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Tomato Handbook (Technical version)



Plant Breeding for Organic Farmers


Tomato Handbook

(Technical version; for those who enjoy science)


Raoul A. Robinson



This series of handbooks is intended for organic farmers who wish to become amateur plant breeders. There is an urgent need for such work if we are to have long-lasting, high quality varieties that can be grown without pesticides. It seems we must do this work ourselves because no one else is prepared to support it, least of all the big chemical corporations. These corporations finance a lot of agricultural research, but the last thing they want are increases in resistance that would reduce the pesticide market.

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 thought of undertaking plant breeding is intimidating. This is because the professional breeders have made the subject so technical, and obscured it by 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.

There were no pesticides either, before 1900, other than the famous Bordeaux mixture (discovered in 1882), and a few natural products such as pyrethrins, rotonone, flowers of sulphur, and soft soap. And, as we all know, heirloom varieties dating from those days are more resistant to pests and diseases than are modern varieties, and their resistance is durable. Heirloom varieties had to have high levels of durable resistance, because virtually all agriculture and horticulture at that time was without pesticides. If the amateur breeders of the nineteenth century could do it, we can too.

If you want to understand the rudiments of plant breeding, particularly for resistance to pests and diseases, my book Return to Resistance is recommended, but is not essential. It is available as a free download from the Open Plant Breeding Foundation (OPBF), or from <>.



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 developed. Colour photographs of pests and diseases will be particularly useful. 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 crop species. Anyone who is breeding a crop for which there is no handbook is invited to write 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.



The tomato (Lycopersicon esculentum) is a member of the botanical family Solanaceae which includes potatoes, tobacco, red and sweet peppers, and eggplant. The tomato is an annual plant with both climbing (indeterminate) and recumbent (determinate) types. Climbing types become vines that do not top off; they continue fruiting indefinitely. They are usually supported by some form of fence, and they are the choice of growers who want ripe fruit throughout the summer. Recumbent types are used by commercial growers who like to harvest the entire crop in one operation. A few varieties are semi-determinate and produce a second crop after the first crop is harvested. Most (but not all) heirloom varieties are indeterminate. 

Tomatoes have stigmas of different lengths.  Ancestral types have a long stigma that protrudes beyond the anthers and this allows natural cross-pollination to occur. Solitary bees are the most common pollinating agents. In any event, ancestral types 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).

The green tomato, or tomatilos, (Physalis philadelphica) is also a member of the Solanaceae but is not a true tomato. It is an important crop in Mexico.



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 they 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.



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 that 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.

Potatoes and tomatoes both originated in South America and blight originated in Mexico; it was consequently a new-encounter disease in both of these crops. It was also a devastating disease, and it caused so much damage that this decade became known as the 'Hungry Forties'. The resulting food shortages were responsible for a series of socialist revolutions in continental Europe, in 1848, but these were brutally suppressed because conservatives insisted on the need for monarchial government. 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.

An interesting thing about the blight fungus is that it has two mating 'types'. Each type is hermaphrodite (i.e., it has both both male and female sexes) but it is self-sterile. This means that both types must be present if sexual fusion is to occur. Sexual fusion results in the production of very tough over-wintering spores called oospores. However, that ship going to New York carried only one mating type (called A1) and, consequently, oospores could not form in either North America or Europe. One of the effects of this was that the blight epidemics could commence 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 imported the second mating type (called A2) from Mexico, and it has now been spread all over the northern hemisphere in certified seed potatoes. As a result, tomatoes can now get the disease directly from oospores in the soil. Blight now starts much earlier in tomatoes, and it has become a much more serious disease, as organic farmers know to their cost. (See Return to Resistance, Chapter 18, for a more detailed account).

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.

(Technically-minded amateurs may know about the need for a designated pathotype when breeding for horizontal resistance. One small advantage of the second mating type is that it eliminates the need for this complication in both tomato and potato breeding, because unwanted vertical resistances will be matched very quickly.) 



List of Parasites


Major fungal pathogens:

Blight (Phytophthora infestans) already described. (Photo needed).

Target spot (Alternaria solani). This disease used to be called 'early blight' to distinguish it from late blight (Phytophthora infestans). Following the introduction of A2 blight, these distinctions are no longer valid, and the name 'target spot' is appropriate. The diseases produces a necrotic spot with concentric circles surrounding the point of infection, reminiscent of an archery target. 

(Photo needed).

Anthracnose (Colletotricum gleosporioides). An anthracnose is a sunken, necrotic  spot with minute black dots on its surface. (Photo needed).

Fusarium wilt (Fusarium oxysporum f.sp lycopersici) and Verticillium wilt (Verticillium albo-atrum). These two wilt fungi produce identical symptoms and can be distinguished only by microscopic examination of the pathogen. The plant wilts even though there is moisture in the soil. Fusarium wilt is more common in warm areas and greenhouses, and Verticillium wilt prefers cooler areas. (Photo needed).


Bacterial pathogens:

        Bacterial wilt (Ralstonia solanacearum, previously Pseudomonas solanacearum). This is a very serious disease of tomatoes, potatoes and other crops in the tropics and, less so, the sub-tropics, but it lacks longterm epidemiological competence in temperate areas which have serious frost. Consequently, resistance to it is important in warm climates but it can be ignored in areas with cold winters. The principle symptom is a massive wilting of the plant even though the soil is moist. The plant then dies. The bacterium is easily cultured and a water suspension can be added to the soil used for tomato seedlings that are to be screened for resistance. (Photo needed).


Virus pathogens:

Tomato leaf curl. (Photo needed). 

Curly top virus. (Photo needed). 

Tobacco mosaic virus. (Photo needed). 

Tomato spotted wilt virus. (Photo needed). 


Major insect pests:

Stink bugs (Pentatomoidea). Also known as shield bugs, these insects have defense glands which produce a nasty smelling liquid. The nymphs are similar to adults except that they are smaller and without wings. The nymphs and adults have piercing mouthparts which they use to suck sap from plants. (Photo needed). 

Cutworms (Agrotis spp.). These insects are the larvae of Noctuidae moths. Cutworms are serious pests on many crops, and they feed on all above-ground parts of herbaceous plants. They get their name from their very destructive habit of cutting young seedlings at soil level. (Photo needed). 

Tomato hornworm (Manduca quinquemaculata) is the catepillar of a  Sphingidae moth and is easily confused with closely related tobacco hornworm  (Manduca sexta). They can be very damaging. (Photo needed). 

Aphids, also known as green bugs or green flies,

  Loopers are moth (Noctuidae) caterpillars that walk by arching their bodies in a characteristic loop. They can be very destructive as they eat leaves. (Photo needed). 

Whiteflies: (Bemisia spp.). A major pest in glasshouses, and in the tropics and subtropics, damaging mainly by transmitting viruses such as tomato leaf curl, but they also cause damage by feeding. They can occur in huge numbers. (Photo needed). 

Tomato fruit-worm: the larvae of a moth (Helicoverpa zea) which has many hosts and names such as cotton bollworm, corn earworm, tomato fruit-worm, and many wild host species. (Photo needed). 

Flea beetles (Chrysomelidae) are so-called because they jump. They eat the surface parts of tomato plants and can be serious pests. (Photo needed). 

Colorado potato beetle (Leptinotarsa decimlineata): A major pests of potatoes, these beetle can also damage tomatoes. (Photo needed). 



Root-knot nematodes (Meloidogyne spp.) form small galls on the roots of many plant hosts leading to the loss of yield and even the death of young plants. This is a good example of inoculating seedlings prior to transplanting in oder to obtain and even distribution of disease in the screening population. (Photo needed). 

Golden nematode (Heterodera rostokiensis). This is a good example of a damaging plant pathogen that has a limited distribution, particularly in North America. It would be utterly foolhardy, and illegal, to import it into an area free of it, for purposes of resistance breeding. (Photo needed). 


Red spider mites:

Mites differ from the six-legged insects by having eight legs. There are many species of red spider mites, the most common being Tetranychus urticae. The mites are quite small and they damage the surface tissues of an extremely wide range of host species. This leads to a loss of water from the plant, and these mites are damaging in dry climates and in greenhouses. (Photo needed). 



Blossom-end rot. This is an example of the limits of resistance breeding. Blossom-end rot is due to a deficiency of calcium, and the only way to control it is to add calcium to the soil. (Photo needed).  



The breeding method is designed to produce new varieties that have high yields of high quality fruit without any use of pesticides. 

These varieties must have: sufficient durable resistance to all locally important pests and diseases to obviate any need for pesticides; high yield; high quality fruit; and agronomic suitability.


Simultaneous selection for all variables

An important aspect of recurrent mass selection is that all variables must be improved simultaneously.


Durable resistance

There are two kinds of resistance to the parasites of crops. For present purposes, vertical resistance has two characteristics. Its inheritance is controlled by single genes which obey Mendel's laws of inheritance; and it is liable to break down to new strains of the pest or disease. It is thus either effective or ineffective, with no intermediates. Its development requires pedigree breeding, and it is definitely not recommended for amateur breeders.

The inheritance of horizontal resistance is controlled by many genes called polygenes. It is thus quantitative in its effects, with every degree of difference between a minimum and a maximum. It is also durable resistance, and its development requires population breeding. It is definitely recommended for amateur breeders, and this handbook is concerned only with horizontal resistance.


Locally important parasites.

Agro-ecosystems vary and the epidemiological competence of various parasites varies with them. You will be breeding for horizontal resistance to the parasites of your local agro-ecosystem. If your new varieties are taken to another agro-ecosystem, they will have too much resistance to some parasites, and too little to others. And the same is true of other people's varieties brought into your agro-ecosystem. Hence the importance of on-site selection. However, most agro-ecosystems are fairly large and this will allow an exchange of breeding material with neighbouring clubs.


On-site selection 

On-site selection means that all screening is conducted (i) in the agro-ecosystem of future cultivation, (ii) in the time of year of future cultivation, and (iii) according to the farming system of future cultivation. For example, if you do all your screening in a greenhouse, you will end up with cultivars suitable for greenhouse cultivation, but probably quite unsuitable for open field cultivation. 

The need for on-site selection is not absolute. In particular, greenhouse screening is permissible during winter when you are trying to get two breeding cycles each year. This may slow down the genetic advance but this effect will be small when compared with halving the duration of the entire breeding program.


High yield

There are two components of yield: fruit size and total number of fruit. You could weigh all the fruit from each of the best plants, but it is probably good enough to work by eye.


High fruit quality

        Flavour: remember that the taste test need not destroy the seeds.


        Shape and size:


Plum tomato. Often called paste tomatoes, these have a high conent of solids and are used in the manufacture of ketchup and tomato paste. They usually have an oblong shape.

Pear tomato. Similar to the plum tomato but pear-shaped, with a richer taste.

Cherry tomato. Small, round and usually sweet, these are often eaten whole in salads.

Grape tomato. Similar to cherry tomato but smaller, but oblong.

Campari tomato. Sweet and juicy with low acidity and lack of mealiness. Larger than cherry tomatoes but smaller than plum tomatoes.

Beefsteak tomato. Large tomatoes generally used in slices in sandwiches.

Slicing or globe tomato. The most commonly used tomatoes for fresh eating, canning, etc.

Ox-heart tomato. Shaped like large strawberries with variables sizes.

San Marzano tomato. A gourmet tomato.


Agronomic suitability

Decide whether you want climbing or recumbent types.


The Breeding Method

The breeding method is recurrent mass selection, often called population breeding. Begin with some 10-20 different parents varieties and cross-pollinate them in all combinations. Use as large a population as possible and keep only a small minority  of the best looking plants as parents of the next screening population.


Choice of Original Parents

Avoid the modern, so-called resistant varieties; their resistance will almost certainly be vertical resistance. Even if already matched, floating vertical resistance  genes are a dreadful nuisance in a recurrent mass selection program. Heirloom varieties were bred before 1900 and do not possess vertical resistance genes and are strongly recommended for amateur breeders.


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.

The breeding cycle

A breeding cycle is the process from one cross-pollination to the next. In the course of a breeding cycle, the seeds from the selected plants of the previous cycle are sown and these plants are screened to produce the parents for the next breeding cycle. There may be other procedures within a breeding cycle such as a multiplication generation to increase seed, or several successive self-pollinating generations to produce pure lines.


Transgressive segregation

This term means that a progeny can have a higher level of a quantitative variable, such as horizontal resistance, than either of the parents. This is crucially important, and it is best explained by a theoretical model.

  Suppose that the horizontal resistance is genetically controlled by one hundred polygenes of small but equal effect. Each gene thus contributes 1% of the resistance. Now suppose a susceptible tomato population in which every plant has only 10% of these polygenes, but that each plant has a different 10%. This would mean that all the genes are present in the population but they are spread too thin for resistance to be exhibited. With random cross-pollination, the most susceptible progeny would have 10% of the genes, if both parents had exactly the same genes; and the most resistant progeny would have 20%, if both parents had totally different genes. This increase in resistance is transgressive segregation. In the next breeding cycle, the parents are the most resistant individuals from the previous breeding cycle, and each individual has 20% of the genes

for resistance. With random cross-pollination, the most susceptible progeny would have 20% of the genes, if both parents had exactly the same genes; and the most resistant progeny would have 40%, if both parents had totally different genes. And so on towards 100%.

This model is theoretical and it is a deliberate over-simplification. In reality, both the genetics and the transgressive segregation are more complicated, and the genetic advance in resistance is slower. In practice, a minimum of 5-10 breeding cycles will probably be necessary, with a maximum of 10-15 breeding cycles. However, useful results can often be obtained quite early in the breeding program, and the whole procedure is one of continuing improvement.


Relative measurements

In each breeding cycle, the plants are screened for relative freedom from pests and diseases, relatively high quality fruit, relatively high yield of fruit, and agronomic suitability (i.e., climbing or recumbent). 

This word 'relative' is used very deliberately, and it is very important. It means the best individual plants within each screening population. And, in the early breeding cycles, the best plants may well look terrible. (There are various reasons for this; see 'Sources of error' below). The danger is that amateur breeders are liable to lose heart and just give up. 

Please, please persevere. 

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


Negative selection prior to flowering.

The purpose of negative screening is to remove the least desirable plants before flowering begins; this will prevent undesirable pollen from entering the gene pool.


Family selection

Family selection means that the progeny of one individual are sown as a single 'family' which is kept separate from other 'families'. During screening, the best families are selected first, and then the best individuals within those chosen families are selected as parents for the next breeding cycle.

Continuing selection during fruit production

Organic farmers and home growers may find some variation within their crops. It is then well worth while to select the best looking plants for seed production.


Hybrid varieties 

There are several advantages in producing hybrid varieties. These include uniformity of size, appearance, and maturity, and increased seedling vigour. Hybrid varieties are produced by crossing two highly inbred pure lines, and the progeny then exhibit hybrid vigour or heterosis. However, considerable experimentation may be required to find good pure lines, and these must be maintained in order to produce new hybrid seed each season.


Elimination of Vertical Resistance genes

When breeding for horizontal resistance, the single genes of vertical resistance can be a positive nuisance. The solution is to use heirloom varieties; they have no vertical resistances, and both their high quality and their relatively good levels of horizontal resistance are useful starting points. If you are using modern varieties, look for hypersensitive flecks which are a sign of an unmatched vertical resistance, and eliminate all plants that have them. With blight (Phytophthora infestans), the presence of the  A2 mating type means that vertical resistance genes will be matched and eliminated quickly.

Single gene resistances occur with the fungal wilts and some nematodes, and their mechanisms are not the easily recognisable hypersensitivity flecks. These vertical resistances can be avoided in two ways. Use parents known to lack vertical resistance genes, and eliminate all plants that appear to be immune. There are no vertical resistance  genes against bacterial wilt and the various insect parasites of tomatoes.



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.


Seed Sowing

Tomato seeds are normally sown in 'flats' and the seedlings are then transplanted in the field. The soil in the flats can be inoculated with various soil-borne parasites to ensure that there are no chance escapes, and that there is an even distribution of parasites in the screening field. These soil-borne parasites include nematodes, the fungal wilts and, in the tropics, bacterial wilt.

The easiest method of inoculating the 'flats' is to mix in soil from a known area of severe parasite incidence


The Breeding Cycle

Each breeding cycle begins with the hand crossing of selected parents. The progeny of these parents are then screened for relatively high levels of the desired variables. The best individuals then become the parents of the next breeding cycle. A breeding cycle may include intermediate operations such as seed multiplication, and selfing for late selection and/or the production of pure lines.



Sources of Error

There are several sources of gross error that have deceived professional breeders for more than a century. These errors have all diminished the apparent value of horizontal resistance to the point where many breeders denied its very existence. Do not let these errors deceive you. It really is very important to understand them.


Parasite interference

If a moderately resistant plant in a screening population is surrounded by susceptible plants, it will be much more heavily parasitised than if it had been surrounded by resistant plants. This effect, which is known as parasite interference, can cause huge assessment errors. This is why it is imperative to use relative assessments of resistance, and to keep only the least parasitised individuals, however susceptible they may seem.

There is another aspect of parasite interference. With vertical resistance, the greater the area of uniform population, the more dangerous vertical resistance becomes. This is because of an increased selection pressure for the matching parasite, and an increased loss when the matching does occur.  The greater the area of uniformity, the greater the danger.

With horizontal resistance, the greater the area of uniform population, the less the parasite interference, and the more effective the horizontal resistance. This is true of interference on any scale e.g., inter-plant, inter-crop, inter-farm, inter-region. Parasite interference is greatest with a single horizontally resistant host individual surrounded by susceptible individuals in a screening population. It is least when the entire crop of a region has a high level of  horizontal resistance in all of its cultivars. The greater the area of uniformity, the greater the security.

The practical importance of parasite interference is that the  most resistant plants in your screening population are much more resistant than they seem.


Biological anarchy

“Little fleas have lesser fleas, upon their backs to bite 'em.”  In other words, all parasites are themselves parasitised with so-called hyper-parasites. This effect is known as biological control, and it can have a major influence on the losses caused by crop parasites.

However, the widespread use of crop pesticides has greatly reduced many hyper-parasites, and the biological control has been diminished accordingly in what is now known as biological anarchy. This is important because the full effect of horizontal resistance is not manifested until the biological controls are restored. Interestingly, the best way to restore the biological controls is to use horizontal resistance, and the best way to maximise the value of horizontal resistance is to restore the biological controls. The two effects are mutually re-inforcing.

The practical importance of all this is that your screening populations probably have much more resistance than you may think.


Chance escape

Many crop parasites have a 'patchy distribution' which means that their population density can vary from intense to zero within one field. Plants in the centre of a concentration of the parasite appear to be susceptible, even thought they may be quite resistant. And plants that are entirely free of parasites appear to be resistant, even though they may be very susceptible. This accidental freedom from parasites is called ‘chance escape’.

Accurate assessments of horizontal resistance can be made only if there is a uniform distribution of the parasite. Unfortunately, a uniform distribution does not often occur. At the very least, there are usually parasite gradients, with a gradual change of parasite population density from one part of a field to another.

There are two approaches to solving this problem of patchy distribution. The first is inoculation of the screening population with populations of the various species of parasite. There are many different techniques, which include spraying the screening population with suspensions of spores, inoculating the soil of seedlings to be transplanted, and the planting of susceptible surrounds or separator rows. Many parasites can cultured and released into the screening population. Professionals should be consulted.

The second approach is to use a grid screening. The entire screening population is divided into a grid of appropriately sized squares, and the least parasitised individuals in each square are selected, regardless of the level of parasitism in that square. Any individual plant that is completely parasite-free, and any square that lacks parasites entirely, is rejected. With a soil-born parasite that occurs in large patches, the screening should be conducted only within those patches.


The vertifolia effect

This effect explains why modern cultivars tend to be more susceptible than heirloom varieties. It was named after a potato variety called 'Vertifolia' which had vertical resistance to blight. When that resistance broke down, the susceptibility of 'Vertifolia' was extreme. The vertifolia effect is thus a loss of horizontal resistance that occurs during breeding for vertical resistance. A similar loss can occur when the breeding is conducted under the protection of pesticides.

Today, the vertifolia effect is at its most prominent in cotton, potatoes, and tomatoes, although it has probably occurred in most crop species, at least to some extent.

The reason for the loss of horizontal resistance is simple. The level of horizontal resistance can only be determined by the level of parasitism. If there is no parasitism, because of a functioning vertical resistance, or because of the use of pesticides, the level of horizontal resistance cannot be determined. Individual plants with the highest levels of horizontal resistance are relatively rare in a screening population. Consequently, the more susceptible individuals are likely to be selected because of other attributes, and because they are common

So, you should never use pesticides on your tomato screening populations, and all your original parents should date from before 1900, and before the craze for single-gene vertical resistances developed. Similarly, you should expose the screening population to wind and rain, and let the pests and diseases do their worst. In the early breeding cycles, the screening populations will look terrible but this is exactly what you want. The least susceptible individuals will produce one or two fruit with enough seed to keep you going. If it seems likely that the entire screening population is about to be totally lost, then (and only then) it is permissible to use pesticides to save the least susceptible individuals.


Population immunity

A plant population may be effectively immune to a crop parasite, even though the individuals in that population are less than immune. This effect means that, when breeding plants for horizontal resistance, we probably need considerably less resistance than we may think.

Population immunity is a consequence of population growth. Unlike an individual’s growth, a population’s growth can be positive or negative. If there are more births than deaths, the population size is increasing, and its growth is described as positive. If the births and deaths cancel each other out exactly, the population size is unchanging, and its population growth is zero. And if there are more deaths than births, the population size is decreasing, and its population growth is negative.

If a parasite population growth is positive, this means that, on average, each parasite individual spawns more than one new individual. Each parasite individual may spawn very many new individuals, in a very short time, and the positive population growth is then so rapid that it becomes a population explosion.

Now suppose that the crop in question has a level of horizontal resistance such that it severely restricts the reproductive rate of the parasite. On average, each parasite individual spawns only one new individual before it dies. The parasite population growth is then zero.

Finally, suppose a slightly higher level of horizontal resistance. On average, each parasite individual now spawns less than one new individual (i.e., most individuals spawn one new individual, while a few spawn none). The parasite population is now decreasing. Its population growth is negative.

An epidemic (or infestation) can develop only when the parasite population growth is positive. And a damaging epidemic can develop only when the population growth is strongly positive. If the parasite population growth is zero or negative, there is no epidemic, and the host population is effectively immune, even though the individuals in it are less than immune. This is population immunity.

One of the dangers of measuring horizontal resistance in the laboratory is that population immunity cannot easily be taken into account. A level of horizontal resistance that looks like susceptibility in the laboratory may prove to be population immunity in farmers’ fields. For this reason, laboratory measurements of horizontal resistance should be relative measurements. That is, the level of resistance should be described as being either higher or lower than that of other cultivars of known field performance.


Epidemiological competence

The epidemiological competence of a parasite is likely to vary from one agro-ecosystem to another. This means that a cultivar that is in perfect balance with your local agro-ecosystem will have too much  horizontal resistance to some parasites, and too little to others, when cultivated in a different pathosystem.

Equally, it means that someone else's cultivars will not be in perfect balance with you local agro-ecosystem. As a general rule, the greater the distance between the two agro-ecosystems, the greater the imbalance. And this is more true of distance in a north-south direction than in an east-west direction.

By all means use other peoples' lines in your breeding program, but use only your own selections as potential new cultivars, unless they originated in an agro-ecosystem  closely similar to yours.


A good source of resistance 

Breeders working with vertical resistance must first find a good source of resistance. That is, they must obtain plants with vertical resistance genes, and then use gene-transfer techniques (i.e., pedigree breeding and back-crossing) to transfer those genes into cultivars. This idea that you need a good source of resistance has become a shibboleth. It is true only if you are working with vertical resistance.

When working with horizontal resistance, you do not need a good source of resistance. You will be accumulating horizontal resistance by transgressive segregation, and it is possible to do this with highly susceptible parents, although it does no harm to use heirloom varieties that already have fairly good horizontal resistance. 


Adult plant resistance 

Vertical resistances protect until a matching infection occurs. After that, the only defence a plant has is horizontal resistance. Because the frequency of matching infection increases as the epidemic develops, horizontal resistance tends to be at a low level in seedlings, and at a high level in mature plants. This is natural; the horizontal resistance increases as the epidemic increases. Many breeders recognised this phenomenon and they called this resistance in mature plants 'adult plant resistance'. Unfortunately, they made little use of it because their vertical resistance breeding techniques relied so heavily on the testing of young seedlings.

When working with horizontal resistance, you should rely on assessments made on mature plants.


Breeding clubs

Many amateur breeders will want to go it alone, doing their own thing, in their own way. There is absolutely nothing to be said against being individualistic. However, there are many advantages in belonging to a breeding club.

The subject of breeding clubs has been dealt with at length in Return to Resistance, available as a free download from this website. The special advantages of university breeding clubs are discussed in a separate handbook.


Cultivar registration

In some countries, new cultivars can be registered as intellectual property. They can then earn royalties for their owners, much as books earn royalties for their authors. The cultivars are identified by DNA finger-printing and this can be expensive. If you plan to put your cultivars in the public domaine, it is still a good idea to register them so that other people do not do so.


Phytosanitary regulations

Phytosanitary regulations are designed to control the movement of crop parasites into areas that are still free from them.

It is very important to stay within the law. Never import crop parasites from another country. Never import host material from another country, except when you have official permission to do so. Never export host material, or parasites, to another country. In other words, obey your country's phytosanitary regulations, and respect other countries' phytosanitary regulations.

Remember also that your aim is local optimisation; you are using on-site selection and transgressive segregation to produce new cultivars that are suited to your own agro-ecosystem; you do not need a good source of resistance; and other people's cultivars will have a reduced value when cultivated in your agro-ecosystem. You are working with horizontal resistance, and you already have all the genetic variability that you may need in your own country.