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CHAPTER 8. DISEASE AND INSECT RESISTANCE IN RICE

CHAPTER 8. DISEASE AND INSECT RESISTANCE IN RICE

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266



GURDEV S. KHUSH



Reduced genetic variability, improved cultural practices, and continuous cropping with rice-factors for increased rice production-have increased the genetic

vulnerability of the crop. Within the last few years serious outbreaks of diseases

and insects harmful to rice have occurred in several countries. Very little

research has been done on the chemical control of rice diseases in the tropics.

Several insecticides have been identified, but chemical control of high insect

populations for prolonged periods under tropical climate-where insect generations overlap throughout the year-is very expensive. Social and economic

conditions in the tropics present other obstacles to the chemical control of rice

diseases and insects.

Research on the control of diseases and insects through host resistance has

been increasingly emphasized in recent years. All international as well as national

rice improvement programs devote major shares of their efforts to incorporating

into their breeding materials genetic resistance t o major diseases and insects. A

large number of pathologists, entomologists, and breeders are engaged in t h s

endeavor throughout the rice-growing areas of the world. As a result, improved

varieties with multiple resistance to several diseases and insects are now available

to rice growers. Those varieties and the others still in the pipeline will play

crucial roles in increasing the world’s food production.

This chapter reviews the progress made in developing rice that is resistant to

diseases and insects. The topics t o be discussed include the nature of the disease

or insect, its distribution, genetic variability of the pathogen, host resistance,

genetics of resistance, and breeding for resistance. The review emphasizes recent

work; it includes references to previous reviews on each subject.

II.



Disease Resistance



Numerous diseases of rice, caused by fungi, bacteria, viruses, and nematodes,

have been recorded in different rice-growing areas of the world. Some diseases

occur wherever rice is grown: some are of both regional and international

importance, others occur in local areas. Some diseases reach epidemic proportions and cause serious crop losses, while others cause only negligible crop losses.

This chapter deals with only diseases of international importance which cause

considerable crop losses. For information on diseases not covered here, the

reader is referred to the excellent treatment of the subject by Ou (1972).



A. FUNGAL DISEASES



Most rice diseases are caused by fungi. Among the 60 rice diseases discussed by

Ou (1972), 37 are fungal diseases. Fungal diseases attack the plant foliage, stems,



DISEASE AND INSECT RESISTANCE IN RICE



267



roots, leaf sheath, or inflorescence, and grains. Some affect only one plant organ.

Four fungal diseases of major economic importance are reviewed here.



I . Blast

The blast disease of rice occurs in all rice-growing areas of the world. It is the

most important disease of the rice plant and causes serious and sometimes total

yield losses. It may infect the leaves, nodes, panicles, and other aerial parts of

the plant. It is also known as leaf blast, node blast, panicle blast, or neck rot,

depending upon the plant part affected.

a. Variation in Pathogenicity. Rice blast is caused by Pyricularia oryzae, a

highly variable organism, The differences in pathogenicity of the fungus strains

were first recorded by Sasaki (1922, 1923). He noticed that rice varieties

resistant to one strain were severely infected by another. Intensive investigations

on pathogenic variability of the blast fungus in Japan were started in 1950 when

some resistant rice varieties, such as Futaba, suddenly became susceptible.

Around 1960, 12 varieties were selected as differentials and 13 pathogenic races

were identified (Goto, 1965). Latterell et al. (1954) reported on pathogenic

races of blast in the United States. With the use of additional isolates from Asia

and Latin America, 15 races were identified (Latterell et aL, 1960). Pathogenic

variability was also reported in India (Padmanabhan, 1965b), Taiwan (Chiu etal.,

1965), Korea (Ahn and Chung, 1962; Lee and Matsumoto, 1966), and Colombia

(Galvez and Lozano, 1968). More than 100 races have been identified in the

Philippines (Bandong and Ou, 1966; Ou and Jennings, 1969), and the number

continues to increase.

Each country has used various sets of differential varieties in identifying blast

races. Therefore, races identified in one country cannot be compared with races

identified in other countries. A cooperative study was started in 1963 between

Japan and the United States to develop an international set of differential

varieties. Hundreds of isolates collected in Japan and the United States were

tested during a 3-year study. Of 39 differential varieties, 8 were selected t o form

an international set of differentials for blast. With the set, 32 race groups were

characterized. The races were called international races and given the designation

lA, lB, etc., to lH, followed by numbers (Atkins et aL, 1967; Goto et al.,

1967).

In the race studies, a pure culture is obtained by isolating a single conidium

from a sample. Inoculum for testing pathogenicity is prepared from the pure

culture. Ou and Ayad (1968) tested 56 monoconidial cultures from the same

crop of spores of a leaf lesion on the Philippine set of differentials and found 14

races; 44 monoconidial cultures from a second lesion were differentiated into 8

races. They also found that 25 monoconidial subcultures from each of two single

conidial pure cultures were differentiated into 9 and 10 races, respectively.



268



GURDEV S. KHUSH



Testing 4 varieties, Giatong and Frederiksen (1969) found 20 monoconidial

lines that separated into 4 to 7 races. In three consecutive generations, the

monoconidial lines continued to change into different races. Similar results were

reported by Chien (1968) and Ou et al. (1971~).

b. Varietal Resistance. Various methods for evaluating resistance to blast

have been developed in different countries. To assess the disease reaction more

accurately and to handle a large number of varieties in a short time, a uniform

testing method was adopted for the International Blast Nursery Program (Ou,

1965a). The blast nursery method of testing allows quick evaluation of the

resistance of a large number of rice varieties to a number of races in the region.

According to Ito (1965), varietal differences in resistance to blast were

observed as early as 1900; varieties Kameji and Aikoku were considered highly

resistant, and Shinriki was thought susceptible. Numerous resistant varietiesmany of foreign origin-have since been identified in Japan and utilized in the

breeding program (Toriyama, 1972). Blast-resistant varieties have been identified

in India (Padmanabhan, 1965a), Thailand (Dasananda, 1965), Taiwan (Chang et

al., 1965), and the United States (Atkins et al., 1965).

Varietal reaction may vary from country to country, from locality to locality,

and from season to season in the same locality. To identify blast-resistant

varieties with a broad spectrum of resistance, the Working Party Meeting of the

International Rice Commission (IRC) held in Sri Lanka in 1959 initiated the

Uniform Blast Nurseries. During the 1963 symposium on the rice blast disease at

the International Rice Research Institute (IRRI), the International Uniform

Blast Nurseries (IUBN), initiated by FAO-IRC, were modified and strengthened,

and leadership for coordination was assigned to IRRI (Ou, 1965a). A total of

258 selected varieties of rice identified on the basis of resistant reactions in

initial tests were included in the IUBN. The results of nursery tests at 50

locations in 26 countries were reported by Dr. Ou and his colleagues in various

issues of the International Rice Commission Newsletter.

In 1966, an additional 321 varieties selected from the IRRI blast nursery were

included in the international tests. More varieties with broad-spectrum resistance

were identified. The most resistant varieties identified from those two groups of

entries in the IUBN are listed in Table I. Very useful donor parents have been

identified through the nurseries. The composition of the nurseries was recently

modified to include improved breeding lines from various breeding programs.

c. Genetics of Resistance. Genetic studies on blast resistance were first

reported by Sasaki (1922). Takahashi (1965) reviewed the work done up to

about 1963. Not much reliance can be placed on the studies because few used

pure fungus strains of known pathogenicity.

Systematic studies using pure cultures of known pathogenicity were begun by

Niizeki (1960) and continued by Kiyosawa and co-workers. The studies were

reviewed by Kiyosawa (1972, 1974). With the use of seven fungus strains of



269



DISEASE AND INSECT RESISTANCE IN RICE

TABLE 1

Blast-Resistant Varieties Selected from International Blast Nurseries'



Variety



Group rb

Tetep

Nang Chet Cuc

C46-15

Tadukan

Trang Cut L. 11

Pah Leuad 111

H-5

R-6 7

(217787

Mekeo White

Ram Tulasi (Sel)

D25-4

M-302

Padang Trengganu 22

Ta-poo-cho-z

Group rrC

C46-15

Mamoriaka

Carreon

Huan-sen-goo

Dissi Hatif

Ram Tulasi

Ram Tulasi (Sel)

Thava Lakkanan PTB 9

Macan Tag0

Ahmee Puthe

Ca 435/b/5/1

DNJ-60

Susceptible varieties

Kung-shan-wu-shen-ken

Fanny



Origin



Total tests

1964-1973



Susceptibility

index



Resistant

frequency (%)



Vietnam

Vietnam

Burma

Philippines

Vietnam

Thailand

Sri Lanka

Senegal

U.S.A.

New Guinea

India

Burma

Sri Lanka

Malaysia

China



302

292

307

309

263

258

3 14

291

278

276

297

292

310

239

277



1.24

1.64

1.56

1.50

1.70

1.57

1.71

1.85

1.83

1.94

1.70

1.73

1.86

1.93

1.61



98.0

88.3

93.8

94.5

94.3

94.3

92.1

92.4

91.7

92.8

91.9

93.6

90.3

87.4

91.3



Burma

Malagasy

Philippines

China

Senegal

India

India

India

Philippines

Burma

Indonesia

Bangladesh



229

227

227

216

223

211

194

222

155

136

205

224



1.5 1

1.48

1.38

1.35

1.51

1.41

1.42

1.52

1.75

1.49

1.56

1.93



97.3

97.8

97.4

96.3

97.3

91.2

97.3

96.9

95.5

97.1

97.1

93.8



China

France



246

252



4.30

4.39



24.4

19.4



aFrom Ou et al. (1975).

bCroup I consists of 258 varieties selected at random and tested in IBN since 1963.

'Group 11 consists of 321 varieties selected from more than 8200 varieties after repeated

tests at IRRI and was entered in IBN since 1965.



varying pathogenicity, the genetic constitution, for blast resistance of several

domestic and introduced rice varieties was analyzed. Ten Ioci are designated:

(1) Pi-a, (2) Pi-b, ( 3 ) Pi-J (4) Pi-i, ( 5 ) Pi-k, ( 6 ) Pi-m, (7) Pi-s, ( 8 ) Pi-t, (9) Pi-tu,

and (10) Pi-z. Some are characterized by multiple allelic series. The Pi-k locus,

originally identified by Yamasaki and Kiyosawa (1966) in the variety Kanto 5 1,



270



GURDEV S. KHUSH



has at least three other distinct alleles-Pi-ks (Kiyosawa, 1969a), P i - k p (Kiyosawa, 1969b), and R-kh (Kiyosawa and Murty, 1969). Similarly, Pi-ta and Pi-ta2

are two distinct alleles at the Pi-fa locus (Kiyosawa, 1966, 1967b, 1969b), and

R - z f are distinct alleles at the Pi-z locus (Kiyosawa, 1967a; Kukoo and Kiyosawa, 1970). The distribution of the resistance genes in different rice varieties is

shown in Table 11.

TABLE I1

Genes for Resistance to Japanese Isolates of Blast Fungus Identified to Date and Their

Distribution in Different Varieties

Gene locus



Allele



Type variety



Other varieties



Pi-a



Pi-a



Aichi Asahi



Akage, Akebono, Akibare, Kinmaze, Norin 17,

Norin 18, Norin 21, Takara Towada, Jae Keun,

Pal tal, Usen, Toto, Blue bonnet, Zenith,

Hokushi Tami, Dawn



Pi-b



Pi-b



BL8



Tjina, Tjahaja, Bengawan, Milek Kuning



Pi-f



Pi-f



Stl



Chugoku 31



Pi-i



Pi-i



Ishikare-shiroke



Asashio, Fujisaka 5, Fukuyuki, Kitaminori,

Yoneshiro, Akishinomochi, Kohi, Miyoshi,

Noruho, Shinsetsu, Doazi chall, Dawn



Pi-k



Pi-k



Kanto 51



Pi-kh

Pi-k p



K3

K2

Shin 2



Koshi-minori, Kusabue, Matsumae, Senshuraku,

Tchi-honami, Yachiho, Dewa-no-mochi, Teine,

Yuukara, Sakaki-mochi, Hakkai 219, Sanpuku

Kongo, Suzukaza, Yakei-Ko, Reishiko, To-to,

Choko-To, Dawn

HR22

Pusur

To-to, Taihung 65, Caloro, Lacrosse, Sha-tiao-tsao,

Ishikarishiroke



Pi@

Pi-rn



Pi-m



Pts



Pi-s



65A15



Pi-t



Pi-t



KS 9



BL10, Tjina



Pi-to



Pi-ta



Yashiro-mochi



Pi-ta2



Pi 4



Pai-kan-tao, Tadukan, Taso-senbon, Shimokita,

Pi 1 , Pi 2

Akiji, Asa-hikari, Pi 3, Satominori



Pi-z



Fukunishiki

Toride I



Pi-z



Pi-zf



Tsuyu-ake, Hokushi Tami, Minehikari



Zenith, Ohy 244, 54C68, Fukei 67

C025, TKM1, C04, Morak Seplai, Kontor,

Leuang Tawang 77-1 2-5, Chao Leuang 11,

Toride 2



DISEASE AND INSECT RESISTANCE IN RICE



27 1



Some genes have been assigned to the respective linkage groups by appropriate

linkage analysis. Thus, Pi-z and Pi-i have been assigned to linkage group I

(Fukuyamz e t al., 1970; Yukoo and Fujimaki, 1970); Pi-m, Pi-k and Pi-A to

linkage group VIII (Toriyama et al., 1968b; Kiyosawa, 1968); Pi-ta, to linkage

group VII (Kiyosawa, 1970; Fukuyama et al., 1970); and Pi-s, t o linkage group

X (Iwata and Omura, 1971).

Some varieties have more than one gene for resistance. The welI-known Zenith

from the United States has Pi-z and Pi-i, and Dawn has Pi-a, Pi-k, and Pi-i. The

Chinese variety Hokushi Tami has Pi-a, Pi-k, and Pi-m. Several Japanese varieties-Kiho, Miyoshi, Naruho, and Shinsetsu-have the combination Pi-a and Pi-i.

The distribution of resistance genes is cosmopolitan. Pi-a is present in varieties

from Japan, Korea, China, India, Pakistan, and the United States; Pi-i in varieties

from Japan, Korea, and the United States; Pi-k in varieties from China;Pi-ks in varieties from Japan, China, and the United States; Pi-kp in varieties from Pakistan;

Pi-kh in varieties from 1ndia;Pi-ta in varieties from the Philippines and ChinaPi-ta2

in varieties from the Philippines; Pi-z in varieties from the United States; Pi-z* in

varieties from India, Thailand, and Malaysia; Pi-b, in varieties from Indonesia and

Malaysia; Pi-t, in varieties from Indonesia; and Pi-m, in varieties from China

(Kiyosawa, 1974).

Outside of Japan, two critical studies on the genetics of blast resistance have

been reported. Pure cultures of known races of blast were employed in both

studies. Atkins and Johnston (1965) reported a single dominant gene in Northrose and Nato that carried resistance to the United States race 1 of blast; they

designated the gene as Pi 1. Zenith and Gulfrose have another dominant gene

that governs resistance to the United States race 6. That gene was designated Pi

6. Pi 1 and Pi 6 showed independent segregation. Hsieh et al. (1967) identified

three dominant genes for resistance in japonica strains. The genes which carried

resistance to races 4, 22, and 25 from Taiwan, were designated Pi 4, Pi 22, and

Pi 25.

Genes for blast resistance identified in Japan, Taiwan, and the United States

have not been related t o each other. Internationally coordinated genetic studies

on blast are badly needed t o identify genes for resistance t o races of blast

prevalent in tropical Asia, Africa, and Latin America. Resistance genes so

identified would be employed in international breeding programs.

d. Breeding for Resistance. Breeding for blast resistance has been in progress

in different countries for at least 40 years. The work done up to 1963 in Japan,

the United States, India, Taiwan, and Thailand was reviewed by Ito (1965),

Atkins et al. (1965), Padmanabhan (1965a), Chang et al. (1965), and Dasananda

(1965), respectively. More recent reviews are those by Ou and Jennings (1969),

Ou (1972), and Toriyama (1972).

To develop blast-resistant varieties, Japanese breeders have incorporated resistance genes from (1) native Japanese lowland varieties, (2) Japanese upland

varieties, (3) Chinese varieties of japonica type, and (4) introduced indica vari-



272



GURDEV S. KHUSH



eties. Blast-resistant varieties Norin 6 and Norin 8 were developed in 1935 and in

1936 from the crosses of lowland Japanese varieties Joshu/Senichi and Ginbozu/

Asahi, respectively. Hybridization of the two produced Norin 22 and Norin 23.

These two varieties and Yamabiko were recommended for southwestern Japan.

Blast-resistant varieties Riku 132 and Fujiminori were developed for northeastern Japan, and Ishikari-shiroke for northern Japan. Yamabiko and Fujiminori

possess the Pi-u gene for resistance (Ezuka et al., 1969). Ishikari-shiroke has Pi-i

which gives moderate resistance to the Japanese races of blast and is still

effective. The development of races virulent to Pi-u has made the gene ineffective. However, varieties with Pi-a continue to show some resistance because of

the presence of polygenes for resistance (Toriyama, 1972).

Several outstanding varieties-Wakaba, Wase-wakaba, Koganenishilu, Ukonnishiki, and Homare-nishiki-with resistance genes from Japanese upland varieties were developed. They were widely planted in the mountainous regions of

southwestern Japan. These varieties have moderately high levels of resistance,

which appears to be stable (Ujihara, 1960). They are now widely used as sources

of resistance in breeding programs in Japan (Toriyama, 1972).

Two Chinese varieties of the japonica type-Reishiko and To-to-that were

found highly resistant to blast in Japan (Matsuo, 1952a) were used in the

breeding programs. Kanto 51 and Kanto 55, two breeding lines with a high level

of blast resistance, were developed and employed in the hybridization programs

to produce several blast-resistant varieties, such as Kusabue, Yuukara, and

Senshuraku. The varieties were very widely planted. However, they became

susceptible 3 to 5 years after their release. Their resistance was conditioned by

the Pi-k gene, and damage to them was due to the development of races virulent

to the Pi-k gene (Matsumoto et ul., 1965). Another Chinese variety, Hokushitahmi, was also used as a source of resistance in the breeding program. Kongo and

Minehikari, highly resistant progenies with the resistance gene Pi-m in addition

to Pi-k (Kiyosawa, 1968) were produced.

Resistance genes Pi-ta and Pi-ta2 from the Philippine variety Tadukan (indica

type) were transferred to varieties of a japonica background by backcrossing

(Shigemura and Kitamura, 1954; Kitamura, 1962). Japonica type lines Pi 1 to Pi

5, with high level of resistance to blast were developed. Pi 1 and Pi 2 were found

to have Pi-ta (Kiyosawa, 1966), whereas the resistance of Pi 3, Pi 4, and Pi 5 was

found to be due to Pi-tu2 (Kiyosawa, 1967b). With the Pi lines as parents,

resistant varieties Satominori, Akiji, Shimokita, and Tosasenbon were developed

and released for cultivation (Kirya e l ul., 1966; Matsuzawa et al., 1968; Toriyama et d., 1968~).Those varieties which possess resistance genes from introduced indica Varieties have been widely grown without a breakdown of resistance (Toriyama, 1965, 1972). Resistance gene Pi-z of Zenith has also been

transferred to varieties with a japonica background. However, the variety Fukunishiki, which carries Pi-3 (Kiyosawa, 1967a), started to show blast susceptibility

after a few years (Hirano et al., 1967). Recently, stronger genes for resistance



DISEASE AND INSECT RESISTANCE IN RICE



273



from indica varieties, such as Pi-zf from TKM 1 and CO 25 (Nagai et al., 1970),

and Pi-b and Pi-t from Tjina, Tjahaja, and Milek Kuning (Fujimaki and Yokoo,

1971) have been transferred to Japanese varieties.

Japanese scientists distinguish between “true” resistance (caused by specific

genes to races of known pathogenicity) and “field” resistance. In blast nursery

tests, varieties having the same true resistance genes sometimes show differential

reactions. The differences are attributed to the differences in field resistance of

the varieties (Hirano et al., 1967; Asaga and Yoshimura, 1969; Hirano and

Matsumoto, 1971). If varieties lack field resistance, they are severely affected by

the fungus races virulent to true resistance genes. Field resistance is estimated by

growing varieties in areas where races virulent t o the true resistance genes that

the varieties possess are prevalent.

Field resistance is also evaluated by spray inoculation with various fungus

strains. Varieties that show few and small lesions when tested by the spray

inoculation method are considered to have field resistance (Niizeki, 1967).

Variety St 1 gave susceptible reactions against seven fungus strains when tested

by the injection method of inoculation. However, it showed high field resistance

when tested by the spray inoculation method (Sakurai and Toriyama, 1967).

Genetic analysis showed that the high field resistance of St 1 was controlled by a

single major gene, Pi-J which is linked to Pi-k with a recombination value of

20% (Toriyama et al., 1968a). The field resistance of St 1 also broke down a few

years after it was bred (Yunoki et al., 1970). Thus, field resistance, as defined by

Japanese scientists, is specific, controlled by major genes, and may not differ in

longevity as compared with true resistance. It differs from horizontal resistance

or field tolerance, which is conditioned by the polygenes and is considered

lasting.

Work on breeding for blast resistance in India was reviewed by Padmanabhan

(1965a). Rapid progress has been made in incorporating blast resistance into

improved materials at the Central Rice Research Institute (CRRI) at Cuttack; at

the All India Coordinated Rice Improvement Project (AICRIP) at Hyderabad;

and at other rice-breeding stations in the country. Local resistant donors (ARC

1250, MNP 36, Mahoharsali, MTU 5, BAM 7, BJ 1, TKM 6) and introduced

donors (Nahng Mon S4, Sigadis, Carreon, Tadukan, Tetep, Dissihatiff, and

Zenith) have been used as sources of resistance. Multilocation testing for blast of

the breeding materials in nationwide coordinated trials has facilitated the work

in resistance and identification of improved plant-type breeding lines with

broad-spectrum blast resistance.

The breeding program for blast resistance at IRRI has emphasized the use of

diverse sources of resistance identified in the IUBN. Tall donor parents are

crossed with improved plant-type varieties or breeding lines, and many better

plant-type lines are selected from the crosses. The lines are continuously

screened in the blast nurseries, two or three times a year for several years. Lines

that show resistance in all the tests are again crossed with improved-plant-type



274



GURDEV S. KHUSH



lines having resistance to other diseases and insects, or some desirable agronomic

or grain quality characteristic. They are again rigorously screened for blast

resistance. In this way, many resistant lines from several resistant donors, such as

H105,Nahng Mon S-4, Dawn, B589A4, Kam Bau Ngan, Cam Pai 15, Sigadis,

and Tetep, have been developed (Table 111).

Recently the composition of the IUBN was modified to include breeding lines

in addition to donor parents. That will facilitate the early identification of lines

with a broad spectrum of blast resistance and their dissemination internationally.

In addition to rigorous screening in the blast nursery, all breeding materials are

carefully watched for the occurrence and incidence of leaf blast and neck rot in

various breeding nurseries. Those that show susceptibility are discarded. Thus,

any breeding lines which reach the varietal stage must be screened at least 15 to

20 times in blast nurseries in as many seasons, and observed in field nurseries for

3 to 5 years at several locations. Highly susceptible combinations that are

encountered are discarded in the early stages. As a result of continuous screenTABLE 111

Some Improved Plant-Type, Blast-Resistant Selections Developed at IRRI

from Various Donor Varieties

Donor parent

H105



Cross



Selection

IR4-93



Hl05lDgwg



Nahng Mon S-4 IR480-5-9



Nahng Mon S-4*/TN1



Dawn



IR759-54-2-2

IR1909-1-3-3



IR8/Peta3//Dawn

IR84 /Dawn



B589A4



IR790-28-1-6



/TNl

Peta4 /TN 1/4/1R8///H105/Dg~g//B589A4~



Kam Bau Ngan



IR1360-85-2-3



IR8/Kam Bau Ngan



Gam Pai 15



IR833-6-2-1

IR206 1-213-2-17(IR34)



Peta3/TN1//Gam Pai 15

Peta3 /TNl//Gam Pai 15/4/IR8/Tadukan//

TKM62/TN1///IR24410.nivara



Tetep



IR1416-128-1-2

IR1416-131-3-6

I R 1544-2 38-2-3

IR 1544-340-6-1

IRl820-52-2-4

IR2035-290-2-3



Peta4/TNl//Tetep

Peta4 /TN 1//Tetep

IR24/Tetep

IR24/Tetep

IR24//Mudgo/IR8///Peta4 /TN1 //Tetep



IR1529-680-3



Sigadis' /TN 1//IR24



Sigadis



Peta4/TN1//Tetep///Peta3/TN1//HR21/4/

IR24//M~dgo/IR8///IR24~/0.

nivara



DISEASE AND INSECT RESISTANCE IN RICE



275



ing, the advanced breeding lines at IRRI have good levels of resistance. Of the

eleven IRRI named varieties three (IR28, IR29, and IR34) have strong blast

resistance, inherited from the common parent Cam Pai 15. High levels of

resistance are not involved in the parentage of the remaining eight varieties, but

some (IR20, IR26, and IR32) show moderately resistant reactions in the blast

nursery. Although IR8 and IR5 have been classified susceptible in the blast

nursery, they have rarely been infected with blast under lowland conditions in

Asia where they were widely grown. It appears that those varieties have adequate

levels of field tolerance that are not detected in the blast nursery.

Serious yield losses from blast in lowland rice in tropical Asia have rarely been

reported in recent years. Apparently, the present varieties have adequate resistance or field tolerance. However, what happened in Japan earlier could occur in

tropical Asia. As the average yields per unit area increase with increased use of

fertilizers and other inputs, blast might become a limiting factor to higher

production. A coordinated international approach t o the development of blastresistant varieties needs to be adopted. It should include: (1) identification of

most widely distributed blast races in the cooperating countries, (2) identification of blast resistance genes through genetic analysis using the identified races,

(3) incorporation of distinct resistance genes into isogenic lines using varieties

with field tolerance as recurrent parents, and (4) programmed release of these

lines, either successively when the previous one becomes susceptible or as

multilineal varieties. Several genes for resistance should also be combined in the

same improved variety.



2. Sheath Blight

Sheath blight is perhaps the second most important fungal disease of rice. It

ranks second only t o blast in the yield losses it causes. The fungus causing the

disease has been called Corticium sasakii (Shirai) Matsumoto or Rhizoctonia

soZani Kuhn, and several other names. Earlier it was thought that the disease

occurred only in Asia, but recently it has been reported in Brazil, Surinam,

Venezuela, and Madagascar (Ou, 1972).

Our observations and reports from our collaborators in other countries indicate that the attacks of sheath blight have greatly increased in tropical Asia in

recent years. The increased incidence is attributed to the greater use of highyielding, high-tillering, short-statured, and early-maturing varieties as well as to

higher plant populations and greater use of nitrogenous fertilizers. The disease

incidence is likely to increase even further with the widespread use of high-yielding varieties and better management practices.

a. Variution in Pathogenicity. Differences in the pathogenicity of fungal

isolates were reported by Chien and Chung (1963). They classified 300 isolates

into seven culture types and six physiological races based on the disease re-



276



GURDEV S. KHUSH



actions of 16 varieties. The susceptible and resistant reactions used in separating

the races were not so clear-cut, however. Differences in pathogenicity of isolates

were also noted by Akai et al. (1960), Tu (1967), and by IRRI pathologists

(IRRI, 1974). Parmeter (1970) also reported that the fungus isolates differ in

virulence and host range.

b. Varietal Resistance. IRRI pathologists have tried various screening techniques to determine differences in varietal resistance-seedling-stage inoculation,

detached flag-leaf inoculation, leaf-sheath inoculation, and adult-plant inoculation. Distinct varietal differences were noted with each method of inoculation,

but the results did not completely agree. Some varieties showed resistance

reactions with one method, susceptible reactions with another. Some inconsistencies in reaction between the seedling stage and the adult plant stage can be

explained by the relation between plant age and disease development. Plants at

the flowering stage are more susceptible than those at the seedling or tillering

stages. Since the disease is more important after the flowering stage, the adult

stage inoculation method has been adopted for screening materials at IRRI

(IRRI, 1974).

The reactions of rice varieties to sheath blight range from highly susceptible to

moderately resistant. Resistant or highly resistant varieties have not yet been

found. Most moderately resistant varieties that have been identified show a

disease index of about 30; the susceptible varieties show a disease index of

80-90. The disease index is calculated as the percentage of infected leaf sheath

area from a 10- to 25-tiller sample. From a sample of over 1000 varieties and

breeding lines screened by plant pathologists at IRRI, several moderately resistant entries have been identified: Kataktara Da-2, Ta-poo-cho-z, Charnock,

F’tbl8, Carreon, Bahagia, Colombia 1, CS2, OS4, Mehran, Sinaloa A68. Several

improved plant-type breeding lines such as IR442-2-58, IR533-1-89, IR1330-3-2,

IR1360-87-1, and IR1093-148-3 have good levels of resistance.

On the basis of field tests in Japan, Hashioka (1951a) reported that varieties

from India, Thailand, Burma, and Europe were more resistant than local varieties. Varietal differences in reaction were noted by Hseihetal. (1965) in Taiwan.

Screening done in India showed CR1-6, CR44-11, Saket 1, and Sona as moderately resistant (H. K. Saksena, personal communication).

c. Inheritance of Resistance. Hashioka (1951 b) reported that resistance to

sheath blight is dominant over susceptibility. The F2 populations from crosses

between resistant and susceptible parents were either mostly resistant or segregated in a ratio of 3 resistant to 1 susceptible.

d. Breeding for Resistance. Work on breeding for resistance to sheath blight

has not been done anywhere, not even in Japan where the disease has been a

serious problem for many years. The unavailability of donor parents with usable

levels of resistance has been the main cause of lack of progress in that area



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CHAPTER 8. DISEASE AND INSECT RESISTANCE IN RICE

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