. There are five anthers to each male parent flower and, with several flowers, this should provide adequate pollen for all the cross-pollinations to be done in one day.
Some potato cultivars are pollen-sterile, or even female sterile. If it is discovered that a male or female parent never forms fruits, its use must be discontinued. Famous cultivars that suffer sterility problems are Russet Burbank, Bintje and King Edward,
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and the use of these as parents should be avoided by amateur breeders.
Do not waste time labelling flowers, keeping detailed records, or keeping each batch of seed separate from all the others. All the true seed is going to be mixed, and sown as a single screening population.
Potato Fruits
The true fruits of potatoes are similar to a small tomato. They develop from a pollinated flower. When ripe, they are usually a green or bronze colour (see photo), and they are soft and somewhat wrinkled. Each fruit contains up to 300 true seeds. Because of the cross-pollination, these seeds will exhibit great variation in all their quantitatively inherited characteristics, including their horizontal resistances.
Seed
Seed Sowing
When there is no further risk of frost, the seed should be sown in a seedbed with a minimum separation of about two inches between seedlings. Various hand-operated seed drills are available for this sort of work. The seed bed should have a very fine tilth, having been rota-tilled several times in order to germinate and kill weeds. Sow most of the seeds you have been able to produce, keeping back a few as an emergency reserve in case of disaster. It is a good idea to aim at sowing at least 100,000 seeds. In the later
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breeding cycles, the number of seedlings can be gradually reduced, and this is a point that will be determined by experience.
These seedlings can then be screened for horizontal resistance, and the whole idea is to let the parasites do this work for you. Remember that you will only be interested in the survivors, and there will not be many of these.
Seedling Screening
After the seedlings have been showing above ground for a few weeks, blight will begin to attack them, and Colorado beetles will begin to eat them.
In the early breeding cycles, there is a grave risk that the blight and beetles will destroy every single plant. The damage will be comparable to the destruction caused by blight, when it first appeared in Ireland. This is obviously a critical stage, and the relatively few survivors must be rescued in good timer. They should be transplanted into pots, and taken to a greenhouse where they can be nursed back to health. It may also be necessary to use crop protection chemicals, both in the field and the greenhouse, in order to save the few survivors. This is a point that should be noted by breeders working on an organic farm, who may prefer to conduct their early breeding cycles on rented land in a
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conventional farm, where crop protection chemicals can be used without jeopardising their organic certification.
The Later Breeding Cycles
In the later breeding cycles, the numbers of seedlings can be reduced. There may be so many surviving seedlings that the best 10-20% would have to be transplanted to a potato field, with a spacing similar to that of a commercial potato crop, before final selections can be made. Professional potato breeders reckon that about 10,000 seedlings must be screened to get a new clone of commercial quality, but this is without regard to horizontal resistance.
As the breeding progresses, therefore, and the levels of horizontal resistance increase, the number of seedlings screened can be reduced, but the amount of work required for each seedling will increase. It is at this stage that the breeding begins to get exciting.
Inoculation and Infestation
In the later breeding cycles, there will be so much resistance that inoculation may be necessary. One or more rows of susceptible modern cultivars should be planted on each side of the seedbeds (or the transplanted potato seedlings) so that blight and
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beetles can invade the screening populations. It is at this stage that much stronger selection pressures for other selection criteria begin to be applied. These include resistance to other parasites, such as virus diseases, as well as attributes of yield and quality. It will probably be advisable to get technical help with the virus diseases.
Number of Generations
The method of potato breeding recommended here is based on the behaviour of maize in tropical Africa following the accidental introduction of a rust disease from Central America (see Return to Resistance, available as a sharebook at
www.sharebooks.ca for a full description). When this maize disease first appeared, the loss of crop was almost total, and the farmers had barely enough seed to sow the next crop. This represented the minimum level of horizontal resistance. There are two crops each year in this area and, after 5-7 years, the level of horizontal resistance was maximal, and the loss of crop was negligible. This indicates how many breeding cycles will be required, as well as how very effective transgressive segregation can be.
Note that this maize accumulated all the resistance it needed without any help from professional breeders or pathologists. It could do this because maize is open-pollinated, and
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the recurrent mass selection was effectively a natural process, assisted by the wisdom of the farmers who were persisted in trying to preserve their favourite landraces. While this accumulation of horizontal resistance was happening, an official maize breeding program was conducted to produce maize with single-gene resistances, but these vertical resistances broke down so quickly that the new lines never even reached the farmers.
Breeding potatoes for comparable levels of horizontal resistance to blight, beetle, and other parasites, may take a little longer than the African maize, because more species of parasite are involved. But the difference is unlikely to be large.
On-Site Screening
On-site screening means that the screening is conducted in the agro-ecosystem of future cultivation, in the time of year of future cultivation, and according to the farming system of future cultivation. This is important because the epidemiological competence of parasites varies considerably from one agro-ecosystem to another. A cultivar with horizontal resistances that are in perfect balance with one agro-ecosystem will then have too much resistance to some parasites, and too little to others, when taken to a different agro-ecosystem, or when cultivated out of season, or when cultivated with different cultivation practices.
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Fortunately, with potatoes, the agro-ecosystems are usually large, and this is not a serious problem. For example, bacterial wilt (Pseudomonas solanacearum) is a tropical disease which lacks epidemiological competence entirely outside the tropics and sub-tropics, apparently because it cannot survive a winter. There is no need to breed for horizontal resistance to this disease in temperate regions. Equally, the temperate virus diseases of potato seem to lack epidemiological competence in the tropics, and there is no need to breed for horizontal resistance to these diseases in tropical countries.
However, the principle of on-site screening becomes important with greenhouse work. Greenhouse screening is permissible when there is a 180-day breeding cycle, but only if it is alternated with an on-site field-screening at the proper time and place.
Locally Important Parasites
Because of the need for on-site screening, a potato breeding club must be located in the area of future cultivation. The breeding for horizontal resistance can only embrace potato parasites that are locally important. If a parasite species is absent from the agro-ecosystem in question, or if it has a very low epidemiological
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competence, it is not feasible to breed for resistance to it. Nor is there any necessity to do so.
A manual such as this can accordingly make only very general remarks concerning individual species of parasite. An amateur breeding club must make itself informed in this matter, and the members should consult libraries and/or specialists. However, if a parasite is locally important, it will show up quickly enough in the screening population.
The 180-day Breeding Cycle
In the temperate regions, it is normally possible to achieve only one breeding cycle each year, with the cross-pollination and seed production undertaken in a greenhouse, during the winter, and the on-site screening conducted in the field, during the summer. With ambitious breeding targets (e.g., horizontal resistance that is complete and comprehensive against all locally important parasites), this could require 10-15 breeding cycles during the same number of years.
If, however, we could squeeze two breeding cycles into each year, the total breeding time would be halved. This would necessitate a 180-day breeding cycle, with each cycle divided into a 90-day seed production cycle, and a 90-day screening cycle. This would require a lot of careful planning, and hard work, but it is
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feasible. It would be achieved by considerable over-lapping of the cycles.
For example, about five times the required number of parents could be selected about two thirds of the way through the screening cycle (i.e., after 60 days of screening). These would be grafted on to tomatoes (sown well in advance) on the clear understanding that 80% of them will be discarded when the final selections are made. By the end of the screening cycle, the selected parents will be grafted, flowering, and ready for cross-pollination. This would add 30 days to the seed production cycle, which would then be effectively 120 days.
One breeding cycle would obviously have to be conducted during the winter and, in most temperate countries, this would mean a fairly large heated greenhouse, as well as the induction of both insect infestations and disease epidemics. Technical help may be needed here. The screening cycle would begin by sowing the true seed immediately after it is becomes available at the end of the seed production cycle, including seed taken from somewhat immature fruits.
In the temperate regions, one 180-day breeding cycle should start in spring, and allow an on-site screening in the summer. The second breeding cycle would start in the autumn, and this would necessitate a winter screening in the greenhouse.
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