When I was an undergraduate, which was more than 50 years ago, I was taught at universities that breeding plants for resistance to disease was a complete waste of time because all resistance was bound to break down sooner or later, usually sooner. We were told about Dr. Paul Muller in Switzerland who had just been awarded the Nobel Prize for discovering that a chemical called DDT was an insecticide. We were advised very strongly to ignore breeding for resistance and to go with chemicals. This mindset exists very strongly to this day. When I went to Kenya I fortunately had an experience which stood it on its head.

I want to talk about resistance which is not going to break down. You can divide any protection against parasites into one of two classes, stable and unstable. You can have a new antibiotic and within no time a new strain of bacterium which is unaffected by it. So this protection is unstable, it breaks down with new races of the pathogen. DDT, a relatively recent synthetic insecticide, was discovered at the beginning of World War 2. When the allies were in Naples in high summer their medical officers were afraid of major epidemics of typhoid and possibly of malaria. Because they had lots of DDT they sprayed all of Naples and all the people in Naples with DDT and there were no epidemics at all. A few months later Naples was full of house flies which were DDT-resistant. So DDT is an unstable protection mechanism.

Pyrethrins come from a flower that grows wild in what used to be Yugoslavia. People in Dalmatia have been picking these flowers and putting them in their bedding to control fleas and bedbugs for centuries without a suggestion of pyrethrin-resistant fleas or bedbugs. We can conclude after two or three centuries of use that natural pyrethrins are a stable insecticide.

Bordeaux mixture is a fungicide discovered in France in 1882. It has been in use ever since and has never once broken down to a new strain of the fungus. It has been used to control downy mildew of grapes and potato blight and it is a stable fungicide.

Widimil is a systemic fungicide against blight. After only a few years' use there were resistant strains of the fungus. It is an unstable fungicide.

We have two kinds of resistance: vertical resistance and horizontal resistance. Vertical resistance breaks down to new races of the pathogen or insect and is an unstable resistance. This is the resistance that my old professors were talking about. They did not know at that time that there was a second kind of resistance called horizontal resistance and horizontal resistance is stable­it does not break down to new races of the pathogen.

Let's compare the two kinds of resistance. Vertical resistance is controlled by single genes so it is qualitative in its inheritance and in its effects. That means it protects completely or not at all with no intermediates. It is either present and active or not functioning at all. Horizontal resistance, on the other hand, is quantitative which means it can be exhibited with every degree of difference between a minimum and a maximum. In the absence of crop protection chemicals, the minimum level of horizontal resistance will lead to a complete loss of crop and the maximum level of horizontal resistance will lead to a negligible loss of crop.

Vertical resistance is generally described as big space because when it is functioning it is very little affected by climate and can cover a huge ecological area. The classic example was the first wheats of the Green Revolution. They had one vertical resistance all the way from Morocco to China and in theory one new race could have gone right through quickly. Fortunately that didn't happen. Horizontal resistance, on the other hand, is small space. In every agroecosystem it is different. What is different in this case is the epedemeological competence of various crop parasites which varies differentially from agroecosystem to agroecosystem. So that a potato cultivar which is in perfect balance with one agroecosystem will not be in balance with another agroecosystem. If you take it to this latter ecosystem it will have too much resistance to some parasites and too little to others. You have to breed for horizontal resistance to all the important species of parasites. This means a new cultivar bred for our own local agroecosystem is good for this sytem and would be inferior in any other agroecosystem. Most agroecosystems are fairly large, large enough to justify their own breeding programs, but horizontal resistance compared with vertical resistance is small space.

Vertical resistance, with the combination of qualitative effects and big space, is a high profile resistance. It has led to terms such as "Green Revolution", "Miracle wheats", "Miracle rices". Horizontal resistance, on the other hand, is low profile. It is the sort of resistance that groups of amateur plant breeders would produce for the sort of crops we've been talking about today. They might become very well known within their own local agroecosystem but they will never be high profile in the same way as the miracle wheats and the miracle rices.

Vertical resistance is very expensive. It requires a team of highly trained scientists, very expensive laboratories and greenhouses. I think in Mexico City at the International Center for Wheat and Maize they have an annual budget of $35 million a year. Most of this money goes for maintenance research which is just a euphemism for saying "we've got to keep breeding new wheat varieties with vertical resistance to replace ones where the vertical resistance has broken down that are no longer good. Horizontal resistance, on the other hand, is very cheap. If a group of us got together and decided we wanted to breed many of the crops we've been talking about today we could do it and probably spend pennies. We could use all sorts of homemade equipment like toothbrushes. With potatoes I've just used kitchen equipment such as coffee filters and a blender to break up the fruits.

Vertical resistance is very technical. That's why you need the teams of scientists who have to do an enormous amount of very expensive very technical background research which is not recommended for amateur breeders. Because professional plant breeders have been working with vertical resistance for about a century they have quite rightly advised amateurs to have nothing to do with plant breeding because they're talking about vertical resistance. None of them have been talking about horizontal resistance which is very easy to use. It is the sort of techniques you've been hearing about today. Any individual or group of amateurs can get together, possibly with some technical help from friendly people in the nearby university, but by and large can do it on their own.

Vertical resistance is unstable. It breaks down to new races of the pathogen. This is a colossal disadvantage. In Kenya they started breeding wheat for resistance to wheat rust in 1926. In 1966 they found that of all the many varieties they had issued to farmers the average commercial life of each was 4.5 years and it takes 8 years to breed a new wheat variety.

Horizontal resistance is stable. That is critically important. It does not break down to new races or new strains of any parasite. This adds an important dimension to plant breeding because it means that if you get a good new cultivar with good horizontal resistance it need never be replaced. If it does get replaced it will only be with a cultivar which is superior to it probably in all respects. This means breeding for horizontal resistance is cumulative, is progressive and you go from poor to better and better until eventually you reach a ceiling of perfection and it becomes virtually impossible to get any better.

Working with vertical resistance is autocratic. If you were farmers in the third world you would be told that 80% of the wheat being grown in the third world consists of varieties coming out of CMMC. CMMC is extremely proud of this. There are very few varieties with very few vertical resistances and if I were a third world scientist I would be extremely worried. The chief plant breeder in CMMC is the equivalent of some sort of dictator who can tell 80% of the people in the third world what wheat varieties they are going to have to grow. This I think is terrible.

When you have thousands of plant breeding clubs all over the world consisting of amateur plant breeders like yourselves, breeding a wide range of durable resistant varieties for their own local ecosystems, then farmers within every ecosystem will have a wide choice of varieties. They can grow whatever they like and they can even produce their own varieties, that is when plant breeding becomes democratic. This is possibly one of the more important aspects of horizontal resistance.

Q; Big space vs small space. You described the size of agroecosystems as being large enough to justify their own breeding programs. How would you describe the Northeast of the US? Would you consider New England or the 9 Northeastern states to be one agroecosystem? Or, from the perspective of breeding for horizontal resistance, is Northern Vermont one and Southern Vermont another?

A; It is difficult for me to say because I don't know this area well enough. I would say the whole of Ontario is probably one agroecosystem, but recognize also that agroecosystem boundaries are definitely fuzzy. In practice you find where the boundaries are by the practical experience of farmers growing different varieties.

Vertical resistance is single gene resistance and it is due to a phenomenon called the gene-to-gene relationship. This is roughly the botanical equivalent of antibodies and antigens in mammals but don't stretch that analogy too far. In nature each gene for resistance in the host is the equivalent of a tumbler in the lock. It would prevent an unauthorized key from turning that lock. In the parasite each gene is the equivalent of the notch in the Yale key. It will enable the key to turn the lock in spite of those tumblers. Vertical resistance is working as a system of locks and keys and this depends entirely on the diversity. One of my favorite rhetorical questions to students is 'what happens when every door in the town has the same lock and every householder has the same key which fits every lock?' The system of locking is ruined by uniformity; it ceases to exist. Yet that is exactly what we have done in agriculture relying on vertical resistance. We've produced a new cultivar, when every plant within that cultivar has the same lock, it is not very long before the parasite develops an entirely new population in which every individual has the same key which matches that lock and then you've got 100% matching and that's the point where they say vertical resistance has broken down. It is no more use we must now get another new cultivar with a different vertical resistance. This is what we've been doing for the last one hundred years.

What worries me is that when you are breeding for vertical resistance you are all the time losing horizontal resistance. This is true for a variety of reasons, mainly that you can only breed for horizontal resistance if you can see the level of parasitism. If there is no parasitism because of a functioning vertical resistance or because of the use of fungicides or insecticides then you cannot see the level of horizontal resistance and because the majority of plants tend towards susceptibility (the resistant ones are in the minority) then in the course of many plant breeding cycles you get a drift towards susceptibility and in potatoes this has been happening ever since 1882 when Bordeaux mixture was discovered. In cotton its been happening since the 1940s since DDT was discovered, I think in most crops its been happening certainly since the 1940s.

There are two kinds of infection. One called alloinfection is the equivalent of cross-pollination. The allocide is coming from somewhere else, it either matches or does not match the vertical resistance to that host. Autoinfection is when the parasite is produced on that same host, and corresponds with self-pollination. It is only an analogy, don't push it too far.

Q: I assume that you would call transgenics vertical resistanceAre you prognosticating that resistance conferred through transgenics would break down just as rapidly?
A; Absolutely.
Q: And also the carrier genes that go along with the transgenics?
A; Yes
Q: Can you achieve horizontal resistance in all crop species or are some easier to work with than others?
A: Some crops are extremely difficult to breed, amongst them garlic, which never sets seed, bananas which are male sterile and female sterile and triploid as well, and pineapples. But in general all of the major food crops are very easy to breed for horizontal resistance and you can assume that every plant has horizontal resistance to every one of its parasites and as a general rule the maximum level of horizontal resistance will give you complete control of each of those parasites. There are going to be exceptions. No rule in biology is ever universal. A crop that is very difficult to breed is classic wine grapes, cabernet sauvignon, for example, for resistance to downy mildew. I think table grapes might be a bit easier to breed. But probably the cost of spray is relatively minor so the motivation to breed for horizontal resistance is rather low, it would come only from a desire to get away from pesticides.
Q: Is there any role that you see for vertical resistance? Are there examples of vertical resistance that have not broken down over a long period of time?
A: Yes there are. Wheat stem rust in Canada. They've been using that same vertical resistance for 40 years now. No rust flies overwinter in the Canadian winter. By the time the matching race comes up to Canada from Mexico, the crop is harvested. Another one is wart disease in potato in Europe. Vertical resistance combined with savagely restrictive legislation has been effective. The moment you find any piece of wart disease in the field that field may never again be used for growing potatoes. And in any other field you have to grow potatoes with vertical resistance. So the selection pressure of the pathogen has been negligible and hasn't broken down. Resistance to Fusarium wilt of tomatoes in California lasted for many, many years, though it has now broken down.
Q: How does horizontal resistance apply to soil-borne diseases?
A: A gene-to-gene relationship will occur only in seasonal tissue. It will occur only in an epidemic which is intermittent so that there is a period of no parasitism at all and when that period ends then the parasite has to alloinfect the host again so you will get gene-to-gene relationships in the leaves of deciduous trees and all parts of annual plants but in no part of a perennial plant, and bananas, being perennial, have no vertical resistances.
Q; Are there any theories or techniques that mirror horizontal resistance that can be applied to perennial crops?
A: If you're breeding perennial crops for resistance, horizontal resistance is the only resistance you can use.
Q: It seems like a longer-term selection period, for example, the work being done in chestnuts.
A: Yes.
Q: Is the end in sight in a lifetime? Are there any theories for ways to speed it up?
A: One of the problems of chestnut blight is that it is a new encounter disease. The pathogen evolved in Europe and the host evolved in North America. And those are very often more savage and difficult to work with than old encountered diseases or even re-encountered diseases.
Q: Is there any pattern which shows up in your work in terms of the number of plant generations which is required to reach a satisfactory level of resistance?
A: Probably the absolute maximum generations required would be 15 plant-breeding cycles. Depending on how many pathogens or parasites you're breeding resistance for it should usually be much less. Working with potatoes in Africa I got resistance to everything that was important in 8 generations. It is really not a long process. It is quicker than pedigree breeding.
Q: When you went to the various countries that you worked in, did you find that the local farmers had landraces that already had horizontal resistance?
A: It varies a lot. In Latin America, all the new crops like wheat, sugar cane and coffee tended to be very susceptible. Coffee was moved from S. Yemen to Ceylon to Indonesia to Amsterdam to Paris to Martinique to South America. Each time coffee was moved (it is a self-pollinating plant) it became more of a pure line so that all of the coffee in Brazil is practically one pure line and very vulnerable to parasites.
Q: What populations should you start with and what would be a basic strategy of how to start breeding for horizontal resistance?
A: A lot depends on the crop. If its coconuts, for example, having a large population takes an enormous amount of land. If its wheat, on the other hand, you could get a large population in quite a small area. As a rough rule I would say start breeding with between 10-20 modern cultivars and make your breeding population as large as is practical. With wheat or potatoes you could probably make a population of a quarter million but even if you only had 10,000 it would still be well worth doing.

If you're breeding for vertical resistance you must first find the gene that expresses that resistance and that can't always be found. You can't breed for any resistance without first finding a good source of the resistance. If you're breeding for horizontal resistance all the parent cultivars can be pretty susceptible and it doesn't matter because all you need is a reasonably wide genetic base at the start. By a process called transgressive segregation the concentration of polygenes contributing to that resistance gradually accumulates from generation to generation. You might start at a level of about 10% and move up to about 90% and then you're getting very close to the maximum level of resistance.

Q: Are you seeing more acceptance of your ideas by other breeders?
A: When I first started publishing around 1970 I was regarded with extreme hostility. Now I'm getting quite a lot of acceptance but there are still a lot of the old diehards around who think that I am a dangerous lunatic.