Bacterial Leaf Blight Disease of Rice- Discussed ! | Plant Diseases

In this article we will discuss about:- 1. Introduction to Bacterial Leaf Blight Disease of Rice 2. Historical Background and Geographical Distribution of Bacterial Leaf Blight Disease of Rice 3. Economic Importance 4. Symptomatology 5. Causal Organism 6. Cultural, Morphological, Biochemical and Pathogenic Variability 7. Disease Management 8. Chemical Control.

Contents:

1. Introduction to Bacterial Leaf Blight Disease of Rice
2. Historical Background and Geographical Distribution of Bacterial Leaf Blight Disease of Rice
3. Economic Importance of Bacterial Leaf Blight Disease of Rice
4. Symptomatology of Bacterial Leaf Blight Disease of Rice
5. Causal Organism of Bacterial Leaf Blight Disease of Rice
6. Cultural, Morphological, Biochemical and Pathogenic Variability of Bacterial Leaf Blight Disease of Rice
7. Disease Management of Bacterial Leaf Blight Disease of Rice
8. Chemical Control of Bacterial Leaf Blight Disease of Rice

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1. Introduction to Bacterial Leaf Blight Disease of Rice:

Rice is the most important and staple food crop for more than two thirds of the population of India. The slogan “Rice is Life” is most appropriate for India as this crop plays a vital role in our national food security and is means of livelihood for millions of rural households. India has the largest acreage under rice (44.6 m.ha) and with a production of about 90 million tones it ranks second to China.

At the current rate of population growth, rice production has to be enhanced to about 125 million tons by 2020. Diseases and insect-pests constitute serious constraints on rice yield, particularly in the tropical countries.

The distribution of rice diseases in the temperate and tropical regions is governed primarily by temperature and other environmental factors. It is also influenced strongly by rice varieties and cultural practices.

The introduction and wide spread cultivation of dwarf and high nitrogen responsive rice verities helped only for a brief period, because the susceptibility of these varieties to diseases was a more potential danger than the advantage that could be derived from them.

In the quest for increasing rice production, man has resorted to intensive methods of rice cultivation involving high-yielding susceptible cultivars with reduced genetic variability, higher plant population per unit area, high doses of nitrogenous fertilizers and staggered sowing and planting which intensified the severity of bacterial blight rice in most of the Asian countries.

In the present review article, an attempt has been made to compile all the available information on the subject.

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2. Historical Background and Geographical Distribution of Bacterial Leaf Blight Disease of Rice:

Bacterial leaf blight is said to have been first seen by farmers in the Fukuoka area of Japan in 1884. During 1908-10, it was commonly observed in the southwest of Japan and since 1926 it has been recorded in the northeast. The disease increased markedly after 1950, and by 1960 it was known to occur in all parts of Japan except the northern Island of Hokkaido.

The study of the disease in Japan commenced in 1901. It was then believed to be physiological origin, and to be due to acid soil, because dew drops on the infected leaves were acidic in reaction. In 1908, Takaishi found bacterial masses in the dew drops, isolated the organism and successfully inoculated it on to rice leaves, he did not name the organism.

Bokura in 1911 also isolated a bacterium, assumed to be identical with that of Takaishi. He carried out inoculation experiments and concluded that the disease was due to the bacterium and not to the acid soil. After a study of its morphology and physiology, the bacterium was named Bacillus oryzae Hori and Bokura.

Ishiyama (1922) studied the disease further and renamed the bacterium Pseudomonas oryzae Uyeda and Ishiyama, according to Migula’s system. It was later (1927) transferred to Bacterium oryzae (Uyeda and Ishiyama) Nakata according to E.F. Smith’s concept and subsequently to Xanthomonas oryzae (Uyeda and Ishiyama) Dowson. Later on the bacterium has been renamed as X campestris pv. oryzae.

Reitsma and Schure (1950) reported a disease called ‘Kresek’ in Indonesia. The causal organism was called Xanthomonas kresek Schure (1953).

This disease has now been shown to be a severe form of bacterial leaf blight of rice, which is found in various parts of the tropics. The disease occurs globally, from Asia to Africa and the Americas. Its distribution ranges from 20°S in Queensland, Australia, to 58° N in Heilang Jiang, China and from sea level to Tibetan Plateau.

The disease has been reported from Philippines, Korea, USSR, China, Indonesia, Taiwan, Mexico, Nicaragua, Ceylon, Thailand, Bangladesh, Cambodia and Vietnam, Malaysia, Northern Australia, Nepal, Pakistan, Latin American Countries, Niger and West Africa, Senegal River Basin, Upper Volta, and Ukraine.

The disease was first reported in India by Srinivasan in 1959 from Maharashtra, where it was widespread and destructive since 1951. The disease appeared in an epiphytotic form in Shahabad district of Bihar in 1963 on variety BR-34. After the introduction and cultivation over a large acreage of new high yielding but susceptible rice cultivars, the disease has become one of the most serious problems on rice in India.

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3. Economic Importance of Bacterial Leaf Blight Disease of Rice:

The disease is a typical vascular wilt, leaf blight being only the mild phase resulting from secondary infection. Damage is due to the partial or total blighting of the leaves or due to complete wilting of the affected tillers, leading to unfilled grains. In the wild phase, these results from early nursery infection, the crop dry up completely before seed maturation.

Reductions of 20-30 per cent have been observed in grain yields when infection was moderate and over 30 per cent when it was severe. A loss of about 47-75 per cent in yield has been reported in artificially inoculated crop. Bacterial leaf blight caused yield losses upto 50 per cent in paddy field.

Singh and Nanda (1975) studied about the correlations and path analysis of yield, yield components and incidence of bacterial leaf blight of rice. In 6 rice cultivars and their 15 F₁ hybrids, grain yield was positively correlated with number of panicles/plant and number of grains/panicle and to a lesser extent, with panicle length was negatively correlated with incidence of bacterial blight disease.

Pre-boot infection caused more loss in yield than the late infection. Pre-emergence infection of flag leaf reported in 38 to 40 per cent loss in grain yield. Under field conditions, yield varied from 10 to 65 per cent.

Lee (1975) reported that yield reduction was directly related to the amount of infection on the upper leaves at the heading stage. When lesion occupied 60 per cent of the leaf area, 50 per cent of the yield was lost. When 20 per cent was affected the loss was less 45 and 60 per cent losses in TN-1 variety were recorded under low, moderate and severe infection.

Highly susceptible cultivars to the disease with similar disease rating differed considerably in disease severity and consequent losses in grain yield, which ranged between 14.7 and 81.3 per cent. Saket 4 was least affected followed by Jayanti, Sarjoo-50, Sona and IR 24. The disease caused substantial decrease of 50-75 per cent in head rice recovery.

The loss in yield has been attributed due to increase in chaffiness, decrease in grain weight and number of panicle. As a result of reduction in the number of filled grains in diseased panicle, a loss in weight of 20.38 per cent was obtained. In assessing the effect of X. oryzae, 33.2 per cent more chaffy glumes and reduction in 1,000 grain weight were recorded in infected tillers than in healthy tillers.

A significant loss in yield was observed in all the 19 high yielding varieties tested due to the disease. The reduction was maximum in Bala and least in Cr-44-35.

Potential losses in grain yield to the disease was 65 per cent and 10 per cent in highly susceptible Karyona and moderately susceptible IR-8, respectively and insignificant in IR-22 under field conditions. At IRRI, Philippines, losses in yield were 74.89 and 46.88 per cent in IR-1 and Taiwan-8, respectively.

Losses in yield under condition of natural infection by the pathogen were correlated with the severity of kresek phase. When the wilt was severe, chaffy grains increased to 55.59 per cent and 1,000 grain weight was reduced by 49.62 per cent resulting in yield loss upto 94 per cent per hill.

The estimated yield losses due to bacterial blight in tropical Asia vary from 2 to 74 per cent depending on location, season, weather, crop growth stage and cultivar. Grain quality of diseased crops was least affected in Saket-4 and most in IR-24. Protein content in grains of the diseased varieties increased in all the cultivars tested.

Yield loss simulation (YLS) model was used to simulate the effect of bacterial leaf blight on the growth and yield of rice cultivar IR-64. Simulation results of the YLS were then compared to those of the blight model.

The stimulated reduction in the total dry matter due to BLB with the YLS and the blight varied by 17.6 to 22.1 and 6.6 to 15.5 per cent, respectively, compared to the observed reduction of 11.0 to 21.4 per cent in disease treatments.

Crop loss assessment studies have revealed that this disease reduces grain yield to varying levels, depending on the stage of the crop, degree of cultivar susceptibility and to a great extent, the conduciveness of the environment in which it occurs. The disease is known to occur in epidemic proportions in many parts of the world, incurring severe crop loss of up to 50 per cent.

Several epidemics have been reported due to this disease in India. The first epidemic was reported from Shahabad district of Bihar in 1963-64, in 1965-66 from Andhra Pradesh, in 1980-81 from Punjab, Haryana and Western Uttar Pradesh and recently from Kerela in 1998.

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4. Symptomatology of Bacterial Leaf Blight Disease of Rice:

Bacterial blight has three significant symptoms viz. leaf blight, pale yellow and kresek. The leaf blight phase symptoms develop mostly on leaf blades, leaf sheaths and sometimes on grains. The symptom development depends upon the rice variety, physiological condition of rice plant, virulence of the pathogen and climatic conditions.

In rice seedlings small water soaked spots appear on the edges of lower leaves. These spots enlarge and gradually turn yellow. The leaves of the disease affected plants during tillering phase roll up, droop, turn yellow, or grayish brown and finally wither.

Leaf blight phase usually appears as tiny water soaked lesions at the fully developed leaves from the tip. The lesions enlarge both in length and width with a wavy margin and turn yellow within a few days. As the disease advances, in lesions cover the entire blade, turn white and later become grayish from the growth of various saprophytic fungi.

Lesion may also start at any point on the blade if it is injured. On the surface of young lesions, milky or opaque dry drops may be observed in the early morning. They dry up to form small, yellowish, spherical beads, which are easily shaken off by wind and drop into the field water.

The symptom of pale yellowing of rice leaves was first noted in the Philippines. Pale yellow leaves can be found on 3 week old seedlings when artificially inoculated in the field on plants that are tillering. While the older leaves are normal and green, the youngest leaf has either a yellow stripe on the blade or uniformly pale yellow.

On resistant cultivars, a yellow stripe appears just inside the margins of leaf blade, with no formation of necrotic lesions for some time. The stripes may eventually turn yellow and necrotic. On susceptible cultivars, infected blades wilt and roll as the diseased portion enlarges while the leaves are still green.

The entire blade may dry up. The lesion may also extend to the leaf sheath where they may reach to the lower and on these cultivars. In severe infection, symptom on the glumes appears as discoloured spots surrounded by a water soaked halo.

Kresek (wilt) phase of the disease appears due to systemic infection of host by the pathogen. Initially it was described as a separate disease by Reitsma and Schure (1950) but later it was confirmed as an additional symptom of the bacterial blight. The term ‘Kresek’ is Indonesian means a resulting sound of withered leaves.

Initial symptom appears as green, water soaked spot just beneath the cut surface, which soon turn grayish green. Rolling and withering of the entire leaf including the leaf sheath occur later. The bacterium spreads through the xylem vessels and infects the base of other leaves. This is direct relationship of root injury and the kresek phase of the disease.

The greatest incidence of Kresek was reported to occur in 14 and 21 days old seedlings that were exposed to X. oryzae in the seedbed, 24 hours before transplanting. Goto (1964) identified the causal organism as X. oryzae and confirmed that kresek is one of the symptoms of the bacterial blight syndrome. The kresek phase of bacterial blight has been reported in the Philippines, Malaysia, India, Sri Lanka, China and Korea.

Reddy (1983) proved the movement of bacterium upward in rice seeding growing from infected seed, showing that the bacterial wilt is transmitted by seed. In the infected crown region of plant soft rot also develops which is extended to apical part of the clump in the older plants.

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5. Causal Organism of Bacterial Leaf Blight Disease of Rice:

Taxonomy and Nomenclature:

A number of modern approaches to bacterial taxonomy, classification and nomenclature seem to be promising especially with the bacterial blight pathogen. In 1908, Takaishi found bacterial masses in dew drops of rice leaves but he did not name the organism. Bokura in 1991 isolated a bacterium, and after a study of its morphology and physiology, the bacterium was named Bacillus oryzae Hori and Bokura.

Ishiyama (1922) studied the disease further and renamed the bacterium Pseudomonas oryzae Uyeda and Ishiyama according to Migula’s system. It was later transferred to Xanthomonas oryzae.

According to the revision of the international code of nomenclature of bacteria the committee of taxonomy of phytopathogenic bacteria of the International Society of Plant Pathology adopted the name Xanthomonas campestris pv. oryzae Dye. In 1990, the pathogen was elevated to a species status and was named Xanthomonas oryzae pv. oryzae.

Biology and Ecology:

Morphology:

The causal bacterium is gram-negative, non-spore forming, short rods with round ends measuring 1.2 x 0.8-1.0 mm with monotrichous polar flagellum of 6.8 mm. Bacterial cells are surrounded by mucous capsules and are joined to form an aggregated mass, which is relatively stable even in water.

Colonies are circular, convex, whitish yellow to straw yellow later, with smooth surface and entire margin and opaque against transmitted light. The yellow pigments are insoluble in water.

The causal bacterium is a strict aerobe. The optimum temperature for growth is found to lie between 25 to 30°C. It fails to grow at 5°C and at a maximum temperature of 45°C. It cannot tolerate more than 3 per cent concentration of sodium chloride.

The thermal death point is 51 to 53°C. Good growth is obtained at 6.5 to 7.5 pH levels. The isolates vary to a large extent in the production of acid from carbohydrates and related carbon sources.

They produce acid from xylose, glucose, fructose, galactose, sucrose, cellobiose and starch. They fail to utilize inositol, glycerol, mannitol and sorbitol. The bacterial isolates are negative to methyl red and vigor-poskar tests. They can utilize citric, lactic, pyruvic, succinic and fumaric acids but fail to utilize benzoic, oxalic, propionic and tartaric acids. The isolates vary in degree of liquefication of gelatin.

They exhibit proteolysis with alkaline reaction in milk media. Indole and acetone are not produced and they fail to reduce nitrate to nitrite. All the isolates can produce ammonia and hydrogen sulphide but they vary in the degree of hydrolysis of ‘soluble’ starch and potato starch.

They are found to be negative to lipolytic activity but positive to the utilization of uric acid. Nucleic acids are utilized. Lecithinase is not produced. They are weakly cellulolytic, pectic methyl esterase is not produced but poly galacturonase is produced. The isolates of the cfmsal bacterium vary in the rate of utilization of different carbon and nitrogen source.

Host Range:

Xanthomonads have generally very specific pathogenicity under natural conditions. Under artificial inoculation, Leersia oryzoides var.japonica, L. oryzoides, Zizania latifolia and Phalaris arundincicea can become severely infected with bacterial blight. Of these, L. oryzoides var. japonica is commonly found infected in nature.

Over wintering of the bacteria on roots and rhizomes of these plants was confirmed by the phage technique.

Lesion developed much earlier on L. sayanuka than on rice, hence the former was considered to function as an active natural host in Japan. Natural occurrences of bacterial blight on Cyperus rotandus and C. difformins were reported by Chattopadhyay and Mukherjee (1968), while Pandey (1970) tested 32 common weeds and concluded that none of them could be considered active hosts.

Under artificial inoculation, L. lexandra was found susceptible by Rao and Kauffman (1970) and L. lexandra and Paspalum scrobiculatum by Reddy and Nayak (1974). Wild rices Oryza sativa var. spontanea and O. perennis, commonly found in and around rice fields in coastal area of India and act as potential source of inoculum for neighbouring rice crop.

Buddenhagen (1987) examined BB occurrence in rice in Australia, Africa, Latin America and Asia and concluded that the pathogen survives and evolves along with wild rices. The domesticated rice crop gets the infection from the neighboring wild rice plants.

It is presumed the disease has many centers of origin in the rice growing world and that each evolved separately with wild rices present on the different continents, as was seen in O. glaberrima, O. barthii and O. longistaminata in Mali, Cameroon and Niger and O. rufipogon and O. australensis in northern Australia.

Disease Cycle:

The over wintering pathogen is transferred to the nursery by irrigation water, application of diseased straw and sowing of diseased rice grains. The bacteria coming in contact with the surface of rice seedlings become activated and multiply. They invade the tissue through wounds, which are produced at the basal part of the stem by root development, or through hydathodes on the leaf blade (Mizukami, 1961).

Invasions also occur through stomata, which remain open continuously and are distributed on the coleoptile and sheath of foliage leaf. After invasion, the bacteria multiply in the intercellular spaces of parenchyma without showing any symptoms. They are then exuded, thereby increasing the bacterial population in the nursery.

After multiplication in the host, the bacteria reach to a population of several million within 3-4 days. When the bacteria reach a sufficient large population to disrupt the normal physiology of the rice plant, symptoms occur in the form of leaf blight. After that bacteria exit from the plant in ooze and become available for dissemination.

It is known that windstorms and rainstorms increase dissemination and also disseminated by irrigation water and infect young plants, infected plants may die. After the death of these plants bacteria become released and reinfect seedlings generally after 2-4 weeks of transplanting leads to kresek phase which is most destructive phase of this disease.

Perpetuation:

The pathogen perpetuates in diseased stubbles in double cropped areas of rice in India and it cannot survive from season to season in single cropped areas. Ratoons in lowlands with plenty of soil moisture constitute the source of primary inoculum in some parts of the country. The pathogen survives in dry form and growth form.

The dry form is in a dormant state and found on the diseased plants as an aggregated mass (bacterial exudates) or in xylem and xylem parenchyma of the infected tissues. The pathogen survives for longer periods at low humidity and low temperature, while it is killed in a short period at high humidity and high temperature.

There is evidence that seed from infected crop is the major source of primary inoculum in the single cropped area. Infected self-sown rice plants in lowlands serve as a source of inoculum in some of the single cropped areas, whereas, such plants in uplands and medium lands constitute the source of primary inoculum in double cropped areas.

Seed infection occurs through the vascular system and 90 per cent of seed infection is observed immediately after harvesting. There is also possibility of over wintering of the bacterium in the rhizosphere of the crops followed by rice during winter. There is no evidence regarding the perpetuation of pathogenic bacteria in soils of infected rice fields, except in rhizosphere of Leersia plants.

Epidemiology:

Different environmental factors that greatly influence disease development are temperature, relative humidity, rainfall and a little sunshine and strong winds influence bacterial leaf blight during rice growing season. When mean temperature more than 24°C, relative humidity ranges from 64-84 per cent, rainfall more than 200 mm, little sunshine and strong winds coincide, this disease appear in the form of epidemic.

A combination of meteorological factors such as high temperature, high humidity, heavy rainfall, high light intensity and frequent typhoons favoured the outbreak of the disease.

He (1982) studied the onset of an epidemic of Xanthomonas oryzae pv. oryzae could be forecasted by recording the number of rainy days (more than or equal to 1 mm) for 20 days before it began and the average temperature for three months (December to February) in winter.

Mew (1993) studied disease progress of bacterial leaf blight as related to stages of plant growth into seedbed, seedlings, panicle initiation, flowering and mature grain stages and found out that both hill infection/plot and leaf infection increased from seedbed to flowering stage and was found maximum at flowering stage i.e. 100 days after seeding while in the case of mean lesion area the inoculation first increased up to 60 days and thereafter, it decreased from 60-80 days after inoculation and after that these was sharp increase in the mean lesion area.

The reason behind the decrease in the mean lesion from 60-80 days was green area on leaves and change of resistance of the growing rice plants.

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6. Cultural, Morphological, Biochemical and Pathogenic Variability of Bacterial Leaf Blight Disease of Rice:

Cultural and Morphological Variability:

Xanthomonas according to strict latin usage; Xan.tho.mo’nas is commonly used, Gr. adj. Xanthus yellow, Gr. fem. n. monas unit, monad; M.L. fem. n. Xanthomonas yellow monad. Colonies of all species of Xanthomonas are normally yellow, smooth, round, entire and butyrous, at least when young, but may show surface markings such as striations and become lobed when older.

The faster growing pathovars of X. campestris produce visible colonies from single cells in 24-36 hours at 25°C. Slower growing isolates of this become visible. The time taken to produce colonies about 1 mm in diameter at their temperature varies from 2 or 3 days to a week or 10 days.

Nwigwe (1973) observed variation in colony colour and size. He grouped them into yellow to wax yellow glistening type (2.5-3.3 mm) and white spherical butyrous type possessing faster growth potential (3.5-5.5 mm).

Goto and Okabe (1967) differentiated the bacterial colonies as fluidal and smooth types and considered them more virulent and less virulent. The experiment conducted at IRRI, Philippines, also revealed that the mucoid colonies were virulent than the translucent colonies.

In India, Shekhawat and Srivastava (1968) reported identical white yellow colonies among six isolates of X oryzae, varying in pathogenicity, that were cultured on nutrient agar and all the isolates turned to straw yellow later.

Sreeramulu and Nayadu (1987) tested four isolates of X campestris pv. oryzae for various cultural, biochemical and pathogenicity characters. All isolates grew well on YDC medium. The 48 hours old colonies were round, deep yellow, raised muciod measuring 0.1-0.2 cm in diameter.

The colonies of the isolate Bu 26 were larger in size with smooth and entire margins. On Wakimoto’s PSA medium, the bacterium grew well and the colonies were yellow. On tetrazolium chloride agar medium, the growth of the bacterium was restricted and the colonies developed pink centers with white margins.

Biochemical Variability:

Anonymous (1970) reported that sucrose was the best carbon source for the growth of X campestris pv. oryzae followed by glucose, mannose, galactose and maltose. Whereas, Valluvaparidasan and Mariappan (1988) demonstrated that sucrose was the most preferable carbon source for X. campestris pv. oryzae followed by D-galactose, D-glucose, maltose, lactose and succinic acid in the order of preference.

Among the amino acids tested, glutamic acid and cystine were utilized more efficiently than D-L alanine, L-proline and tryptophane. Mukoo and Watanabe (1958), Tanaka (1963) and Watanabe (1963) claimed that the most favourable nitrogen source was glutamic acid followed by cystine.

Hsu (1966) reported L-glutamic acid, L-aspartic acid as the best source of nitrogen and cystine supported some growth at low concentration. Chakarvarthi and Rangarajan (1967) found tryptophane inducing growth of X. campestris pv. oryzae. Rao and Nayude (1978) observed glutamic acid as better source of nitrogen for the bacterium.

Pathogenic Variability:

Variation in virulence of X. oryzae has been studied by several workers. The breakdown of resistance of the variety Asakaze’ in Kyuchu district of Japan in 1957 was the first indication for existence of new strain in the pathogen. Extensive studies were made to group the strains or races of the bacterium in terms of ‘virulence’ or ‘pathotype’.

Kuhara (1965) classified few isolates of Japan into three pathogenic groups based on qualitative disease reaction OR’ and ‘S’ types) on resistant and susceptible varieties. But a collection of 118 isolates of the pathogen was divided into two groups (A and B).

The isolates of the bacterium in A group were capable of infecting all the 20 tested varieties of rice whereas B group isolates did not invade appreciably in six resistant varieties. The two pathogenic groups were found distributed or existed together in the same locality.

In Philippines, Ou (1977) observed variation in the pathogenicity of the bacterial isolates but did not recognize distinct races among them whereas further study revealed the existence of its races. The differential reaction of ‘Isabela strain’ and strains 1 and 2 of the pathogen on certain cultivars of rice also showed the presence of distinct races in the bacterium.

In Korea, among the five pathotypes recognized, four have been recovered from a single cultivar Milyang 23 itself. Existence of races/pathotypes in the pathogen has been indicated in other countries also. The Asian isolates have been considered as vertical pathotypes. Conversely, in Indonesia both specific (vertical) and non-specific (horizontal) reactive groups of the pathogen have been recorded.

Devadath and Padmanabhan (1969) reported pronounced variation in virulence among nine isolates of the pathogen, which reacted differentially on the seedlings of 20 varieties. The variation in the virulence has also been revealed from the experiment conducted with 13 isolates on 10 varieties.

Mohiuddin and Kauffman (1977) grouped the isolates of X. oryzae into eight virulence pattern based on reactions to seven varieties.

In the endemic regions of the country, such as Aduthurai, Hyderabad, Maruteru, Cuttak, Chinsurah and Faizabad, both the virulent and avirulent strains of the pathogen are distributed and 72 isolates, obtained from such locations have been grouped into seven pathotypes.

The grouping of isolates of X. oryzae into strains/pathotypes varied from country to country and uniformity has not been observed as the differentials used by different workers are not the same. The virulence of the pathogen is also influenced by the variety on which the infection has occurred.

Durgapal (1985) obtained isolates from rice in this epiphytotic year were highly virulent and showed a pattern of enhanced aggressiveness toward cv. Jaya, the most popular in the region. On the basis of reaction of BJ-1, TKm-6, T-65 and TN-1, the 1980 isolates could be differentiated into 5 pathogenic groups designated pathotypes I-V.

Based on the reactions on a set of 9 cultivars and lines, DV 85 (Xa-5, Xa-7), CAS 209 (Xa-10) and IR 1160-8-6-1 from IRRI; Kinmaze, Kogyoku (Xa-1, Xa-3) and Wase Aikoku 3 (Xa-3) from Japan and 3 Indian breeding lines, B 76, ARC 10464 and CNGS 20083, 11 different pathotypes were identified.

Sahu (1988) screened 200 rice cultivars using 4 X. campestris pv. oryzae pathotypes. The cultivars were grouped into 7 genotypically different classes. It is suggested that groups 1, 2, 3 and 6 had genotypes with Xa4, Xa5, Xa10 and Xa5 genes, respectively.

Group 7 cultivars conferred resistance due to recessive gene Xa5 and dominant gene Xa7. Groups 4 and 5 posses 2 dominant genes (Xa4 and Xa10) and group 5 conferred resistance due to Xa5 and a recessive gene closely linked to it. Groups 8 and 6 indicated resistance at tillering and panicle initiation stages, respectively.

Reddy and Reddy (1989) tested virulence of 150 isolates of X. campestris pv. oryzae in glasshouse, representing 25 endemic locations in India, using clip inoculation on a selected set of rice differential varieties. The population first divided into two distinct groups.

Pathotypes I did not behave as a homogeneous population. But IR 20, with the Xa4 gene, responded differentially on repeated tests of isolates of pathotype I. Isolates from eastern U.P., Maharashtra and Punjab belonged to pathotypes la, to which IR 20 was resistant. Isolates from Andhra Pradesh, Bihar, Gujarat, Haryana, Kerela, Orissa, Tami Nadu and Western U.P belonged to pathotypes lb to which IR 20 was susceptible.

Noda (1990) distinguished seven races using 5 Japanese differentials but when 18 differential varieties from several Asian countries including Japan, were used, races I, II and III could be subdivided further into 10, 17 and 3 subgroups, respectively. Five cultivars were resistant to all the isolates tested.

The resistance of rice to pathotype V of X. oryzae pv. oryzae in Guangdong, China was studied. Exotic and domestic germplasm (1076) were screened of which 29 were resistant.

Of these IRBB 7, IRBB 5, 8072-2, MI 045 and RP 2151-21-2 were highly resistant to pathotypes I, II, III, IV and V and had good agricultural characteristics. Among the 14 bacterial blight resistance genes identified, Xa-3, Xa-5, Xa-7 and Xa-13 gene -were effective against pathotype V.

Qian (1997) studied 108 isolates of X oryzae pv. oryzae. They were identified using 5 basic differentials by clip inoculation. They were identified as pathotypes 0, I, II, III and IV according to disease reaction.

Among these isolates, the frequency of pathotype IV was the highest (28.7%) and the distribution of pathotype IV was the greatest (61.2%) followed by pathotype III and pathotype II. Results showed that most currently used rice varieties have no resistance to pathotype IV.

Hifni and Kardin (1998) isolated 106 isolates of X. oryzae pv. oryzae and grouped into 12 pathotypes. The simplest pathotype (pathotype I) has atleast three genes for virulence to overcome Xal, Xall and Xal4 resistance genes, respectively. Pathotype V, the most dominant pathotype (46.23%), has atleast seven virulence genes to overcome Xa1, Xa2, Xa3, Xa4, Xa10, Xall and Xa14.

Adhachi and Oku (2000) studied a detection method specific forX. oryzae pv. oryzae, based on the PCR and designed by amplifying the 16s-23s rDNA spacer region.

The nucleotide sequence of the spacer region between the 16s and 23s rDNA, consisting of approximately 580 bp from X. oryzae pv. oryzae, X. campestris pv. alfalfae, X. campestris pv. campestris, X. campestris pv. cannabis, X. campestris pv. citri, X. campestris pv. cucurbitae, X. campestris pv. pisi, X. campestris pv. pruni and X. campestris pv. vitians, was determined sequences had more than 95 per cent identity. Therefore, a pair of primers, XOR-F and XOR-R₂ was designed and found to specifically amplify a 470 bp fragment from all strains of X. oryzae pv. oryzae isolated from diverse regions in Japan.

Aansari and Sridhar (2001) tested for their virulence using a set of IRRI rice differential varieties, IR 24, IR 8, IR 20, IR 1545, DV 85 and Cas 209 carrying Xal8, Xall, Xa4, Xa5rn, Xa5 and Xal and XalO resistance genes, respectively.

In addition, there ability to cause kresek was also observed in the same differentials by crown inoculation method. The isolates showed distinct differences in their virulence pattern and based on the pathogenicity characters these isolates were classified into nine races/pathotypes.

Madhiazhagan (2001) determined the existing pathotype of X. oryzae pv. oryzae in Union Territory of Pondicherry and Tamil Nadu, India. Ten plants of each of differential rice cultivars IR 8, IR 20, BJ 1, DV 85, Cempo selak and Java 14 were separately inoculated with 25 isolates.

The reactions of cultivars were observed and classified as resistant and susceptible, following 0-9 scale. Sixteen isolates produced susceptible reaction on IR8, IR 20, Cempo selak and Java 14 and resistant reaction on BJ 1. Seven isolates produced susceptible reaction on IR 8, IR 20, BJ 1 and DV 85 and resistant reaction on Cempo selak and Java 14 and two isolates produced susceptible reaction in all the cultivars.

Noda (2001) observed that the strains of X. oryzae pn.oryzae collected in Yunnan province, China, during the period from 1994-1996 were polymorphic for virulence to the 12 near isogenic lines harboring the resistance genes Xa1, Xa2, Xa3, Xa4, Xa5, Xa7, Xa8, Xa10, Xa11, Xa13, Xa14 and Xa21 and the three check varieties, IR 24, Toyonishiki and Sigadagabo.

One hundred thirty eight strains were classified into 10 pathogenic groups (pathotype A to J) based on their pathogenicity.

The races and differential rice cultivars of bacterial leaf blight were selected based on relationship between 40 major rice cultivars and 150 isolates of X. oryzae, which were collected from different ecological regions of Korea. Results indicated that the pathogen has four races and its differentials are classified into four groups.

Genetic Variability:

There are many methods for assessing genetic variation in the germplasm, but no single method is adequate for this purpose because different methods have sample variations at different levels and differ in their genetic resolution power as well as the quality of information content.

Restriction fragment length polymorphism and virulence analyses used to evaluate the population structure of X. oryzae pv. oryzae from several rice growing countries in Asia.

Two DNA sequences from X. oryzae pv. oryzae, IS 1112, an insertion sequence and avr Xa10, oryzae, IS 1112, an insertion sequence and avr Xa 10, a member of family of avirulence genes, were used as probes to analyze the genomes of 308 strains of X. oryzae pv. oryzae collected from China, India, Indonesia, Korea, Malaysia, Nepal and the Philippines.

On the basis of the consensus of three clustering statistics, the collection formed five clusters. Genetic distances within ranged from 0.48 to 0.64. Three of the five clusters consisted of strains from a single country. Strains within two clusters, however, were found in more than one country, suggesting patterns of movement of the pathogen.

Chunlian (2000) collected strains of X. oryzae pv. oryzae from 11 provinces in China and assessed by using the inoculation method and insertion sequence based PCR. Some 114 were classified into different pathotypes by inoculation of 5 differential rice varieties.

Most strains were classified into pathotypes IV, II and 0. A new pathotype was identified IS-PCR showed that the genetic diversity of the total X. oryzae pv. oryzae population was H = 0.82.

Genetic diversity of the pathotype was highest for pathotypes V, VII and the new pathotype (H = 1.0). Genetic diversity was 0.91 for pathotype 0, 0.80 for pathotypes II, 0.73 for pathotype I, 0.71 for pathotype IV, 0.62 for pathotype III and 0 for pathotype VI.

Gupta (2001) analyzed 16 isolates of X. oryzae pv. oryzae representing different geographical locations in India and 2 isolates from the Philippines using RAPD. The primers OPA-03, OPA-04, OPA-10, OPA-11, OPK-7, OPK-12 and OPK-17 generated simple, specific and reproducible fingerprint patterns, indicating usefulness of RAPD markers in differentiating X. oryzae pv. oryzae isolates.

At a similarity index of 0.5 the isolates were grouped into only 2 clusters, suggesting these primers are not very efficient in grouping isolates. Primer PJEL-1 and PJEL-2, used in insertion sequence IS 1112 based PCR, also generated specific and reproducible fingerprint patterns for the same set of the isolates.

Based on the RAPD-PCR (7 primers) and IS 1112-PCR (2 primers) data, at a similarity of 0.57, sixteen and of 18 isolates were grouped into 5 different clusters and 2 isolates were loosely grouped with them.

Jalaluddin (2005) studied pathogenic variability and genetic diversity of 35 isolates of X. oryzae pv. oryzae. Virulence analysis of 35 isolates through inoculation on 11 near isogenic lines and rice varieties/strains IR 24, TN-1, Asominori and M 95, each carrying a specific BLB resistance gene showed high level of virulence diversity among the populations, 23 races were identified.

Among these races, nine (1 to 9) were virulent on 9 to 14 of 15 differential rice varieties. Two highly aggressive races (race 1 and race 2) have overcome the resistance of 14 differential rice varieties. PCR based fingerprinting of 20 selected Bangladeshi strains and five Japanese races showed to distinct major groups.

Most of the Bangladeshi races belonged to group I while all five Japanese races and a few Bangladeshi races belonged to group II.

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7. Disease Management of Bacterial Leaf Blight Disease of Rice:

Host Resistant:

Genetic resistance is the most effective, practical and economical method of disease management. Considerable emphasis is being given on identification and incorporation of bacterial blight donors in commercial varieties using conventional breeding methods and molecular approaches.

Most of the work done on host resistance in India and elsewhere has been reviewed by several workers including Mizukami, 1966; Khush, 1977; Devadath,1985; Gangopadhyay and Padmanabhan, 1987; Mew, 1987 and Chaudhary and Nayak, 1987 giving detailed information on the mechanism and genetics of resistance.

Methods of Inoculation:

Although varietal resistance can be tested in the field under natural infection, the results may not always be consistent because of seasonal variation. Artificial inoculation minimizes such problems. To evaluate bacterial blight resistance, four methods have been devised and commonly used.

(a) Needle-Pricking Method:

The needle-pricking method was widely used in Japan until the clipping method was introduced. The method was found to produce lesions identical to natural infection. Four large-scale mass screening in the field, the needle-pricking method has proved to be too laborious.

(b) Clipping Method:

The clipping method takes advantage of the fact that bacterial blight is a vascular disease; by clipping off the leaf tip, inoculation in directly deposited in the infection court. In this method, a minimal inoculum is 10⁴-10⁶ cells/ml, but under field conditions of more uniform result is obtained with an inoculum of 10⁸-10⁹ cells/ml.

Experience at IRRI indicated that different levels of resistance can be measured by varying the inoculum density.

(c) Dipping Method:

In the dipping method the root and basal part of the seedlings are clipped in a bacterial suspension before transplanting. This method was unique because resistance of the cultivars to the kresek symptom can also be evaluated, but a fairly large number of plants per entry are needed to achieve reliable results.

(d) Spraying Method:

In the spraying method a bacterial suspension is sprayed directly onto the plants. Keeping the inoculated plants in a moist chamber may be necessary in the dry season or when humidity is low. Disease development was usually slower than with the clipping or the needle-pricking method.

Varietals Resistance:

As a result of efforts on breeding and selection by various workers, some resistance donating lines and a few commercial varieties have been identified.

However, none of the varieties tested so far have been found immune. Among the popularly grown rice cultivars IR 20, IR 22, DV 85, DV 86, TKM 6, BJI, DZ 78, PR 109 and Ratna have been identified as resistant while IR 8, IR 24 and Jaya as mildly susceptible and Padma, Pankaj, Jagannath, Bala, Krishna, Cauvery, Yamuna, Kanchi, Vijaya, Karuna and Dulari as susceptible.

Reports are on hand indicating resistant varieties viz., IR 20, IR 36, Sasyasree, Sona Mahsuri, Vijaya Mashuri, Samba Mashuri, Swarna, Janaki, Radha, Sujata, Asha, Deepti, Usha, Ratnagiri 68-1-1, Rama Krishna, Udaya, Bharathidasan, PR 4141, CO 43, CO 44, IR 50, Saket 4, Prasad, Govind, Pant Dhan 4, Pant Dhan 6, Biraj, Suresh, Asha, Ananga, Deepti, Jaya, Gayatri, Govida, IR 36, IR 20, Jayati, Purijat, PR 4141, Ramkrishna, Shnkan, Sufala, Seema, Usha, Pant Dhan 4, Pant Dhan 12, Ratna, Saket and Bhuban released in India.

Important rice varieties developed by IRRI include, IR 8, IR 20, IR 22, IR 24, IR 26, IR 28, IR 29, IR 30, IR 32, IR 34 and IR 36. Out of these IR 20, IR 22, IR 26, IR 28, IR 29, IR 30, IR 32 and IR 34 are resistant to bacterial blight.

Biological Control:

Reddy (1977) studied the effect of bacteriophage on bacterial blight of rice. When the intact and injured root portions of 7 days old rice seedlings were immersed in a bacteriophage suspension, the phages were translocated through the vascular system.

Lesion development in the leaf due to X. oryzae pv. oryzae was completely inhibited when uprooted plants were supplied with phage through the roots for 12 h before transplanting and bacterial inoculation.

Saikia and Chowdhury (1993) reported the influence of phylloplane microflora on bacterial leaf blight development using a range of inoculation methods. Erwinia herbicola was found most effective even at the lowest ratio (1:1), and gave 90 per cent disease reduction at a ratio 50:1. Bacillus subtilis was the second most effective antagonist.

Sindhan (1997) reported that the phylloplane organisms isolated from rice leaves, Pseudomonas acidovorus, Aspergillus ochraceus, A. flavus, A. niger, Fusarium pallidoroseum, Penicillium janthinellum, Streptomyces sp. and Micrococcus sp. Inhibited X. oryzae pv. oryzae in vitro.

Islam and Bora (1998) showed the efficacy of 2 plant growth promoting rhizobacteria, Azospirillum brasilense and Bacillus polytnyxa on bacterial leaf blight. They found that individual application was better as compared to combined application. Nayar (1998) demonstrated that talcum powder based formulation of the Pseudomonas fluorescens strain PI caused highest inhibition of X oryzae pv. oryzae.

Appreciable control was obtained with foliar spray at 0.1 per cent (3.6 x 19⁹ cfu/1). Combined application of the product by seed treatment, seedling root dipping and foliar spray, induced 60.70 per cent control of Xanthomonas oryzae pv. oryzae in the field.

Hong (1999) studied that Bacillus subtilis B56 showed antagonistic activity against Xanthomonas oryzae pv. oryzae.

Studies conducted on in vitro and in vivo evaluation of four antagonists namely Bacillus subtilis, Pseudomonas fluorescens, Trichoderma harzianum and Penicillium notatum against Xanthomonas oryzae pv. oryzae by dual culture method revealed that Bacillus subtilis, P. fluorescens and Trichoderma harzianum were inhibitory to the test bacterium.

Forty day old potted rice cv. IR 8 plants were inoculated with X. oryzae pv. oryzae and Bdellovibrio bacteriovorus at 1 : 1, 1 : 9 and 1 : 99 ratios of determine the efficacy of B. bacteriovorus in controlling X. oryzae pv. oryzae causing blight in rice.

Lesion length, kresek incidence and foliage blight in inoculated seedling decreased with increasing proportions of B. bacteriovorus in the inoculum. Vasudevan (2002) observed the mechanism of suppressing bacterial blight (caused by Xanthomonas oryzae pv. oryzae) by biological agents (e.g. Trichoderma and Pseudomonas spp.).

Modifications in Cultural Practices:

Increased productivity brought about by modern practices (modern varieties, high levels of applied N, etc.) tends to create ideal conditions of the development of diseases and insect pests. The problem is particularly acute in monoculture areas where two or more rice crops are planted in succession.

Cultural practices like proper leveling of field, good drainage and irrigation have been observed to keep down the incidence of bacterial leaf blight. Among the factors favouring bacterial blight development, application of high N levels is important in increased disease incidence and severity.

High N levels either favour pathogen multiplication and lesion enlargement or through promoting increased vegetative growth of the plant influence the microclimate in favour of the pathogen.

Reddy (1979) demonstrated that increased N levels increased bacterial blight and reduced yield. High disease severities negated N response and beyond a critical N level, the law of diminishing returns operated in susceptible cultivars. The optimum level of N application to derive maximum yield (with minimum disease effects) was 76 kg/ha rice for susceptible cultivars.

To cut costs and increase productivity in bacterial blight prone areas a positive prognosis was suggested involving omission of top dressing of rice with N when bacterial blight is severe at panicle initiation to minimize disease effects and optimize N response.

To manage bacterial blight various researchers have suggested integrated control measures, including using resistant cultivars, removing inoculum sources, using healthy seeds, avoiding flooding in nurseries and main fields avoiding excess N and applying N in splits.

Baruah (1991) studied that in areas where disease caused by X. oryzae pv. oryzae is endemic, basal application of urea in the form of neem cake coated urea and Mussoorie rock phosphated urea reduced disease incidence and gave higher grain yields than 3 split applications of prilled urea and basal placement of urea super granules with increased incidence.

Devadath and Padmanabhan (1976) observed that blight severity can e reduced through agronomic practices such as wider spacing (30 x 15 cm) and N application (80 kg/ha), especially in a variety with a very high tillering ability such as Taichung (native)- 1. A high K level is also essential for low incidence.

Have and Kauffman (197?) compared with a single N application; 2-3 top dressings decreased infection and increased yields. Yields of rice were similar when ammonium sulfate and green manures from water hyacinth (Eichhornia) and Ipomoea carnea were used as fertilizer, while development of the disease caused by Xanthomonas campestris pv. oryzae and Helminthosporium oryzae (Cochliobolus miyabeanus) was generally less plots receiving the green manures.

Recommendations of this type and many others that involve non­monetary crop management inputs to control bacterial blight are not readily accepted in rice farming communities, because the farmers do not see immediate benefits. Psychologically farmers accept from scientists and extension specialists ready made solutions in the form of agrochemicals to combat the disease when they see it.

Das (1998) conducted a field experiment over a period of 3 seasons to study the efficacy of some natural products like fresh cowdung, hing and antibiotics (Plantomycin) to manage bacterial leaf blight of rice in Bhubaneshwar, Odisha, India.

Foliar spraying of fresh cowdung suspension at 50 kg cowdung/ha reduced the incidence of BLB of rice significantly showing the lowest percentage of leaf area infected (18.53%) compared with 38.03 per cent in the unsprayed control coupled with the greatest mean grain yield of 41.36 q/ha.

The next in order of effectiveness was hing solution (0.2g/liter) and antibiotics (Plantomycin-copperoxychloride mixture at 2 g+1.5 g/liter) which were statistically at par with each other and minimized disease incidence moderately 22.0 and 23.56 per cent mean leaf area infested with average yield of 35.0 and 34.9 q/ha, respectively, compared with 25.0 q/ha in the untreated control.

Use of Natural Products/Botanicals:

Mary (1986) observed that a foliar spray of cowdung extract (20 g/lit) was as effective as Penicillin, Pushamycin and Streptomycin in controlling Xanthomonas campestris pv. oryzae. Leaf of Artabotrys hexapetalus and seeds of Moringa oleifera were found inhibitory to Xanthomonas oryzae pv. oryzae.

Grainge and Alvarez (1987) found that leaf extracts of Artabotrys hexpetalus formed 29 mm zone of inhibition against Xanthomonas oryzae pv. oryzae.

It was observed that plant extracts of rice cv. TKM 6 inhibited growth of Xanthomonas campestris pv. oryzae. Eswaramurthy (1995) demonstrated that incorporation of leaves of Azadirachta indica, Prosopis juliflora and Ipomoea carnea was highly effective in controlling rice bacterial blight.

Narasimhan (1995) studied inhibitory effect of Curcuma longa and Azadirachta indica against Xanthomonas oryzae pv. oryzae and found that active principle for antibacterial activity of Curcuma longa and Azadirchta indica was the protein part of the plant extract.

Buffalo urine at 10 per cent concentration was found to completely inhibitory to the mycelium of Macrophomina phaseolina and Fusarium oxysporum lycopersicae.

Das (1998) evaluated hing solution against bacterial leaf blight and they found that sprays of hing solution (0.2 g/lit) was very effective. Gangopadhyay and Gangopadhyay (1998) studied the effect on disease development of turmeric, aqueous solution of sodium bicarbonate (10 g/lit)] in field. None of the turmeric treated plants developed disease symptoms.

Aqueous extracts of few plant products coded as A and B and a mixture of A and B in 9:1 proportion were sprayed on bacterial leaf blight affected rice crop. The spray restricted the spread of disease and increased yield of 10 q/ha.

Neem gold (20 ml/lit) was found most effective against bacterial leaf blight. Madhiazhagam (2002) worked on the biological efficacy of five botanicals viz., Adhatoda vasiaca, Allium cepa, Azadirachta indica, Curcuma longa and Prosopis juliflora against Xanthomonas oryzae pv. oryzae.

Among the leaf extracts Adhatoda visiaca was found most effective in reducing the disease incidence upto 25.27 per cent as compared to the control that was 83.72 per cent. Meena and Gopalakrishnan (2004) evaluated 14 plants extract against Xanthomonas oryzae pv. oryaze by paper disc diffusion and food poison methods.

Maximum inhibition of bacterium was observed at 20 per cent (15.6 mm) and 15 per cent 12.2 mm) concentrations of Datura stramonium (Datura) followed by Azadirachta indica at 20 per cent (8.6 mm) and 15 per cent (6.8 mm) concentrations.

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Chemical Control of Bacterial Leaf Blight Disease of Rice:

A variety of chemical including antibiotics, fungicides, herbicides, antiseptics and other miscellaneous compounds have been tested in vitro against the X. oryzae pv. oryzae.

The effect of various antibiotics on the sensitivity of different X. oryzae isolates in vitro showed that 7 isolates were sensitive to Achromycin, Agrimycin-100 to varying degrees. Achromycin gave maximum inhibition zone to all the four isolates of X. oryzae followed by Chloromycetin, Streptocycline and Agrimycin-100.

The effect of various antibiotics on the sensitivity of 17 isolates of X. oryzae in vitro showed that all the isolates were sensitive to Chloromycetin to varying degrees. Some strains were insensitive to Chloromycetin. Chloromycetin was also observed to be effective against four isolates of the pathogen in vitro.

In vitro maximum inhibition zone was observed in Tetracycline-HCl followed by Terramycin, Ledermycin, Erythromycin and Chloramphenicol. Sodium Penicillin-G has given maximum inhibition at 100 and 500 ppm to the pathogen in vitro.

Variation in sensitivity to streptocycline against 17 isolates of X. oryzae was observed in vitro. Streptocycline inhibited the growth of four isolates of the pathogen when tested in vitro. Isolates of X. oryaze showed different degree of inhibition in Streptomycin sulphate. Penicillin at the rate 500 ppm was most inhibitory to the growth of X. campestris pv. oryzae in vitro.

Several groups of fungicides have been tested against X. oryzae in vitro including copper, sulphur, mercurial, benzimidazoles and other compounds. Copper acetate inhibited the growth of X. oryzae in vitro. Fytolan (copperoxychloride) gave good inhibition at 50 and 100 ppm to isolate of the bacterium from Philippines, but it was tolerated by an isolate from Hyderabad.

Kocide (Copper hydroxide) showed good inhibition of the growth of bacterium in vitro. Blitox-50 (Copper oxycloride) showed same inhibition of growth of the bacterium at higher (1000 ppm) concentration.

Dithane M-22 inhibited the growth of X. oryzae at 200 ppfn. Dithane Z-78 (zineb) tested at different concentrations of in vitro inhibited the growth of the bacterium except the isolate from Hyderabad.

On agar, Thiram effectively inhibited the growth of bacterium at 250 ppm and at higher concentrations, killing the bacterium at 600, 200 and 100 ppm with 6, 12, 24 hours contact, respectively in aqueous suspension at pH 7.0. Combination of Thiram with antibiotics like Agrimycin 100 and streptocycline were highly effective against the bacterium.

In addition, some systemic fungicides, metallic elements and other miscellaneous compounds have also been evaluated against X. oryzae pv. oryzae in vitro and some of them have been reported to be highly effective against the bacterium. Difolatan and Terraclor inhibited the bacterial growth in vitro.

Duter (fentin hydroxide) was next to mercuric chloride in effectiveness against X. oryzae in vitro (Balaraman and Rajagopalan, 1978Some synthesized organic arsine xanthate, had an antagonistic effect on the growth of the bacterium. Momilactones A and B prepared from extracts of rice husks were not effective against X. oryzae pv. oryzae in three different tests.

Of the 31 compounds tested against the bacterium lead chloride, cadmium nitrate, stannous chloride, cobalt nitrate, silver nitrate, alizarin, methyl blue, picric acid, cinnamic acid, P-phenyl diamine, phloroglucinol, 8-hydro quinoline, benzimidezole, sodium oxychalate, zinc chloride evans blue, Sudan III, rose Bengal and crystal violet inhibited the growth at higher concentration but cobalt and cadmium nitrate were toxic to the bacterium at 0.01 M.

Of 235 compounds evaluated, the most active against the X. oryzae pv. oryzae were zinc oxide, S-hydroxyqunoline and methyl- glyoxal at 0.0005 to 0.001 per cent, cadmium salt, o-phenanthronin, CGA 78039 and formaldehyde at 0.0001 to 0.0005 per cent, salt of Cu, Ni, Co were effective for all the strains of the bacterium at concentration between 0.001 to 0.005.

Zinc sulphate at 2.5, 1.25 and 0.625 per cent concentrations was tested against X. oryzae pv. oryzae in vitro, inhibited the growth of the bacterium. The inhibition zone decreased as the concentration of zinc sulphate decreased from 2.5 to 0.625 per cent.

Despite previous reports that Fentiazon which an efficient agent of the control of rice bacterial blight, has no activity against the causal organism in vitro, Fentiazon at 100 ppm was inhibitory to X. oryzae pv. oryzae on Suwa’s synthetic medium inoculated 10⁹ cells/ml.

The bacteriostatic activity is thus only detectable under condition of low population level and low growth rate of the bacterium. Similar activity in vitro to that of Fentiazon against X. c. pv. oryzae was also detected with probenazole which is a systemic fungicide.

Oryzamate (probenazole) had no inhibitory effect on the bacterium in vitro. The multiplication of the pathogen was significantly inhibited at 100 mg/ml of Propanil, Monlinate, terbutryn and aglypt, which are herbicide. The effect was mainly bacteriosatatical rather than bactericidal. Only propanil and Molinate at concentration of 100 mg and 200 mg/ml were bactericidal.

There are several reports indicating that chemicals including antibiotics, fungicides, herbicides, insecticides and other miscellaneous compounds have been tested and recommended for the control of bacterial blight of rice. Krishnappa and Singh (1977) observed that Agrimycin-500 (Streptomycin) significantly controlled the disease resulting in 51.34 per cent increase in yield.

When four antibiotics were evaluated in field of the control of the disease, Agrimycin-500, agricultural terramycin-17, Brestanol and Agrimycin- 100 + Fytolan, Agrimycin 100 + Fytolan gave good control and an economic return on rice cv. Sona. As antibiotic which is identical with formycin produced by a strain of Nocardis spp. showed a curative effect against X. oryzae pv. oryzae on rice.

The ability of nickel salts to control bacterial blight of rice was established by Rajappan and Vidhyasekaran (1996). A three per cent solution of nickel nitrate gave the most effective control followed by potassium nitrate, magnesium nitrate, calcium nitrate and sodium nitrate.

This suggested that only the nickel portion was effective in controlling disease. Chandrasekaran and Vidhyasekaran (1988) observed the effect of Chloramphenicol against bacterial blight of rice. In treated plants, 11 per cent of the leaf area was affected compared with 60 per cent in the untreated control.

Balasubramanian (1971) showed that in field trails with rice cv. Taichung Native 1, Streptocycline or Agrimycin, each applied as a foliar spray at 50 ppm, decreased Xanthomonas oryzae pv. oryzae infection by 50 percent. Yields were also increased by 250 ppm Agrimycin and 50 to 100 ppm Erythromycin. Natrajan (1988) tested several chemicals to control Xanthomonas oryzae pv. oryzae on rice plants.

Applications were made at 30 and 45 days after transplanting the crop. Bleaching powder was the most effective in reducing bacterial leaf blight followed by Plantomycin, Paushamycin + Copperoxychloride and Paushamycin alone. Mariappan (1988) conducted trials to test the efficiency of antibiotics and antibiotics + fungicide for the control of bacterial leaf blight.

Agrimycin 100 + copper oxychloride and Agrimycin 100 alone gave equal control and were significantly superior to control the disease with treatments involving Paushamycin and Streptomycin. Bhapkar (1960) suggested that spraying with copper fungicides before flowering for the control of the disease.

Fytolan gave good control of the disease and increased yield. Combination of Fytolan with antibiotic (Agrimycin-100) was effective against the blight phase of the disease and gave good control with economic return on susceptible cv. Sona.

Application of probenazole (Oryzamate) which is a non-fungicidal agent against bacterial leaf blight of rice, induced resistance mechanism in rice plant and controlled the disease. Among the 5 fungicides tested for the lesion length of bacterial leaf blight, MBC and Oryzamate (Probenazole) significantly reduced the lesion length over the control.

Foliage application of a new systemic bactericide shirahagens (Techlofthalam) to plants inoculated by spraying gave good control, but on plants inoculated by needle prick method, it failed to suppress the lesion development. No effect on plant physiological activity was observed. Soil application of Techlofthalam 0.3 and 0.6 q/m² inhibited lesion formation by the pathogen though the effect was somewhat delayed.

ADTA (2-amino-1-3, 4 thiadiazole) was bacteriostatic agent to some Xanthomonas spp. and was effective against the pathogen when applied as foliar spray or to water in the field. The effect of the former was rapid, but short lived, that of the later was slow but lasted longer. Fentin hydroxide controlled the disease in experiment conducted in pot culture.

Preventive spray of Fentiazon gave better control than curative ones. Spray applied immediately after infection gave better result than the spray given at later stage. Reports are on hand indicates that spray of ammonium sulphate, 5 days before inoculation significantly decreased the intensity of the disease.

The effect of several herbicides has also been tested for the control of the disease. Out of seven herbicides, Simetryn and Nitrogen due to effect on the host plant reduced disease severity and Propanil was effective only if applied immediately or up to one day before inoculation.

Studies on the effect of herbicides and insecticides on 12 rice varieties resulted that MCPA (2-methyl, 4-2 chlorophenoxyacetic acid) and 2, 4-D reduced severity of bacterial blight on IR-28 and TKM-6. The effect of four herbicides on the lesion length of the disease showed that it was reduced significantly with Machet as compared to remaining treatment Dath 1983.

Integrated Disease Management:

To achieve the desired level of rice production economically and safely, it is essential to combine all suitable techniques and methods of suppression in a judicious manner, which maintain pest population at levels below those causing economic injury.

Therefore, options like utilization of the resistant genes of the crop varieties, conservation and augmentation of natural enemies, suitable cultural practices and the need-based application of eco-friendly pesticides forms the essential components of the integrated disease management.

The integrated disease management (IDM) requires selection of cultivars possessing moderate resistance to the disease and capacity for leaf area compensation to overcome the yield reduction due to the disease and moderate levels of nitrogen application to delay in disease onset and spread.