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Tomato Handbook (Non-Technical Version)

Introduction

There were no professional plant breeders before 1900, and all plant breeding was carried out by amateurs, even by English country parsons, who often had time on their hands. Nowadays, the very thought of undertaking plant breeding has become intimidating. This is because the professional breeders have made the subject so technical, and obscured it with a jargon so abstruse, that no outsider can understand it. The purpose of this series of handbooks is to help people to return to the simple, but effective, pre-scientific plant breeding.

Wiki

These handbooks are all WIKI files. This means that anyone may contribute information, or improve the files, as a result of their personal experience. In this way, the handbooks will be continuously enlarged and improved. However, the editors reserve the right to question doubtful information, and to remove comments that they consider facetious or unhelpful.

The total number of handbooks is limited only by the total number of different species of crop. Anyone who is breeding a crop for which there is no handbook is invited to begin that book, and to invite comments from other breeders of that crop.

A Golden Rule

Whenever you happen to taste a really delicious tomato, keep the seeds for cultivation and/or breeding. To destroy those seeds is a small but very real impoverishment. Equally, when you see an unusually healthy tomato plant in your garden, keep some of its seeds for further exploration. You might even get results more quickly this way than you would with a more formal breeding program.

Botany

Tomatoes (Lycopersicon esculentum) belong to the same botanical family (Solanaceae) as potatoes, tobacco, red and sweet peppers, and eggplant. The tomato is an annual plant with both climbing and recumbent types.

Tomatoes have stigmas (i.e., pollen receptors) of differing lengths. Ancestral types have a long stigma that protrudes beyond the anthers (i.e., pollen producers) and this allows natural cross-pollination to occur. Ancestral types also tend to have self-incompatible pollen, and this also encourages cross-pollination.

Modern types have a short stigma which remains buried in the anther cone, and both self-compatibility and self-pollination are normal. This has the advantage that pure lines can be produced which breed true; but it has the disadvantage that cross-pollinating in a breeding program must be done by hand (see below). Many heirloom varieties are modern types.

History

Tomatoes originated in South America, in the general area of Peru and Ecuador, but they were first domesticated in Mexico. The Spanish took domesticated forms to Europe in 1523 and they had reached Italy by 1544, and England by 1597. The Spanish also took them to the Philippines and they were recorded in Malaysia in 1650. Tomatoes were taken to North America from Europe in the late eighteenth century.

Today tomatoes are grown all over the world in areas that have at least three frost-free months each year. They are also one of the more popular greenhouse crops.

Blight

Tomatoes are susceptible to the potato blight pathogen called Phytophthora infestans.

This microscopic fungus is a natural parasite of wild potatoes in Mexico, and it was totally confined to this country until it was accidentally carried in a ship's galley to New York in about 1840. From there, it was accidentally taken in another ship's galley to Europe, and it reached Ireland in 1845.

It was a devastating disease, and it caused so much damage that this decade became known as the 'Hungry Forties'. Food shortages led to a series of socialist revolutions in continental Europe, in 1848, and these were brutally suppressed. It is estimated that, in Ireland, about one million people died of starvation, and another million and a half emigrated, mainly to North America. Comparable numbers died in eastern Europe and western Russia, where potatoes had replaced rye as the basic staple.

In those days, the blight epidemics could begin only from rather rare blighted potato seed-tubers in the soil. This meant that the blight epidemics started slowly, and potato blight was known as 'late blight' for this reason. It also meant that the only way that tomatoes could get blight was by spread from nearby potato crops, rather late in the season.

Quite recently, some incredibly careless people in Europe inadvertently introduced another mating type from Mexico and, as a result, the fungus is able to form tough over-wintering spores. Tomatoes can now get the disease directly from these spores in the soil, and blight now starts much earlier, and it has become a much more serious disease, as organic farmers know to their cost. Breeding for durable resistance to blight is the main reason for amateur breeders to work with tomatoes.

As a temporary measure for home-growing, it is worth knowing that blight spores can only infect tomatoes when the plants are wet. A small garden plot can easily be protected from rain with a temporary plastic-sheet housing like a small greenhouse. The sides can be open and the plants should be watered by furrow irrigation. Even if small amounts of blight develop, they will not spread so long as the stems, leaves, and fruit remain dry.

List of Parasites

Non-technical amateurs should not worry too much about identifying the many pests and diseases that attack tomatoes. Just select for good health and high yields.

The Breeding Objectives

The aim of this breeding is to produce new tomato varieties that organic farmers can grow without any use of crop protection chemicals, and without any significant loss from pests or diseases. This is necessary because none of the professional breeders seem to be interested in such work, and none of the funding organisations seem willing to finance it.

The Breeding Method

The breeding method is called 'population breeding' to distinguish it from the more usual 'pedigree breeding' used by scientists working with resistance to pests and diseases. The difference is that pedigree breeding produces resistance that is likely to break down to new races of the pests and diseases. Population breeding, on the other hand, produces durable resistance that does not break down in this way.

Population Breeding

Population breeding requires fairly large tomato populations in which every plant is different form every other plant. The initial population is created by cross-pollinating some 10-20 heirloom varieties that are significantly different from each other. Each variety must be crossed with all the other varieties. However, you can use much smaller numbers if you wish, but this will require more breeding time.

Cross-pollination

The technique of pollinating tomato flowers requires only a little practice and is soon learned. Select a closed bud which is about to open the next day. Using a large darning needle, separate the petals from each other in order to open the flower. Then use the needle to break off each yellow anther by bending it away from the centre of the flower. It can be allowed to fall to the ground. On the following day, the emasculated flower will be fully open, easily recognised, and it can be pollinated. This flower becomes the female parent.

Each day, you use pollen taken from a different variety. In this way, each variety can become a male parent. The best technique for pollinating is to hold a mature anther taken from the male parent in a pair of forceps. Touch the stigma of the each emasculated flower with that anther. A small spot of yellow pollen should be visible on the stigma of the female parent. The emasculated flower is easily recognised by the absence of anthers. You may care to label the each cross-pollinated flower with a light, tie-on label which records the identity of the male parent, but this is not essential provided that all the unwanted, self-pollinated flowers are removed.

Remember that, if you start with ten varieties, the first male parent must pollinate nine varieties on the first day because we do not want it to self-pollinate. The second male parent must pollinate only eight varieties on the second day, because it was itself cross-pollinated on the previous day. And so on down to the last male parent which must pollinate only one variety on the last day. In this way, every variety will have been crossed once with every other variety, and there will be no self-pollinations.

Choice of Parents

As a general rule, it is probably better to work with heirloom varieties. Choose the varieties you like best, with regard to taste, fruit shape and colour, yield, plant shape, and so on. Do not try to use only resistant parents. The whole object of this exercise is to accumulate resistance by breeding within susceptible but otherwise high quality varieties.

Seed Preparation

Cut open the ripe fruit and remove the seeds. Each seed is protected with a layer of transparent mucus, and this should be removed by fermentation. Place the seeds in a glass jar and cover them with water. Leave them to ferment for about twenty four hours. When the mucus has gone, wash them thoroughly with water running through a domestic sieve. Spread them on a paper towel to damp-dry, and then on a clean plate to dry completely. When dry, the seed should be stored in an air-tight container in the kitchen fridge. It is a good idea to place one of those silicon gel bags in the container. These bags come with new electronic or photographic equipment, and friendly storekeepers give them away. The bag can be activated by exposing it to the dehydration cycle in a domestic convection oven.

For the first breeding cycle only, the seed of each cross-pollinated fruit should be treated and stored separately. There are two important reasons for this.

First, the original parents were pure lines. This means that there will be no variation within the first generation. So the first seed should not be used as a screening population. It should be used only as a seed-multiplication generation in which the fruit are allowed to self-pollinate. The seed from these fruits will be highly variable and can be used for screening.

Second, you want each of the original parents to be equally represented in the subsequent screening population. The easy way to do this is to take an equal quantity of seed derived from each of the original crosses. There is a good deal of latitude here and I would just thoroughly mix those equal quantities and sow as many of the mixture as I can handle. The limiting factor here is the amount of land you have available. Remember that most of this first screening population will be wiped out by pests and diseases. So you can plant the tomatoes much more closely than a commercial crop. Remember, the bigger the population, the better the chance of obtaining survivors which will become the parents of the next screening generation, and the more likely that those parents will be of high quality.

Seed Sowing

Tomato seeds are traditionally sown in 'flats' and then transported to the field for transplanting when the seedlings are about six inches tall. An alternative is to sow the seeds in peat pots. The entire pot is then planted in the field, and the tomato roots grow through the peat into the soil. If you have very many seedlings, it might pay you to contract the sowing to a commercial greenhouse operation to sow them for you. These people have many kinds of machine that will sow seeds accurately, quickly and cheaply.

Inoculation with Soil-Borne Parasites

It is often a good idea to inoculate the soil used for seedlings with soil-borne parasites, such as wilt pathogens, nematodes and insects. Just add some soil taken from a bad patch in a field where the parasite in question is prevalent. It should be well mixed into the seedling soil and, with the transplanting of the seedlings, there will be a relatively uniform distribution of the parasite in your screening field. However, this type of operation is best left to the later screening generations because of the risk of the total destruction of the screening population. Remember also that a commercial seed sowing operation will not thank you for introducing heavily contaminated soil into their premises.

The Breeding Cycle

The breeding cycle involves all operations between one cross-pollination and the next. The biggest genetic improvements occur in the early breeding cycles. Usually some 10-15 breeding cycles are necessary to complete a breeding program, although you can continue longer if you wish, but expect to get smaller and smaller improvements with each cycle.

The main feature of each breeding cycle is the screening population (see below).

The breeding cycle may include other operations, such as seed multiplication or a few self-pollinated generations to produce pure lines. However, the longer the breeding cycle, the longer the breeding program. If you have one breeding cycle each year, the program will require some 10-15 years. Obviously, if you can squeeze two breeding cycles into each year, you will get results twice as fast. This is not difficult in the tropics but, if you have winters, you will need a fairly large greenhouse. Remember also that you may well get useful results in some of the early breeding cycles.

The Screening Population

The screening population is produced from the seeds of each cross-pollinated fruit. Because tomatoes produce many seeds, you will usually have more seed than you need. Keep the surplus just in case of some disaster affecting the screening population. The screening population should be as large as you can manage; the more plants that are screened, the more rapid the genetic advance.

Improvement

The genetic improvement is quantitative, with small increases in every important variable with each new breeding cycle. In practice, the best plants of one breeding cycle become the parents of the next breeding cycle. The best plants are those with the least pests and disease, the best yields, and best-looking and best-tasting fruit.

Improvement comes from further cross-pollinations. The best plants in the screening population are selected and marked, possibly with a brightly painted stick, or a piece of brightly coloured wool. At least one flower of each selected plant should be cross-pollinated with pollen taken from another selected plant. It is probably not necessary to do this comprehensively, as with the original cross-pollination. But you can introduce an element of randomness by using different plants as male parents, or by mixing pollen from several plants. If each selected plant has only one fruit, produced by cross-pollination, all other flowers having been removed, there should be ample seed for the next breeding cycle.

Remember also, that a few fruits which been self-pollinated will do little harm. A self-pollination in a plant produced by cross-pollination will still produce considerable variation, although less than a new cross-pollination would.

You have more resistance than you think

There are several reasons why tomato plants growing in a screening population get much, much more pests and diseases than they would when they are grown as a single variety in a farmer's field. And, in the early breeding cycles, the best plants may well look terrible. The danger is that amateur breeders are liable to lose heart and just give up.

Please, please persevere.

Remember that the most susceptible plants will have disappeared entirely. The early survivors may look ghastly, but they are at least alive and surviving. They have significantly more resistance than those susceptible plants which died of disease. Their progeny will be somewhat better, and there will a continuing improvement with each breeding cycle. Eventually, you will be amazed at how much improvement you have produced.

On-site selection

On-site selection means three things. It means that the selection is done in the area of future cultivation, at the time of year of future cultivation, and according to the farming practices of future cultivation. For example, if you do all your screening in a greenhouse, you will probably produce varieties suitable for greenhouse cultivation but possibly unsuitable for open-field cultivation. This is because the pests and diseases tend to be different in a greenhouse, compared with open fields.

So, whenever possible, you should do on-site selection. However, there is an obvious exception to this rule. If you are breeding for open-field tomatoes in an area with winters, and you want two breeding cycles each year, then one of these breeding cycles will have to be in a greenhouse. This may mean that the rate of improvement is reduced somewhat, but this is acceptable if the total breeding time is almost halved.

Patchy distribution

Some pests and diseases have a patchy distribution, which means that some parts of your screening population will be heavily affected while other parts will escape entirely. Plants which escape may appear to be resistant when, in reality, they are quite susceptible. Try to do your screening within the heavily affected areas rather than outside them.

New pure lines

In order to produce a new variety, you must make it into a pure line. To do this, you allow your final selection to self-pollinate. Plant some of its seeds, select the best plant, and allow it to self-pollinate. Repeat this process for at least four (and preferably six) generations. By then all significant variation will have stopped, and the line will “breed true”.

In practice, you are likely to have several, even many, lines that show promise of becoming new varieties. And you will be testing them in the field. The two operations of pure line formation and field testing can be combined with 'family selection'. About a dozen seeds from each self-pollinated selection are planted in a row which is called a 'family'. You choose the best families first; and then you choose the best individual in each of the selected families. That individual is allowed to self-pollinate to produce seed for the next generation of field testing and pure line formation.

Variety registration

The purpose of registering a new variety is the equivalent of registering a book for copyright in order to earn royalties. Not all countries have legislation for this, and those that do have widely differing laws. If you want to register new varieties, check with your agricultural department. Increasingly, registration involves taking a DNA fingerprint, and this is expensive. So check on the cost of registration. Even if you want your new varieties to be in the public domain, you may still want to register them in order to prevent someone else from registering them, and claiming royalties for your work.