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CHAPTER 8. DISEASE AND INSECT RESISTANCE IN RICE
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.
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
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.,
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.
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
DISEASE AND INSECT RESISTANCE IN RICE
Blast-Resistant Varieties Selected from International Blast Nurseries'
Nang Chet Cuc
Trang Cut L. 11
Pah Leuad 111
Ram Tulasi (Sel)
Padang Trengganu 22
Ram Tulasi (Sel)
Thava Lakkanan PTB 9
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,
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.
Genes for Resistance to Japanese Isolates of Blast Fungus Identified to Date and Their
Distribution in Different Varieties
Akage, Akebono, Akibare, Kinmaze, Norin 17,
Norin 18, Norin 21, Takara Towada, Jae Keun,
Pal tal, Usen, Toto, Blue bonnet, Zenith,
Hokushi Tami, Dawn
Tjina, Tjahaja, Bengawan, Milek Kuning
Asashio, Fujisaka 5, Fukuyuki, Kitaminori,
Yoneshiro, Akishinomochi, Kohi, Miyoshi,
Noruho, Shinsetsu, Doazi chall, Dawn
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,
To-to, Taihung 65, Caloro, Lacrosse, Sha-tiao-tsao,
Pai-kan-tao, Tadukan, Taso-senbon, Shimokita,
Pi 1 , Pi 2
Akiji, Asa-hikari, Pi 3, Satominori
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,
DISEASE AND INSECT RESISTANCE IN RICE
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
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
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-
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
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
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
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
Nahng Mon S-4 IR480-5-9
Nahng Mon S-4*/TN1
Peta4 /TN 1/4/1R8///H105/Dg~g//B589A4~
Kam Bau Ngan
IR8/Kam Bau Ngan
Gam Pai 15
Peta3/TN1//Gam Pai 15
Peta3 /TNl//Gam Pai 15/4/IR8/Tadukan//
I R 1544-2 38-2-3
Peta4 /TN 1//Tetep
IR24//Mudgo/IR8///Peta4 /TN1 //Tetep
Sigadis' /TN 1//IR24
DISEASE AND INSECT RESISTANCE IN RICE
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-
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
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