OEPP/EPPO B u l l . 3 ( 3 ) : 89-93 (1973)

Breeding Plant Varieties Resistant t o Pests11

by J. SNEEP 21 and F.L. DIELEMAN 3'

ABSTRACT

In The Netherlands at present the research on resistance against pests is aimed at red spider mite in cucumber, glasshouse whitefly in tomato, carrot fly, aphids in lettuce. In addition projects are planned with a view to breeding resistance against Myzus persicae Sulz. in paprika, and aphids in barley.

Tolerance can hardly be considered as a type of resistance, since it does not control the parasite population. Non-preference has some advantages, but does not reduce the size of the insect population and causes pests to migrate to other plants and possibly to the neighbours. Antibiosis is the more desirable type of resistance. To prevent it from being broken, antibiosis ought to be based on polygenes. Another method of prevention is to breed multiline varieties.

When creating new varieties that possess certain valuable qualities in combination with a resistance based on polygenes, the breeder has to test yearly thousands of plants. Rapid methods for mass-testing for resistance are there- fore urgently needed.

Breeders have found high-yielding varieties, which help to feed the world population. However, the desired resistance against pests must frequently come from low-yielding varieties. To combine the indispensable high yields with resistance against pests is a time-consuming work. The process will be shortened as better screening methods become available.

Although in the last few decades the know!edqe of insect-plant relations has greatly increased (the Dutch entomologists have added their quota to this increase), is was not until 1970 that in The Netherlands an official co-operation materialized between entomo!ogists and plant breeders, aiming at intensifying the breeding of cul- tivated plants resistant to pests. If this breeding research is to b: successiul, co-operation is indispensable.

Within the framework of this combined research co-ordinated by The Nether- lands Organization for App!icd Scientific Research (TNO), the Agricultural University, the Institute of Phytopathological Research (IP3) and the Institute for Horticultural Plant Breeding (IVT) have set up seven projects:

1. Red spider mite (Tetranvchus urticae Koch) in cztcurrzber

In 1970 KOOISTRA (IVT) observed differences between varieties in the degree of infestation by the spider mite. The less infested varieties all appeared to possess the bitter principle. After the varietal assortment had for the greater part just been cleared

1) Paper presented at the Joint EPPO/lOBC/WPRS Conference on Integrated Approaches in Plant Protection, Vienna, 12-15 June, 1973.

2) Institute of Plant Breeding, Agricultural University, Wageningen (The Netherlands). 3 ) Dept. of Entomology, Agricultural University, Wageningen (The Netherlands).

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off this bitter principle, one could fall back, if necessary, on a type with bitter substance in the leaves only and not in the fruit. However this would not be very satisfactory.

These initial trials were the reason why DE PONTI (IVT) studied the develop- ment of mite populations on susceptible and resistant varieties in order to establish a basis for rational breeding methods.

2. GZasshouse whitefly (Trialeurodes vaporariorum Westw.) on tomato

This pest causes a lot of trouble in greenhouses. By estimating the number of adult insects following a controlled artificial infection HOGENBQOM and DE PONTI (IVT) developed a reliable test method for the purpose of selection.

3. Carrot f l y (Psila rosae Fab.) in carrot

As the pressure of natural infestation is sufficiently high and consistent, possible differences among varieties under field conditions are currently being investigated (BRADER, IPO, and NIEUWHOF, IVT).

4. Aphzds in lettuce

a) Nasonovia ribis-nigri Mosley

The tests are carried out in conditioned chambers because the infection pressure in the field varies too much. The test method is based on a combination of the mean relative larval growth rate (RGR) and the estimate of the reproductive capacity (DIELEMAN, Agr. Univ., and EENINK, ITV).

The mean relative growth rate is utilized when investigating varietal differen- ces in seedlings. The differences in resistance of individual plants are based on the strength of the fecundity over a period of 8 days.

These methods are suitable for testing varieties of the self-fertilizing species of lettuce, in which all plants of a variety have the same genotype. The methods are not applicable in testing thousands of individuals in a segregated population of a bree- ding program.

b) Greelz pedcb aphid (Myzus persicae Sdz . )

For this pest, tests must be carried out on older leaves, which is time- consuming. The test is applied only to determine the growth of the population. Popula- tions of M . persicae vary considerably, which interferes with reliable and rapid trials.

In addition the following are in a preparatory stage: M. persicae on paprika, aphids in barley, and possibly onion fly on onion.

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As far as entomologists are concerned, by resistance they also mean tole- rance. Most plant breeders disagree with them on this point. Tolerance is not much used in breeding, as it does not control the parasite population. On the contrary, it creates a chance for the insects to develop to an even greater extent on hosts other than the crop already suffering from their attack. Tolerance in one crop often consti- tutes a danger for other crops. True enough, tolerance has the advantage of avoiding the danger of a break-through by new biotypes. However, in the view of many breeders this does not outweigh the disadvantages.

The breeder likes to class non-preference and antibiosis with true resistance. Non-preference has the advantage that there is only a small chance oi the resistance being broken. Unfortunately, it does not reduce the size of the insect population, while it causes the pests to migrate to other varieties. For the latter this migration may lower the general result of a non-preference resistance already obtained.

Antibiosis is the more desirable type of resistance for breeding, particularly as it starts to decelerate the growth of the lariae from the beginning of its life-history. With antibiosis all the eggs are laid that otherwise would have been laid on non- resistant plants. The number of offspring will then be reduced, and as a result the adjacent crops are not exposed to extra damage. Another advantage of the retarded growth of the larvae is the extended period of development during which they are exposed to detrimental factors.

Admittedly, with antibiosis there is danger of resistance being broken, but this risk can be minimized by creating a resistance based on polygenes. It is therefore worthwhile to obtain such resistance even though the work involved is laborious.

A third advantage may be mentioned. Antibiosis, which damages the young larvae, creates at the same time an opportunity for a multiline variety to achieve fa- vourable results and nowadays this point receives a great deal of attention in the breeding world.

A multiline variety is a mixture of lines of a cultivated crop. It is homogeneous from an agricultural point of view (same plant length, same time of maturing, etc.), but each line carries another gene with resistance against one biotype of the pest. The infestation is hereby diluted, and this effect is repeated with the swarming of the next generation. As a consequence the attack is weakened, even though the parasite population consists of more than one biotype.

The procedure might be defined as follows : instead of introducing the polygenes for resistance into one plant, which is always more difficult, they are now built-in in the compound of the multiline. The multiline could be a modern version of the primitive cultivation of mixtures instead of one genotype, without involving a reduc- tion in yield.

To prevent a break-through soon after it has been obtained, insect resistance of plants must be based on the largest possible number of genes. The same is true for resistance against fungi, viruses, bacteria and nematodes. Polygenes, too, underly the performance of a plant with regard to yield, quality and early maturing.

Proceeding from this broad and by no means exhaustive recapitulation, what are the consequences as far as the breeding of a self-fertilizer is concerned ? A simple

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mathematical calculation will give us some idea assuming a variety is to be bred which has resistance to one insect and one fungus combined with reasonable yield, quality and early maturing. When each of these characteristics is governed by two loci and no other characteristics such as resistance to frost, drought, lodging, viruses, bacteria, etc., p:ay a role, then an ideal and simple situation exists. Supposing two genotypes can be found that between them possess all the desired characteristics, their hybrid will provide the breeder with good initial material. Since in the Fa he can expect the first ideal plant, how big must this Fa be in order to come across at least one such plant, assuming the genes are inherited independently ? The answer is compara- tively simple :

1,048,576 plants.

The smallest complete Fz population will therefore consist of more than one million individuals. In these calculations the factor chance was not yet included. It is quite obvious that even in the simplest case of selection it is almost impossible to cope with such numbers, especially if the tests have to be carried out on fully grown p:ants.

Consequentiy the entomo!ogists will have to find test methods that provide the opportunity for mass screening at the seedling stage if possible. Mycologists and viro!ogists have been successful in their efforts in a number of cases. A plant breeder can proceed along other ways than those just mentioned. However, these methods do not fall within the scope of the present meeting. A complication in breeding for resistance to insects is the unreliability of spontaneous infection in the field for most European countries. This is another reason why the breeder asks for reliable and easy screening methods in the seedling stage.

Sometimes we hear the reproach that breeders have paid too much attention to high yieId. Notwithstanding the often astronomical quantities of plants with which t h g have had to work, breeders have managed to increase production considerably in order to make mechanical harvesting possib!e. In doing so, they have frequently intro- duced early maturing and resistance to one or more fungi and viruses. Since not all types of resistance could be combined, however, they have too long relied on inexpen- sive and effective insecticides to control insects.

The yield of our varieties cannot be compared with those of the pri- mitive and wild relatives of our cultivated crops which have a certain resistance to pests. With proper cultivation the primitive wheat, for instance, could produce only 4 '/. of what iS regarded as normal. Owing to present day high yields the world population is able to survive at its present size, though in certain areas, on the verge of famine.

If indistinct, short-term measures should demand varieties that are resistant to insects and other parasites, a decrease in yield is sure to be the result. The breeder is unable to combine everything and for what he can combine he needs time. He should therefore be given the necessary time. His willingness stands beyond questioning. He must first of all be provided with rapid methods for mass screening in order to obtain the much desired resistance and with all the other characteristics necessary for a world popu:ation, now out of control, to live in a sound environment as well as being protected from starvation.

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OEPP/EPPO Bul l . 3 ( 3 ) : 89-93 (1973)

RESUME

Selecti’on de varietes resistantes aux ravageurs

par J. SNEEP 1 ) et F.L. DIELEMAN 2 )

Bien que la connaissance des relations existant entre l’insecte et la plante ait considkrablement progress6 au cours des dernikres dkcennies, ce n’est qu’en 1970 que la coop6ation entre entomologistes et shlectionneurs a pu Ctre concrktishe officielle- ment aux Pays-Bas. Le programme de recherche ktabli en commun doit permettre de s6Iectionner des variktks rksistantes aux ravageurs suivants : 1. Acarien jaune sur concombre : Tetranychus urticae Koch. 2. Mouche blanche des serres : Trialewodes vaporariorum Westw. i . Mouche de la carotte: PJila rosae Fab. 4. Pucerons des laitues,

a) Nasonovid ribis-nigri Mosley, b) Myzur persicae Sulz.

Les entomologistes souhaitent gknkralement voir inclure dans la sklection l’ob- tention dune tolkrance a certains ravageurs. Cet avis n’est pas partage par les selection- neurs qui redoutent I’effet incomplet sur les populations et le risque que celies-ci ne passent alors sur dautres cultures. I1 est vrai que la tolerance a l’avantage d’kloigner le danger dapparition de souches virulentes, mais malgrh cela la plupart des specialis- tes pensent que les inconvknients l’emportent sur les aspects positifs. En revanche, les dlectionneurs estiment que la << non pr6fkrence )> et l’antibiose font partie intkgrante de la resistance. Malheureusement, si la (< non prkfkrence > ne rkduit pas la densitk des popu!ations qui se porteront sur d’autres varietks, elle kcarte nkanmoins les risques d’un effondrement de la rksistance.

Lantibiose constitue le type de rksistance que les sklectionneurs prhfkrent puis- qu’elle agit dhji sur les larves. De plus, les e u f s sont normalement dkposks sur la plante plutbt que sur une autre variktk, et enfin cette technique de sklection est interessante puisqu’elle permet d’obtenir des variktks a lignkes multiples, chaque lignke &ant por- teuse d u n gene de rhsistance contre un biotype du ravageur.

Ces pratiques entrainent la nkcessitk pour l’entomologiste de tester a l’ktat de semis de grandes populations de FZ, mais ici surgit m e s6rieuse difficult6 car les me- thodes rapides font gknkralement dkfaut. Les mycologues et les virdogues, en revan- che, ont pu mettre au point ce genre de technique dans bien des cas, ce q u i facilite grandement le travail.

REFERENCES

KOOISTRA, E. (1971). Red spider mite tolerance in cucumber. Euphiticu 20 : 47-50.

1) Institute of Plant Breeding, Agricultural University, Wageningen (Pays-Bas) 2) Dept. of Entomology, Agricultural University, Wageningen (Pays-Bas).

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