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In this article we will discuss about:- 1. Introduction to Rice Knot Disease 2. Distribution of Root Knot Disease of Rice 3. Symptoms 4. Pathogen 5. Life Cycle 6. Resistance 7. Biological Control.
Contents:
- Introduction to Rice Knot Disease
- Distribution of Root Knot Disease of Rice
- Symptoms of Root Knot Disease of Rice
- Pathogen of Root Knot Disease of Rice
- Life Cycle of Root Knot Disease of Rice
- Resistance of Root Knot Disease of Rice
- Biological Control of Root Knot Disease of Rice
1. Introduction to Rice Knot Disease:
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Rice (Oryza sativa L.) is one of the most important food crops of the human race. The nutritional quality and high caloric food makes it a staple food of nearly half of the world population. It has been estimated that 8 billion people will populate the earth by the year 2020, and the expected 4.8 billion rice consumers will need 760 million tons of rice annually.
This means that rice production must be increased by 2 % per year to meet future demand. Presently, rice ranks third after wheat and maize in terms of worldwide production. Asia accounts for 90 and 92 per cent of the world’s rice area and production, respectively.
Thus, rice production, consumption and trade are concentrated in Asia. One third of Asia’s rice production is consumed in China and one fifth in India. Among the rice growing countries in the world, India has the largest area under the rice crop and ranks second in production after China.
Rice is cultivated in India in about 45 million hectares under irrigated (46%), rainfed lowland (28%), rainfed upland (12%), and flood-prone (14%) ecosystems.
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Like other crops, rice is also affected by a large number of diseases i.e. Blast of rice (Pyricularia oryzae), Bacterial leaf blight (Xanthomonas oryzae pv. oryzae), Bacterial leaf streak (X. oryzae pv. oryzicola), Sheath blight (Rhizoctonia solani), Brown leaf spot (Ilelminthosporium oryzae), Stem rot (Sclerotium oryzae), Narrow brown leaf spot (Cercospora janseana), Kernal bunt (Nevossia barclayana), Black sheath rot (Gaeumannomyces graminis var. graminis), Leaf smut (Entyloma oryzae) and Bakanae disease (Fusarium fujikuroi) that cause economic damage to the crop.
In addition to these various diseases, root knot nematode disease of rice caused by Meloidogyne graminiola has become an important disease of rice which threatens rice production. This disease is most severe in nurseries, direct sown and transplanted rice in upland and medium low land irrigated fields.
This disease is characterized by abnormal swelling of roots (known as root knots or galls), yellowing, stunting and wilting of plants depending on the initial population density (Pi) of the pathogenic nematode in soil.
Plowright and Bridge (1990) reported that a high initial population density of M. graminicola caused wilting of seedlings along with severe reduction in growth parameters while low population caused only reduction in growth parameters.
2. Distribution of Root Knot Disease of Rice:
Root knot disease of rice is widely distributed in rice growing areas of the world, viz., India, U.S.A., South America, Burma, Vietnam, Taiwan, Indonesia, Philippines, Laos, Thailand Bangladesh and Pakistan.
However, this nematode disease occurs in its most severe form in Asia. In India this disease was first reported by Israel in 1963, subsequently occurrence of the disease was reported from different states by several workers, viz., West Bengal, Assam, Tripura, Andhra Pradesh, Haryana and Bihar.
It has also been reported from Uttar Pradesh. This disease was also recorded in severe form in rice nurseries at Agricultural Research Farm -of Banaras Hindu University, Varanasi, India.
3. Symptoms of Root Knot Disease of Rice:
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The above ground symptoms of root knot disease of rice are yellowing, dwarfing, drying of leaves and wilting whereas, below ground symptoms include formation of hook, spindle or club shaped galls on the roots system of rice plants (Fig. 29.1 a-e).
The seedlings become yellow to deep yellow in the beginning followed by drying of leaves. The drying of leaves starts from the leaf tip and progresses downward resulting in wilting of plants. The root systems of such seedlings are usually heavily laden with root galls (Fig. 29. If).
Such galls are usually smaller in size. The degree of symptom manifestation differs with time of infection, age of the plants and load of inoculum, etc.
Losses:
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The losses caused by M. graminicola may vary from negligible to heavy depending on the severity of disease. Biswas and Rao (1971) reported a loss of 15 to 17% in rice yield by M. graminicola in pot condition. However, Rao and Biswas (1973) further reported 21 to 64% yield loss due to this nematode.
Jairajpuri and Baqri (1991) reported grain yield losses from 16 to 32%. Netscher and Erlan (1993) reported that M. graminicola caused 28 to 87% yield loss in upland rice in Indonesia.
Losses caused by M. graminicola also differ with agro-ecosystems in which the rice crop is grown. 20% losses have been reported under intermittent flooding, 30% in semi deep water and 70% under saturated condition when the nematode infects young susceptible seedlings at transplanting stage.
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Soriano (2000) recorded 11 to 73% yield losses by this nematode under simulation of intermittently flooded rice, whereas under simulated upland conditions, yield loss varied between 20 and 98%. Padgham (2004) also reported 16 to 20% yield loss caused by M. graminicola in low land rainfed rice in Bangladesh. Jaiswal (2006) has also recorded 2.5 to 72% yield loss in upland field conditions in India.
4. Pathogen of Root Knot Disease of Rice:
The pathogen of root knot disease of rice is the nematode M. graminicola. It is a semi-endoparasite, facultative meiotic parthenogen, with a haploid chromosome number of 18. The morphological descriptions of different stages of M. graminicola are as follows.
Female:
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Female body white, globose to pear-shaped with varying neck length (50-215 µm). Female size variable, 490-910 µm long and 300-590 µm wide. Body cuticle annulated. Head not distinctly set off from neck and without annules. Cephalic frame work not prominent.
Cephalids not observed. Stylet delicate, 11.2 µm long with rounded knobs. Esophagus well developed with elongate, cylindrical procorpus and large, rounded metacarpus provided with heavily sclerotized valve.
Orifice of the dorsal esophageal gland 3.0 µm posterior to base of stylet. Excretory pore very distinct, generally located about one and one-half stylet length or more from base of stylet. Ovaries two, convoluted.
Vulva and anus terminally located. Perennial pattern prominent with distinct and characteristic striations. Eggs deposited in gelatinous matrix but egg sacs absent. Absence of egg sac is characteristic feature of this nematode.
Male:
Body cylindrical, vermiform, tapering gradually at both ends. Males are usually 1.2- 1.4 mm long and 32.0-35.0 µm wide. Head not offset from body. Cephalic framework prominent. Cuticular annulations distinct, each annule measuring 2.0 µm in width.
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Lateral field wide consisting of 4-8 lines depending on young or older specimens at mid-body. Stylet robust, 15.4 µm long with rounded knobs. Opening of dorsal esophageal gland nearly 3.5 µm posterior to base of stylet.
Median bulb elongate with well-developed sclerotized valve. Length of esophagus (from anterior end to base of esophagus) 224.0 µm, distinct nerve ring encircling isthmus just posterior to median bulb. Cephalids located in anterior portion.
Hemizonid very distinct located about two annules anterior to the excretory pore. Spicules arcuate, 27.0 µm long. Testis 1 occasionally reflexed a short distance from its anterior end. Gubernaculum 6.1 µm in length. Tail end round, 11.2 µm long.
Second Stage Juveniles:
Second stage juvenile are vermiform, cylindrical, tapering more prominently toward posterior end. Head not offset from body with slight cephalic framework, bearing three faint post labial annules. Body annulated, cuticular annulations distinct and fine, each annule measuring about one micron in width.
Lateral lines variable but four or more up to 8. Stylet small 11.1 µm with rounded knpbs, sloping posteriorly. Dorsal esophageal gland opening nearly 3.0 µm posterior to base of the knob. Metacarpus spherical with prominent sclerotized valve. Tail 71.0 µm long, hyaline tail terminal 18.0 µm in length, without distinct annulations.
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Eggs:
Eggs cylindrical, averaging 92.5 µm long and 39.6 µm wide with hyaline shell culture marking, with different embryonic stages. First moult larvae commonly seen in a population which give rise to second stage larvae after hatching.
5. Life Cycle of Root Knot Disease of Rice:
The first and second stage juveniles develop inside the eggs. The second stage juveniles emerge from the egg into the soil. This is the only infective stage of the nematode. The second stage juveniles are attracted to the roots and usually penetrate the roots closely behind the root tip by inserting their stylets.
The juveniles then migrate first towards the root tip, where the absence of differentiated endodermis allows them to reach the vascular cylinder. This migration happens intercellularly by mechanical and possibly enzymatic softening of the middle lamella. The parasites finally start feeding on three to ten cells, which are rapidly turned into multinucleated giant cells, by endomitosis and cell hypertrophy.
At the same time as the giant cells are formed, the cells of the neighbourng pericycle start to divide, giving rise to a typical gall or root knot. Rice seedlings shows first visible symptoms in the form of swelling of roots 40 hours after inoculation and usually one or two root galls appear 48 hours after being exposed to nematodes.
The nematode then undergoes a second molt and gives rise to the third-stage juvenile (J3), which is stouter and lacks a stylet. The third-stage juvenile goes through the third molt and give rise the fourth-stage juvenile (J4), which can be distinguished as either male or female.
A male fourth-stage juvenile undergoes the fourth and final molt and emerges from the root as the worm-like adult male, which becomes free living in the soil. The fourth-stage female juvenile continues to grow in thickness and becomes an adult female, which appears pear shaped.
The female continues to swell and with or without fertilization by a male, produces eggs that are laid in a gelatinous matrix. However parthenogenesis is often encountered in root knot nematodes. The newly hatched second stage juveniles remain in the maternal gall or migrate intercellularly through the aerenchymatous tissues of the cortex to new feeding sites within the same root.
This behaviour appears to be an adaptation by M. graminicola to flooded conditions enabling it to continue multiplying within the host tissues even when roots are deeply covered by water. Thus the life cycle of M. graminicola is completed after passing through different stages.
Several workers have studied the life cycle of M. graminicola and reported varied results on number of days required for completion of its life cycle. Rao and Israel (1973) reported that the life cycle of M. graminicola was completed in 26-51 days during different months.
They reported that 4th stage was seen on day 9 and adult male and female with ovisac were seen on day 24 and 27, respectively. Yik and Birchfield (1979) reported that an isolate of M. graminicola. from the USA completed its life cylcle in 23-27 days at 22- 29°C. Bridge and Page (1982) however, reported that the life cycle of M. graminicola was completed in 19 days at an ambient temperature of 22-29°C.
They further noted that the generation time of M. graminicola could be as little as 13 days at an ambient temperature of 25-35°C. Khan (1995) reported that females laid eggs 20 day after inoculation of J2ofM. graminicola. Halbrendt (1997) reported thatM. graminicola reproduces rapidly and can complete its life cycle in as little as 15-19 days.
Dabur (2004) reported that life cycle of this nematode was completed in 24 days. They reported that adult male and females were observed on day 10 and egg laying and release of juveniles were first observed on day 20 and 24, respectively. Recently, Jaiswal (2006) reported that the M. graminicola completes its life cycle within 15 days at 27-37°C.
Survival and Dissemination:
M. graminicola is well adapted to flooded conditions and can survive in waterlogged soil as eggs in egg masses or as juveniles for long periods. Numbers of M. graminicola decline rapidly after 4 months but some egg masses can remain viable for at least 14 months in waterlogged soil.
M. graminicola can survive in soil flooded to a depth of 1 in for atleast 5 months, it cannot invade rice in flooded conditions but quickly invades when infested soils are drained. M. graminicola can be spread in soil and on seedlings of other crop hosts planted in infested field. Since M. graminicola occurs in flooded rice too, there is additional possibility of dissemination in irrigation and run-off water.
Pathogenic Response to Host:
Nematodes remove plant nutrients, alter nutrient flow patterns and retard root growth, all of which may contribute to suppressed plant yield. It is established that root growth and rate of root extension is frequently suppressed by nematode infection.
Root growth of infected rice plants is drastically reduced by M. graminicola. Reduction in root length and its biomass is essentially the result of nematode feeding on giant cells, causing root growth to stop and tips to swell.
The reduced root system affects the growth and development of the rice plants by limiting its ability to extract water and nutrients from the soil, eventually resulting in stunting and loss in grain yield of rice. Hunter (1958) reported that the primary cause of poor nutrient uptake and suppressed growth of root knot infected plants is related to the reduced root system.
Meon (1978) reported that disruption and development of abnormal vessel elements caused by M. javanica infection resulted in restricted water flow. Gergon (2002) has also reported that M. graminicola reduced the root length of onion (Allium cepa L.) and consequently the yield.
M. graminicola reduces the total chlorophyll content in rice plant particularly chlorophyll b, whereas chlorophyll a remained unchanged. Reduction in chlorophyll b might be causing a lower rate of photosynthesis and poor grain formation.
Mohanty and Rao (1978) observed a reduction in chlorophyll a and b fractions by 20 to 39.5% due to M. graminicola infection in susceptible rice varieties. Swain and Prasad (1988) have also reported decrease in chlorophyll content of infected rice seedlings by M. graminicola; however, they have reported decrease in both types of chlorophyll.
Meloidogyne infection decreases the rate of photosynthesis in leaves. Loveys and Bird (1973) reported that high inoculum levels of M. javanica caused a decline in the net photosynthetic rate within two days after inoculation in tomato plants.
They concluded that the nematode interfered with the production and translocation of root derived factors regulating photosynthesis. Wallace (1974) also reported that Meloidogyne infection caused lower photosynthetic rate.
M. graminicola impair the growth and yield of infected rice plants which is attributed to derivation of plant nutrient for the growth and development of nematodes which consequently cause nutritional stress to the plants. Thus, it is the nutritional stress to the plant that is expressed in the form of yellowing.
Grain yield loss is attributed to reduced number of tillers per heel, reduced number of grains per panicle, reduced length of panicles and reduced test weight of grains.
Meloidogyne spp. in associated hypertrophied root tissues are metabolic sinks and receive photosynthates translocated from other parts of the plant via the root system for their growth and development. In the process, the nematodes deprive the plant of nutrients causing reduced root and shoot growth.
The magnitude of symptom production thus depends on the degree of alterations due to nematode infection by number of penetrating and establishing nematodes within the root system and the ratio between the number of nematodes and food resources supplied by the plants.
Oteifa (1952) reported that a high inoculum level of the species M. incognita suppressed shoot growth of Lima bean (Phaseolus lunatus) accompanied by a decrease in shoot-root ratio. Recently, Singh (2006) established that nematode-to-root biomass ratio determines the degree of symptom manifestation in root knot disease of rice caused by M. graminicola.
The increased metabolic activity of giant cells stimulates mobilization of photosynthates from shoot to root and, in particular, to the giant cells where they are removed and utilized by the feeding nematodes. Mobilization and accumulation of substances reaches a maximum when the adult females commence egg laying stage and declines thereafter.
It is important to emphasize that due to growth of individual J2 in roots and their subsequent increase in biomass, the photosynthates produced by seedlings are utilized in higher proportion by the feeding nematodes.
The adult female with a biomass several hundred times that of the invading J2 likely utilizes far more nutrients. Further the utilization of the nutrients by adult females at the egg laying stage must be still higher on account of production of approximately 300 eggs/female.
Dropkin (1972) estimated the biomass of adult females of Meloidogyne spp. which were in egg laying stage and reported that an infective juvenile increases its size by about 1000 times during that period. To accomplish this tremendous growth rate, the nematode must draw the bulk of the nutrients and act as a metabolic sink.
Nematode-to-root biomass ratios are inversely proportional to the degree of symptoms meaning that lower the nematode-to-root biomass ratio higher would be the degree of symptoms. Thus it is the nematode-to-root biomass ratio that determines the degree of symptom production in root knot disease of rice caused by M. graminicola.
6. Resistance of Root Knot Diseases of Rice:
A number of rice cultivars and breeding lines have been recorded as resistant to Meloidogyne species although only a small number of these are truly resistant. Oryza glaberrima is resistant to M. graminicola, and some progeny from interspecific crosses with O. sativa appear to be less susceptible.
The majority of O. sativa cultivars are susceptible to M. graminicola. However, there are a number of cultivars from India, Thailand and USA which are reported to be resistant to M. graminicola.
Crop Rotation:
Continuous growing of a highly susceptible host greatly increases population of a nematode pest and cause damage to the crop; conversely, growing a poor host will significantly suppress the nematode population. Crop rotation has received considerable attention as a pest management tactic for managing plant parasitic nematodes, including M. graminicola.
Since rice and wheat both are good hosts of M. graminicola, there is need to change this crop rotation or grow a non-host/ poor host crop in between this crop rotation. Some corps viz., castor, cowpea, sweet potato, soybean, sunflower, sesame, onion, turnip, jute and okra are poor host of M. graminicola, hence, could be used in rotation to reduce this nematode population.
Rahman (1990) reported that soil population of M. graminicola are reduced when rice is preceded by planting of mustard (Brassica campestris subsp. oleifera) and guzitil (Guizotia abbysinica). Jaiswal (2006) has also reported that some of the crop rotations viz., rice- berseem, rice-mustard, rice-linseed, rice-lentil and rice-pea are very effective in arresting the population of M. graminicola.
Introducing a fallow into the crop rotation, free of weed hosts will also give control of the nematodes but this practice is impractical in most circumstances. However, Eclipta alba, a weed which is toxic to M. graminicola could be grown and incorporated into the infested field to kill the nematodes.
Weeding:
Some of the dominant weeds viz., Echinocloa colonum, Phalaris minor, Cyperus rotundas, and Cynodon dactylon, etc., serve as reservoirs for M. graminicola. The presence of such weeds helps in increasing the population of M. graminicola in soil. The weed hosts are usually infected by second stage juveniles which grow within root galls.
The developed females lay eggs in larger number, which are mostly retained within galls even after hatching. Thus a large number of infective J2 s from soil are trapped within roots of these weeds. Weeding of such hosts, therefore, removes the population of nematode and thereby a good proportion of the inoculums are reduced in soil. This will obviously reduce the build-up of inocula of M. graminicola in rice fields.
Soil Flooding:
Soil flooding is used in rice production to control M. graminicola, although feasibility of this tactic is limited by water availability and the potential for damage to the soil structure.
It is known that M. graminicola can survive in intermittent flooding as endoparasite of rice roots but its population remain inactive in the flooded soil. Bridge and page (1982) reported that juveniles of M. graminicola that migrate from rice roots in flooded soil cannot reinvade the root system. Kinh (1982) reported that permanent flooding reduced the damage to rice caused by M. graminicola.
Damage to the crop can be avoided by raising rice seedlings in flooded soils, thus preventing root invasion by the nematodes. Therefore, flooding can be an important tool for the management of root knot disease of rice.
Puddling:
The root knot disease of rice is adversely influenced by the practice of puddling and submergence. Puddling reduces aeration and provides high moisture levels for prolonged periods which allow poor respiration and movement of nematodes resulting in reduction in population densities of M. graminicola in puddled field.
Shallow flooding reduces the percentage of root damage by the nematode, resulting in an absence of yield-reducing effect by nematode under irrigated conditions.
Organic Amendments:
The use of organic matter is a well established and ancient agricultural practice used by farmers for the management of plant parasitic nematodes. Organic amendments improve the nutrients and water holding capacity of the soil and thereby improve plant growth, hence increase tolerance to nematodes.
Organic amendments significantly reduced the root knot severity, population of M. graminicola and increased the length and weight of shoot and roots and the growth of rice seedlings. Decomposition of organic residues results in the accumulation of specific compounds that may be nematicidal.
The green manure crops (leguminous crops) Sesbania rostrata and Aeschynomere afaraspera, when grown in rotation have been shown to significantly increase yields of irrigated rice in the presence of another nematode pest of rice, the rice root nematode (Hirschmanniella oryzae) by acting as trap crops of the nematode.
Unfortunately, S. rostrata is a very good host of M. graminicola and cannot be used against this nematode.
The use of decaffeinated tea waste and water hyacinth compost has been suggested to control M. graminicola, and some reduction in populations is reported following the incorporation of other chopped ‘botanicals’; Polygonum, Ageratum, Mikania and also water hyacinth.
Amendments of soil with cow dung manure also increase the population of nematode trapping fungus (Arthrobotrys dactyloides) which in turn may reduce the population of root knot nematode.
Soil Solarization:
Soil solarization can give a reduction in the population of Meloidogyne spp. in rice beds of over 80% and improve the seedling growth. However, this practice can be effective only on a small scale such as on nursery beds.
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Chemical Control:
Several successful attempts have been made in chemical control of root knot nematodes in rice in India by seed treatments, root dips, soil drehcnes and soil incorporation but their practical and economical applicability has not been determined.
Kinh (1982) reported that carbofuran and diazinon gave effective control of M.graminicola in Vietnam when applied to irrigation water, however this means of application has environmental concern. Das and Deka (2002) reported that seed treatment with neen based pesticides reduced the population and damage caused by M. graminicola.
7. Biological Control of Root Knot Disease of Rice:
In nature when the pathogen population increases, its natural enemies are also activated that kill them and consequently causing a decline in pest population. This is possibly the nature norms that does not allow the soil to be populated by a single dominant species rather it maintains a good balance of soil biodiversity.
Jaffee (1992) reported that the disease dynamics in soil microcosms exhibited both temporal density-dependent parasitism and a host threshold density parasitism.
Kerry (1987) reported that continuous cropping of cereals which are attacked by the cereal cyst nematode caused soil to become suppressive to that pest. Suppression followed by inductive phase during which large densities of pest nematode supported an increase in parasites of those nematodes.
Similar observations have been made with a bacterial parasite (Pasteuria penetrans) of nematodes. Several biocontrol agents are reported for capturing and killing of plant parasitic nematodes in vitro or in vivo.
Singh and Gupta (1986) reported Catenaria anguillulae Sorokin on Heterodera sorghi in epidemic form in a sorghum field severely infested by cyst nematode disease, which resulted in steep decline in the population of this nematode.
Barooti (1985) also reported natural parasitism of plant parasitic nematodes by C. anguillulae to the extent of 35%. Further, Gupta (2003) recorded natural parasitism of nematodes by C. anguillulae in the rhizosphere soils of different fruit plants and reported that it varied between 0.60 to 36%.
Recently, Singh (2007a) reported that C. anguillulae parasitized and killed the second stage juveniles of M. graminicola under natural condition. The degree of parasitism of eggs and juveniles by C. anguillulae varied with severity of root knot disease.
The root system showing higher root gall index recorded higher percentage of infection in eggs and juveniles of M. graminicola. Thus natural enemies like C. anguillulae modulate the population of M. graminicola in the soil. Further, Singh (2007 b) reported association of nematophagous fungi with root galls of rice caused by M. graminicola and its biological control by Arthrobotrys dactyloides and Dactylaria brochopaga.
They reported that number and frequency of colonization of root galls by nematophagous fungi was higher in old galls obtained from severely infested rice fields.
They also reported that mass culture of A. dactyloides and D. brochopaga reduced the number of root galls, number of females and number of eggs and juveniles and consequently improved the growth parameters of rice in pots. Stirling (1998), Stirling and Smith (1998) and Kumar and Singh (2006) also reported the control of root knot nematode of tomato by application of A. dactyloides.
From these studies it is evident that all these practices can be integrated for the effective management of root knot disease of rice caused by M. graminicola.
