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IV. Developing Varieties with Multiple Resistance

IV. Developing Varieties with Multiple Resistance

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DISEASE AND INSECT RESISTANCE IN RICE



0



m

1969 70



1969 7 0 71 72 7 3 74



323



71 72 73 74



FIG. 4. Change in the proportion of F, populations and entries in the replicated yield

trials at IRRI with resistance to important insects and diseases (Khush, 1977). Reproduced

by permission of the New York Academy of Sciences.



60% in the wet season of 1974. Similarly, only 17% of the entries in replicated

yield trials of 1969 wet season were resistant to bacterial blight. The proportion

increased to 98% in the 1974 wet season (Fig. 4). Progress with tungro, grassy

stunt, brown plant hopper, and green leafhopper has been equally dramatic

(Fig. 4).

The main thrust of the program, of course, has been to combine resistance to

all those diseases and insects with improved plant type. Equally rapid progress

has been made in this direction. About 87% of the entries in the replicated yield

trials of the 1969 wet season were either susceptible to all six diseases and

insects (blast, bacterial blight, tungro, grassy stunt, brown plant hopper, and

green leafhopper) or resistant to only one of them (Fig. 5). Only 2% of the

entries were resistant to three diseases and insects. The proportion of entries

with multiple resistance gradually increased, and in the 1974 trials, 90% were

resistant either to five diseases and insects or to all six. Six of the multiple



324



GURDEV S. KHUSH

Resistonce to a wrnber of d i m s and insects (%)



FIG. 5. Change in proportion of entries in annual replicated yield trials with multiple

resistance to important diseases and insects (blast, bacterial blight, tungro, grassy stunt,

brown plant hopper, and green leafhopper) at IRRI. Each year’s trial consisted of about

185 entries (Khush, 1977). Reproduced by permission of the New York Academy of

Sciences.



resistant lines were named varieties and recommended for cultivation in several

countries. Table XVII shows the disease and insect reactions of all IRRI named

varieties and indicates the progressive increase in the levels of resistance of the

newer rice varieties. The breakthrough in developing the improved plant-type

germ plasm with multiple resistance to major diseases and insects was achieved

through a well-planned breeding program and through a liberal exchange of ideas

and materials between IRRI scientists and rice scientists in other rice-growing

countries. Salient features of the breeding methodology and procedures employed and international cooperation are discussed in the following section.



A. BREEDING METHODS AND PROCEDURES



I . Choice of Parents



A major strength of the breeding program at IRRI has been the well-stocked

germ plasm bank. From 256 accessions in 1961, the germ plasm collection



DISEASE AND INSECT RESISTANCE IN RICE



325



increased to 6900 entries a year later, and to 23,560 entries in 1972 (Chang et

al., 1975). It now has about 40,000 accessions from 73 countries of the world.

Another major strength has been the presence of highly competent pathologists

and entomologists on the Institute staff. As soon as a serious disease or insect

problem was identified, scientists started developing screening techniques and

evaluating germ plasm for resistance. The identified sources of resistance were

immediately introduced into the crossing program. The donor parents generally

had poor plant type characterized by tall stature, droppy leaves, weak stems, and

consequently, low-yield potential. The first step therefore was to transfer those

sources of resistance to an improved plant type (short stature, erect leaves,

sturdy stems, high tillering). This “conversion” was achieved by crossing the

donor parents with an improved plant-type parent, growing large Fz populations

(2000-5000 plants), and selecting improved plant-type segregates. The selected

plants were examined for grain quality, and F3 progenies were grown from those

with good grain quality. The F3 progenies were evaluated for the resistance trait

under study, and several resistant selections (generally up to 100) with good

grain quality, improved plant type, and appropriate growth duration were saved

for further evaluation in the F4 and F5 generations. Through repeated evaluations, a few (2-10) true-breeding, improved plant-type selections with resistance

to given traits were selected. Between 1965 and 1969, IR8, several selections of

IR262 (IR262-43-8 in particular), and IR24 were used as improved plant-type

parents. Several donor parents were used for each disease and insect.

2. Crossing Program



During the period when emphasis was on germ plasm conversion, single crosses

were made. Poor progenies were obtained in a multiple cross if more than one

parent had poor plant type. Since very few improved plant-type parents were

available in the initial stages, only single crosses were feasible. About 200 to 400

such crosses were made each year. By the end of 1970 a large number of

improved plant-type breeding lines with resistance to one or two diseases and

insects became available. To combine resistance to all the major diseases and

insects together, the crossing program was expanded and a large number of

multiple crosses were made employing those breeding lines. A large number of

single crosses between those lines were made each season. The following season

either the two F l s were crossed with each other or an Ft was crossed with a

third breeding line. A fairly large number of F, seeds (300-400) were obtained

from multiple crosses. This allowed the gametic variability of single-cross F, s to

be sampled.

In producing single-cross F, s, each breeding line was crossed with a number of

other breeding lines. Thus, a set of single-cross F1 progenies were available for

making double crosses or topcrosses in the next season, and appropriate combinations could be selected t o combine the resistance to given diseases and insects.



326



GURDEV S. KHUSH



That also allowed the rapid determination of the combining ability of the

breeding lines. A breeding line that yielded poor progenies in a number of cross

combinations was assumed to be a poor combiner and was removed from the

crossing program.

3. Handling Segregating Populations

The pedigree method of breeding was employed almost exclusively in handling

the segregating materials. Selection work was based on comprehensive records on

the disease and insect reactions of each line and, in the case of F4 and advanced

generation lines, on the reaction of the ancestral lines as well. The bulk method

of breeding was not used because it does not permit concurrent screening for a

number of diseases and insects. The backcross method was not used for lack of

suitable recurrent parents. A few backcrosses were made in the crosses with

Oryza nivara for the program on resistance to grassy stunt. IR8, IR20, and IR24

were used as recurrent parents. After 3 to 4 backcrosses, we obtained breeding

lines similar to IR8, IR20, or IR24, but they lacked resistance to other important diseases and insects, such as tungro and brown plant hopper, and were again

entered in the crossing program. Now varieties and breeding lines with multiple

resistance are available, and we are using the backcrossing program to incorporate resistance to white-backed plant hopper.

For traits under polygenic control, the pedigree method is not as suitable. At

IRRI, the diallele selective mating system, originally suggested by Jensen (1970),

is being tried for combining minor genes for resistance to stem borers and whorl

maggot from several sources. This method involves: (1) crossing a number of

moderately resistant parents (generally 5-6) in all possible combinations, (2) intercrossing the F1s so obtained in all possible combinations, ( 3 ) screening the

double-cross F1 progenies for resistance, and (4) intercrossing the selected plants

that have better resistance than either parent. This crossing, screening, and

selection process is continued until the minor genes from different sources are

accumulated and the intensity of the trait is built up. We are in the third cycle of

this type of recurrent selection program for stem borer resistance and the second

cycle of selection for the whorl maggot program. The success of the method is

difficult to prognosticate at this stage.



4. Screening Segregating Populations

The success of the disease- and insect-resistance breeding program depends to a

large measure upon the fidelity, speed, and practicability of the screening

technique. Various greenhouse and field screening methods employed at iRRI

have been tailored to accommodate large volumes of breeding materials. About

50,000 pedigree rows are grown each year at IRRI. Most are screened for



DISEASE AND INSECT RESISTANCE IN RICE



327



reaction to all the major diseases and insects. If possible, the breeding materials

are exposed t o artificially created or naturally occurring disease and insect

epiphytotics. Previously, breeding nurseries were grown with full insecticide

protection; however, since 1970 most of the nurseries are grown without

insecticide treatment to allow a buildup of huge populations of plant hopper,

leafhopper, and stem borers. Rant hoppers and leafhoppers in large numbers also

insure the spread of virus diseases in the nurseries. Sometimes artificially virusinoculated plants are planted around the borders of nurseries t o provide a source

of inoculum. The insect populations are manipulated by applying selective

insecticides. At IRRI farm, Diazinon has no toxic effect on the brown plant

hopper but it kills all predators and other natural enemies of t h s insect. By

judicious application of Diazinon, an outbreak of brown plant hopper has been

induced in the IRRI nurseries. This has also led to an increased incidence of

grassy stunt.

All nurseries are artificially inoculated with bacterial blight in the field and

tested for reaction to blast in the blast nurseries. Data on green leafhopper and

brown plant hopper reactions are also obtained from greenhouse tests. Selected

materials are planted at other locations in the Philippines under disease and

insect pressures different from those at IRRI.

Every effort is made to eliminate the susceptible materials in the early

generations. Screening begins in the F1 generation of multiple crosses. For

example, consider a double cross between four parents; A is resistant to bacterial

blight, B is resistant t o grassy stunt, C is resistant to brown plant hopper, and D

is resistant to green leafhopper. All these traits are controlled by single dominant

genes, whch segregate independently of each other. About 400 seeds from the

double cross A/B//C/D are obtained. The most logical system for screening the

progenies would be to germinate and inoculate all 400 seedlings with grassy

stunt in the greenhouse. Half of the seedlings would be susceptible and will be

eliminated. The remaining 200 would be transplanted in the field and inoculated

with bacterial blight; half of the 200 that would be susceptible would be rogued

out. Seeds from the remaining 100 plants would be harvested individually. Two

small seed samples would be taken out from each and the progeny tested for

resistance to brown plant hopper and green leafhopper. Those carrying the

brown plant hopper resistance gene (50%) and the ones carrying the green

leat3opper resistance gene (50%)would be identified. F2 populations would be

grown only from those carrying both genes (25-30 plants). Thus, by judicious

and timely screening, the original F, sample of 400 would be reduced to 25 to

30 plants, and F2 populations would be grown from these plants. All these F2

populations would be segregating for the four resistance genes. They could be

subjected to appropriate disease and insect pressures. Agronomically desirable

plants with multiple resistance would be selected and rescreened in the F, and

F4 generations to obtain true breeding lines.



328



GURDEV S. KHUSN



A large number of multiple crosses between breeding lines of improved plant

type, known combining ability, and resistance to a number of diseases and

insects are made each season and screened according to the outlined procedure.

New parents with different sources of resistance are constantly included in the

crossing program. This integrated varietal development program has resulted in

superior germ plasm with resistance to all major diseases and insects. Newer lines

with different genes and gene combinations for resistance should continue to

come from the program.



B. INTERNATIONAL COOPERATION



International and interdisciplinary cooperation has been the key ingredient of

the varietal development program at IRRI. Liberal exchange of ideas and

materials between different programs, cooperative testing for disease and insect

resistance in the Philippines at the Bureau of Plant Industry Stations, and in

several other countries such as India, Sri Lanka, Bangladesh, Thailand, and

Indonesia has contributed greatly to the development of germ plasm that is

resistant to diseases and insects.

The pedigree of IR28, IR29,and IR34 (Fig. 6 ) illustrates this international and

interdisciplinary approach. To develop these high-yielding, good grain quality,

and multiple disease- and insect-resistant varieties, eight varieties from six different countries were used in the crossing program. The seeds of these varieties and

40,000 others were supplied by scientists from those countries. This germ plasm

was evaluated by pathologists and entomologists for disease and insect resis-



1



I



IR1561

BB

BPH



1

IRE33

BL T GLH



IRR37

0s



GLH



I

IR2M2

EL BEGS BPH QLH



n

IRL0,IRLoBIRW

BL EBT GS BPH Wli



FIG. 6. Pedigree of IR28, IR29, and IR34. The progress in combining together the

resistance to six major diseases and insects from several parents is indicated. BL = Blast

disease; BB = bacterial blight disease; T = tungro virus disease; GS = grassy stunt virus

disease; BPH = brown plant hopper; and GLH = green leafhopper (Khush, 1977). Repre

duced by permission of the New York Academy of Sciences.



329



DISEASE A N D INSECT RESISTANCE IN RICE



tance, respectively. The breeders combined the identified sources of resistance to

diseases and insects with improved plant type. The segregating populations were

tested at IRRI and the Philippine Bureau of Plant Industry Station at Maligaya,

and in Indonesia.

The seeds of the improved germ plasm as well as of the entries in the germ

plasm bank are shared with scientists all over the world. Up to the end of 1975

more than 95,000 seed samples of breeding lines were supplied to requesting

parties in 80 countries of the world. The breeding lines are used as parents in the

crossing programs, and some have become named varieties. To date 40 breeding

lines from IRRI have been named varieties in other countries. The recently

expanded international testing program will facilitate the exchange and dissemination of germ plasm between the various rice improvement programs.



V. Stability of Resistance



There is growing support for the contention that the resistance governed by

polygenes-also referred to as general resistance or horizontal resistance-is more

lasting than resistance governed by major genes (specific or vertical resistance).

When the program on breeding for disease and insect resistance in rice was

initiated at IRRI, little was known about the genetics of resistance. Available

donor parents were used as sources of resistance and we developed the improved

plant-type breeding lines and varieties with multiple resistance to as many as

four diseases and four insects (Table XVII) within a short period of 7 to 8 years.

TABLE XVII

Disease and Insect Resistance Reactions of IRRI Named Varieties

Disease and insect reaction'



Variety



Blast



Bacterial

bIight



IR5

IR8

IR20

IR22

IR24

IR26

IR28

IR29

IR30

IR32

IR34



MR

S

MR

S

S

MR

R

R

MS

MR

R



S

S

R

R

S

R

R

R

R

R

R



Grassy

stunt

S

S



S

S

S

MS

R

R

R

R

R



Tungro



Green

leafhopper



Brown

plant hopper



Stem

borer



Gall

midge



S

S

MR

S

S

MR

R

R

MR

MR

R



R

R

R

S

R

R

R

R

R

R

R



S



MS

S

MR

S

S

MR

MR

MR

MR

MR

MR



S

S

S

S

S

S

S

S

S

R

S



S

S

S

S

R

R

R

R

R

R



'S = Susceptible; MS = moderately susceptible; MR = moderately resistant; R = resistant.



330



GURDEV S . KHUSH



During this period at IRRI, by investigating the mode of inheritance of resistance to some diseases and insects, it was found that resistance to most diseases

and insects with the exception of stem borer, is controlled by the major gene.



A. VERTICAL RESISTANCE



Information on the stability of vertical or major gene resistance in rice is

meager. As discussed earlier, bacterial blight-resistant IR20 and IR26, which

have Xa4, have been widely grown in the tropics. Their resistance has held up

except in a small area of the Philippines where a strain of bacterium that is

moderately virulent to Xa4 has appeared. This strain has remained localized and

causes only slight damage to rice varieties with Xa4. Several bacterial blight-resistant varieties, such as Benong, Sigadis, Syntha, and Dewi Tara, have grown in

Indonesia for 10 t o 20 years. The resistant TKM6, MTU15, and CO 21 have

grown in India for many years. The occurrence of bacterial strains virulent to

varieties with host resistance has not been reported.

Several varieties resistant to green leafhoppers-Peta, Intan, and Bengawanwere widely grown in Indonesia and the Philippines for 30 to 35 years. Several

improved-plant-type varieties-IR5, IR8, IR20, IR26, and C4-63, which inherited GZh3 for resistance to green leafhopper from Peta-have also been grown for

several years. No clear-cut evidence for the origin of green leafhopper biotypes

that are virulent to GZh3, under the influence of host resistance has been found.

However, varieties resistant t o brown plant hopper became susceptible within

1.5 years of their introduction into the British Solomon Islands, because of the

appearance of a new brown plant hopper biotype. Similarly, within 2 years of its

large-scale cultivation in the Philippines, IR26 was attacked by new biotypes of

the insect in several localities. The germ plasm for resistance to brown plant

hopper comes from South India and Sri Lanka where biotypes of the insect are

virulent t o those varieties. These biotypes probably originated under the influence of host resistance.

The influence of host resistance on the insect populations of brown plant

hopper and green leafhopper is obviously different. The difference may be due

to the differential selection pressure exerted by the resistant varieties on insect

populations. The level of resistance to brown plant hopper conveyed by Bphl

and bphZ is sufficiently high that the insect cannot perpetuate itself on resistant

varieties. It either changes or is eliminated. On the other hand, the level of

resistance to green leafhopper conditioned by GZh3 is only moderate. The insect

feeds on resistant plants and reproduces, although at a much lower rate than it

can when feeding on susceptible varieties. Thus, it can perpetuate itself on

resistant varieties but chances for the origin of more virulent biotypes are

considerably lower than those for the brown plant hopper. Thus, the useful life



DISEASE AND INSECT RESISTANCE IN RICE



33 1



of the Glh3 gene may be considerably longer than that of either Bphl or Bph2.

The other genes for resistance to the green leafhopper-Glhl and Glh2-convey

higher levels of resistance comparable with those of Bphl for brown plant

hopper. When varieties having either gene are grown widely, new biotypes of the

green leafhopper might arise rapidly. Differences in the inherent capacity of the

two insect species to change under the influence of host resistance may also be

responsible for the differences in longevity of resistance to the two insects.

The strategy at IRRI to utilize major gene resistance in rice is twofold. The

short-term strategy aims at incorporating the known major genes for resistance

to different diseases and insects into the improved plant-type background,

combining these genes in different combinations, and sharing the resulting germ

plasm with other programs. IRRI is close to meeting this goal. The long-term

objective is t o identify several genes for resistance to each disease and insect,

particularly bacterial blight and the brown plant hopper. As soon as a new gene

is identified, it is transferred to an improved plant-type background.

When a number of genes become available it would be possible to adopt any of

the following approaches to utilize these genes for vertical resistance:

1. Release one gene for resistance and wait until it becomes ineffective; release

the second gene, and so on. This approach was adopted to control stem rust of

wheat in Australia between 1938 and 1950 (Watson and Luig, 1963). This

approach is being taken with respect t o brown plant hopper resistance. IR26 was

released during the brown plant hopper outbreak of 1973 in the Philippines.

This variety and IRI 561-228-3, another brown plant-hopper-resistant selection,

were grown widely in the Philippines in 1974 and 1975. Both cultivars have

Bphl for resistance. Toward the end of 1975 and in 1976, hopperburn on these

varieties was reported in two locations in the Philippines. IR36 and IR38 which

have bph2 for resistance to brown plant hopper, were hastily released by the

Philippine Government in March 1976. IR36 and IR38 are expected to hold for

a couple of years. By that time varieties with Bph3 and bph4 would be available.

2. Pyramid two, three, or even more major genes together in the same variety,

as suggested by Watson and Singh (1952). Several wheat varieties that combined

up to five genes for resistance to stem rust were developed.

Canadian breeders have adopted the same procedure for developing oats that

are resistant to crown rust (Knott, 1974). Several scientists, most notably Nelson

(1972), favor this approach. This approach depends upon the existence and

availability of several races or biotypes capable of distinguishing between genotypes with various numbers of resistance genes. Otherwise the breeding procedure becomes too lengthy. The availability of several biotypes of brown plant

hopper makes it feasible to pyramid genes for resistance to this insect.

3. Develop multiline cultivars, as proposed by Jensen (1952) and Borlaug

(1958). This approach was followed for crown rust resistance in oats in Iowa

(Browning et al., 1969), and a program to develop multiline stem-rust-resistant



332



GURDEV S. KHUSH



cultivars of wheat is under way at Centro Internacional de Mejoramiento de Maiz

y Trigo. The development of multilines involves an extensive program of gene

identification and backcrossing. As suggested in an earlier section, this approach

merits serious consideration for an international project on blast.

4. Develop resistant varieties with different resistance genes and recommend

them for different geographical regions of the country where the crop covers a

sizable area. As pointed out by Nelson (1972), this type of gene deployment is

essentially a geographical multiline. A formal plan for regional deployment of

genes is in effect for resistance t o crown rust in oats in Iowa (Frey et al., 1973).

This approach may be followed for either rice diseases or insects when enough

genes are identified.



B. HORIZONTAL RESISTANCE



At IRRI the search for horizontal or polygenic resistance in rice continues.

The Institute program on stem borer resistance deals with polygenic systems.

Polygenic variation has been noted for the resistance to brown plant hopper,

grassy stunt, and bacterial blight. However, there are practical difficulties in

exploiting this variation. One concerns the breeding system of the crop. In an

outcrossing species a number of cultivars, with minor genes that are desirable for

accumulation, can be mixed, planted, and allowed to interbreed for several

generations. Appropriate disease or insect pressure is applied to each generation,

and individuals with higher levels of resistance are selected for growing the next

generation.

Random mating permits the formation of new gene combinations at each

generation, and recurrent selection changes the gene frequency for higher levels

of resistance. The process cannot be followed with rice because of its self-pollinating nature. However, a usable source of male sterility that can be employed

for inducing high rates of outcrossing in a composite population with several

sources of polygenic resistance is being sought. Pending the availability of a male

sterile, the diallele selective mating system discussed in an earlier section is being

used.

The second difficulty concerns the screening techniques. Most artificial screening techniques fail to detect polygenic differences. During the brown plant

hopper outbreak of 1973 at the IRRI farm, several selections with tolerance to

the insect were identified. They withstood the insect attack longer than the

susceptible varieties did, but were eventually killed. However, when they were

tested in the greenhouse, these differences could not be detected.

At IRRI, a breeding program on horizontal resistance to brown plant hopper

was initiated, using these sources and following the diallele selective mating

system. When the F1 progenies from the double crosses were ready for testing,

there were no brown plant hoppers in the field.



DISEASE AND INSECT RESISTANCE IN RICE



333



It is important to develop horizontal resistance to the brown plant hopper, and

efforts are being made at IRRI to screen the breeding population in other

countries, such as the British Solomon Islands, where field populations of the

brown plant hopper are always high.



VI. Conclusions



Among cereal crops, rice is the host of the largest number of diseases and

insect pests. These cause serious yield losses annually.

The magnitude of losses caused by the diseases and insects is likely to increase

as the level of rice production per unit area increases.

The germ plasm resources for disease and insect resistance are vast, but only a

portion has been collected and evaluated for resistance.

Much germ plasm remains to be collected and catalogued. It should be

collected before it becomes extinct through the adoption of high-yielding

varieties.

The germ plasm that has not been evaluated, especially the national germ

plasm collections, should be evaluated to identify more sources of resistance.

Different sources of resistance should be genetically analyzed to identify

diverse genes for resistance.

A systematic international survey of races or biotypes of major diseases and

insects should be carried out with the use of differential varieties.

Sources of resistance to all races or biotypes should be identified and genetically analyzed.

The different major genes for resistance should be utilized according to needs

of each program. Various alternatives are discussed.

Greater efforts should be expended in studying and utilizing horizontal resistance, although vertical resistance will continue to be useful for years to come.

Interdisciplinary cooperation among the pathologists, entomologists, and

breeders is essential for the speedy implementation of host resistance progcams.

International cooperation is essential in collecting and evaluating germ plasm,

studying the races and biotypes, identifying the diverse genes for resistance,

cooperative testing for disease and insect resistance, and liberal exchange of

improved germ plasm.



REFERENCES

Abeygunawardena, D. V. W. 1967. Proc. Symp. Rice Dis. Their Control Growing Resistant

Varieties Other Measures pp. 171-179. Agric., For. Fish. Res. Counc., Tokyo.

Abeygunawardena, D. V. W., Bandaranayaka, C. M., and Karandawela, C. B. 1970. Trop.

A@. (Ceylonj 126, 1-13.



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