Rice

Description

There are only two species of cultivated rice in the world, Oryza sativa (Asian rice) and Oryza glaberrima (African rice). Both species are annual grasses (except in the tropics, where the plant can be perennial) belonging to the family Poaceae which are cultivated for their grain which is considered a staple food in most parts of the world. Asian and African rice plants are morphologically very similar and can be difficult to tell apart. They have rounded stems (called culms) which are divided into nodes and internodes. The plant leaves are borne on the nodes of the stem and are long and slender with a pronounced midrib. The plant produces three flowers, two of which are reduced, on a spikelet on the terminal (last) internode of the stem. The rice grain is formed by the ripened ovary of the flower and is between 5 and 12 mm in length. Rice is an annual plant, harvested after one growing season and can reach a height of between 1 and 1.8 m (3.3–5.9 ft) depending on variety. Generally, African rice tends to form smaller, pear shaped grains with a red bran and olive to black seed coat and has several disadvantages compared with its Asian counterpart e.g. seed is easily scattered, the grain is difficult to mill and the crop gives a lower yield. The growing of Asian rice has therefore begun to supercede that of African rice in West Africa. Asian rice originates from China and African rice is believed to have been domesticated in areas around the Niger river in Africa.


Uses

After removal of the rice grain from the chaff, the rice can be used as brown rice or further processed to remove the bran to produce white rice. Brown and white rice may be consumed after cooking or may be ground into rice flour. Rice can be puffed under low pressure to produce puffed rice that is commonly used as a breakfast cereal. Rice bran oil can be produced from the inner husk and can be used as a cooking oil. Rice starch can be extracted and fermented to produce rice wine or brewed to produce sake.


Propagation


Basic requirements
Rice is mainly grown in warmer regions as the plants will not grow at temperatures below 10°C. Rice is considered to be a semi-aquatic annual grass and is commonly grown in paddies in wetlands or under shallow water. Several new methods of propagating rice have been developed which allow rice to be cultivated in less conventional areas e.g. drought resistant varieties are grown in upland areas.

Propagation
Rice is propagated directly from seed and sown on wet or dry seed beds or used to grow transplants in a nursery bed. Transplants produced in a nursery are always planted in dry seed beds when they are transferred to the field. Seeds may be sown in the field by broadcasting or by mechanical drilling. Three main methods are used for direct seeding rice in lowland areas: dry seedbeds, wet seedbeds and dapog. Dry seedbeds are prepared in close proximity to a water source. Each bed is generally 1.5 m (4.9 ft) in width and is seeded at a rate of 10 kg of seed for every 10 square meters. The seeds are covered with a light layer of soil and the bed is saturated with water until the seeds germinate. Wet seedbeds are raised nursery beds in which pre-germinated seeds are sown. The bed is puddled or wet with water and the germinated seeds are spread out uniformly and kept wet. When the seedling reach 2-3 cm (0.8–1.2 in) in height, shallow irrigation is used and the water levels in the bed are increased as the plants grow. Dapod is a method of producing transplants on a mat made of banana leaves of plastic sheeting which is used in some areas of the Philippines. Pre-germinated seeds are spread out on cement of wet soil and covered with the leaves or plastic sheeting. The cover is pushed down onto the seeds and continuously watered. After 11 days, the mat is rolled up and transferred to to the field.

Cropping systems
Rice can be be grown in an irrigated system, as a rainfed crop or in an upland systems in managed fields known as paddies. In irrigated systems, supplemental water is provided during the dry season alone or in both the dry and wet seasons depending on the amount of local rainfall. Irrigated systems are required in areas where rainfall is very low. Irrigated fields are bunded and the water levels are carefully managed prior to harvest. Rainfed systems are generally a poorer alternative to irrigated systems as they are unstable and prone to drought or floods. Rainfed fields are also bunded but are only flooded for short periods and water depths never exceed 50 cm (20 in). In some parts of Asia, Africa and Latin America, rice is grown in upland areas in acidic and nutrient depleted soils. Fields are either planted on flat land in valleys or on mountainside terraces where the land is sharply sloping. Fields in upland systems are generally unbunded and surface water does not usually accumulate as in the other growing systems.

Harvesting
In many areas of the world rice is still harvested using the traditional method of cutting by hand. Rice should be cut when 80–85% of the panicles have turned from green to straw colored. When harvested by hand, the rice is cut using sharp knives or sickles. In some areas, rice is mechanically harvested using threshers or combines.


Physiological disorders

Physiological disorders are quite common in rice crops grown under different conditions of soil. It is also called physiological diseases. It is frequently occurs in field with poor drainage and accumulation of toxic products from anaerobic decomposition due to reductive soil conditions.

Important physiological disorders are
Deficiency of major or minor nutrients like N, P, K, Fe, Mn, Zn, Si etc.
Toxicity of elements like Fe, Mn, B, Al, etc.
Accumulation of toxic substances due to poor soil conditions like sulfide, organic acids, carbon dioxide etc.
Injury due to excess salt like Na salts etc.

Main disorders:

Nitrogen deficiency
It is one of the common disorder found in rice and usually occurs at tillering and panicle initiation stage. The symptoms occur in older leaves (sometimes in younger leaves also) where we will see appearance of light green to chlorotic tip. The leaves may die out under sever condition, where as the deficient young leaves are narrow, short, erect, and lemon yellowish. Some times the whole field look yellowish.

Zinc deficiency
Symptoms occurs on young or middle-aged leaves. The plants are stunted in growth with bronzing upper leaves and chlorotic basal leaves. Stacking of leaf sheath or joints are also common.

Boron deficiency
Symptoms usually appear first on young leaves. Tips of emerging leaves are white and rolled. In severe conditions, growing points can die, but new tillers continue to emerge. Plants shows reduced height. When infected at panicle initiation stage, plants won’t produce panicles.

Phosphorus deficiency
The main symptoms are stunted dark green plants with erect leaves, thin stem and with few tillers. The infected leaves are narrow, short, very erect, and ‘dirty’ dark green. Some time you will see red and purple colors on leaves if variety produce anthocyanin.

Potassium deficiency
Symptom first appears on older leaves where you can see tips showing yellowish brown margin or dark brown necrotic spots. Symptom occur from tip, then along leaf edges and spreads to leaf base. Younger leaves are droopy, short and dirty green color, whereas older leaves show yellowish to brown discoloration along intervenes. Some time this symptom is mistaken with tungro virus.

Calcium deficiency
The symptoms are chlorotic white discoloration at tips of youngest leaves. Deficient plants are weak rooted. Necrotic tissue may develop along the lateral margin of the leaves. In acidic soils we can see premature leaf senescence.

Magnesium deficiency
The older leaves shows pale discoloration with interveinal chlorosis. Green coloring appears as string of beads. The leaves are wavey and droopy.




References

Akira Tanaka and Shouichi Yoshida. Nutritional Disorders of the rice plant in Asia. Technical Bulletin (1970). International Rice Research Institute.

CABI Crop Protection Compendium. (2014). Oryza sativa (rice) datasheet. Available at: http://www.cabi.org/cpc/datasheet/37964. [Accessed 07 April 15]. Paid subscription required.

Lisu Chen, Lin Lin, Guangzhe Cai, Yuanyuan Sun, Tao Huang, Ke Wang, Jinsong Deng (2014). Identification of Nitrogen, Phosphorus, and Potassium Deficiencies in Rice Based on Static Scanning Technology and Hierarchical Identification Method. PLoS ONE 9(11): e113200. doi:10.1371/journal.pone.0113200.

Rice Knowledge Bank. International Rice Research Institute. Available at: http://www.knowledgebank.irri.org/. [Accessed 07 April 15]. Free to access

Nutrition deficiency in rice. Better Crops International. Vol. 16, Special Supplement, May 2002.

Webster, R. K. & Gunnell, P. S. (Eds.) (1992). Compendium of rice diseases. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopap.... Available for purchase from APS Press.









Common Pests and Diseases

Bacterial blight
Xanthomonas campestris

Symptoms
Water-soaked stripes on leaf blades; yellow or white stripes on leaf blades; leaves appear grayish in color; plants wilting and rolling up; leaves turning yellow; stunted plants; plant death; youngest leaf on plant turning yellow
Cause
Bacterium
Comments
One of the most important diseases of rice; disease found in tropical and temperate regions; greatest economic impact in Asia
Management
Bacterial blight can be effectively controlled by planting resistant rice varieties; avoid excessive nitrogen fertilization; plow stubble and straw into soil after harvest

Bacterial leaf streak
Xanthomonas oryzae

Symptoms
Small, water-soaked streaks between leaf veins which are initially dark green and then turn translucent; streaks grow larger, coalesce and turn light brown in color; tiny beads of yellow colored bacterial exudate are common on the surface of the streaks; leaves turn brown and then gray-white in color before they die
Cause
Bacterium
Comments
Bacteria survive on infected seed and straw; bacteria may enter the plant through wounds; bacterial exudate can be spread in irrigation water; emergence of the disease is favored by high humidity and high temperatures; bacterial leaf streak is widespread in tropical Asia and West Africa
Management
Control of bacterial leaf streak is dependent on the use of resistant rice varieties and on planting of treated seed

Sheath blight
Rhizoctonia solani

Symptoms
Circular, oblong or elliptical, green to gray water-soaked spots on leaf sheaths; lesions with pale green or white center and purple-brown margin; lesions covering leaf sheaths and stems; poorly filled grains
Cause
Fungus
Comments
Occurs in all areas where rice is grown; second only importance to rice blast; most damaging in intensive rice production; spreads rapidly via irrigation
Management
Avoid overfertilizing plants as excessive nitrogen application has been shown to increase susceptibility to the disease; applications of foliar fungicides may be required; two applications are recommended and should be timed so that the first application is made between the early internode elongation and the second application made on emerging panicles 10-14 days later

Rice blast
Magnaporthe grisea

Symptoms
Lesions on all parts of shoot; white to green or gray diamond-shaped lesions with dark green borders; death of leaf blades; black necrotic patches on culm; rotting panicles
Cause
Fungus
Comments
Most important disease of rice worldwide; causes most damage in areas of intense cultivation; disease emergence favors high soil nitrogen content
Management
If disease is not endemic to the region, blast can be controlled by planting resistant rice varieties; avoid over-fertilizing crop with nitrogen as this increases the plant's susceptibility to the disease; utilize good water management to ensure plants do not suffer from drought stress; disease can be effectively controlled by the application of appropriate systemic fungicides, where available

Bakanae
Fusarium moniliforme

Symptoms
Seedlings are elongated, slender and pale; seedlings are stunted and chlorotic; death of seedlings; abnormal elongation of older plants which often makes them visible as they grow taller than uninfected plants in the field; sterile plants which do not produce panicles or produce empty panicles
Cause
Fungus
Comments
Disease transmitted through infected seed; disease emergence favored by high temperatures
Management
Treating seeds with appropriate fungicides prior to planting can be very effective at controlling the disease; less susceptible rice varieties should be grown in areas where fungicide-treated seed is not available

Brown spot
Cochliobolus miyabeanus

Symptoms
Circular, brown lesions on seedling; distorted primary and/or secondary leaves on seedlings; black discoloration of roots; death of seedlings; circular or oval lesions with gray center and reddish-brown margin on older plants; death of large areas of leaves; brown or black spots on grain; reduced number of grains; reduced kernel weight
Cause
Fungus
Comments
Occurs wherever rice is grown; fungus overwinters on plant debris; disease emergence favored by water on surfaces of plant
Management
Ensure plants are provided with correct nutrients and avoid water stress; chemical seed treatments are effective at reducing the incidence of the disease

Narrow leaf spot (Cercospora leaf spot)
Cercospora oryzae

Symptoms
Short, elliptical or linear brown lesions on leaves; necrosis of leaves; blotchy pattern on leaves; premature ripening of kernels
Cause
Fungus
Comments
Disease occurs in major rice growing regions in tropical and sub-tropical Asia, Africa, Australia, North America, South America and Central America
Management
There are no chemical controls currently recommended for the treatment of the disease; treating seeds with hot water or appropriate fungicides prior to planting can reduce the incidence of the disease

Stem rot
Magnaporthe salvinii

Symptoms
Symptoms generally begin to appear after the mid tillering stage; black lesions appear on outer leaf sheath at the water-line; lesions expand and begin to infect inner leaf sheaths and culm begins to rot; infections which reach the culm can leaf to lodging of plants, unfilled panicles and death of tillers
Cause
Fungus
Comments
Fungus survives in crop debris in soil after harvest; fruiting bodies are carried to the surface when fields are flooded where they then infect leaf sheaths at the water line
Management
Bury crop residue deeply in the soil after harvest; avoid excessive nitrogen fertilization; plant less susceptible rice varieties

False smut
Ustilaginoidea virens

Symptoms
The pathogen infect the rice plant during flowering stage and causes chalkiness of grain. The individual grains were covered with orange fungal mass in the beginning, later turns into greenish velvet color during sporulation stage and finally into charcoal black during spore maturation stage. It infect only few grains in spikelet. Recorded disease loss up to 75% in severe incidence. It also reduces market value of rice.
Cause
Fungus
Comments
It is seed born disease and may also affect seed germination.
Management
Treat seeds with hot water (52 C) for 10 min. Roughing the infected plants from field and from harvested grains. Keep the rice field and surrounding clean. Use resistant varieties. Maintain humidity in field by alternate wetting and drying.

Leaf scald
Microdochium oryzae

Symptoms
As name suggest we will see scalded appearance on leaves. The lesion is marked with different zone from alternating light tan and dark brown from leaf tips or edges. As the leaves mature the lesion is with light brown halos. Margins and leaf tips are translucent.
Cause
Fungus
Comments
The symptoms may vary depends on cultivar, growth stage and plant density. The disease is severe in Latin America and West Africa.
Management
Grow resistant varieties. Treat the seeds with suitable fungicide. Apply only recommended quantity of nitrogen fertilizer in split dose.

Stem borers (Yellow stem borer, Striped stem borer, White stem borer, etc.)
Scirpophaga incertulas
Chilo suppressalis
Scirpophaga innotata

Symptoms
Longitudinal white patches on leaf sheaths; central leaf whorl drying out and turning brown; tillers drying out without producing panicles; panicles may dry out or may produce no grain; adult insects are nocturnal moths which lay their eggs on the leaves or leaf sheaths of the rice plants; larvae are legless grubs which feed on leaf sheaths before entering the stem of the pant
Cause
Insect
Comments
Stem borers are generally considered to be the most damaging insect pest of rice
Management
Stem borers are difficult to control with insecticides as once they bore inside the stem they are protected from chemical sprays; in order for chemical control to be successful, repeated applications of appropriate insecticide must be made to the foliage; granular formulations give better control than sprays; clipping seedling prior to transplanting can successfully reduce moth numbers as eggs are laid at leaf tips; harvesting plants at ground level can remove the majority of larvae from the field; plowing or flooding the remaining stubble will kill off most of the remainder of the larvae in the field

Leafhoppers & planthoppers
Nephotettix spp.
Recilia dorsalis
Nilaparvata lugens
Laodelphax striatellus
Sogatella furcifera

Symptoms
Plants may show no symptoms of leafhopper of planthopper damage; feeding punctures can leave the plants susceptible to bacterial or fungal infections; insects transmit many rice viruses; if infestations is severe, insects may cause plant to completely dry out; adults insects are pale green or brown winged insects with piercing-sucking mouthparts
Cause
Insect
Comments
Leaf and planthoppers transmit many rice viruses, including grassy stunt and tungro virus
Management
Rotating crop for a period of one year is an effective and economical method of controlling hopper numbers; natural enemies and predators are often very successful at controlling hoppers and should be conserved by avoiding inappropriate use of insecticides which can damage their populations; planting resistant varieties is a very effective control method; chemical control with an appropriate insecticide may be necessary but should only be applied if the insects have reached an economic threshold

Rice case worm
Paraponyx stagnalis Stagnalis (Lepidoptera : Pyralidae)
Nymphula depunctalis (Guenee)

Symptoms
Case worm larvae scraps chlorophyll from leaves. Another important symptom is the larvae cuts off leaf tips and make cylindrical tubes around them. In infected field you can see cylindrical tubes attached to plants or floating on water surface.
Cause
Insect
Comments
The damaged leaf tissue looks like ladder.
Management
Drain water to remove floating larvae from field. Follow proper cultivation practices like nitrogen application and spacing. Encourage biological control agents like snails, spiders, lady bird beetles, dragon flies in rice field.

Rice mealy bugs
Brevennia rehi

Symptoms
Both adult and nymphs feed on rice plant by sucking sap. The main symptoms are stunting and wilting of plants, yellowing and curling of leaves.
Cause
Insect
Comments
We can see white wax covered eggs, nymphs and adults on infected plants. The insects are common in field with well drained soil.
Management
Augment biocontrol agents (like lady bird beetles, chloropid flies, spiders, small encyrtid wasps) in rice fields.

Rice gall midge
Orseolia oryzae

Symptoms
Gall midge maggot bore into bud or stalk of rice plant and feeds on internal content which leads to formation of tubular gall at the base of the tillers. This leads to elongation of leaf sheath which is commonly called as silver shoot or onion shoot. The leaves will be wilted, deformed and curled up.
Cause
Insect
Comments
It is common during tillering stage of rice crop.
Management
Grow available gall midge resistant cultivars. Follow proper cultural practices like crop rotation, ploughing the ratoon crop and other alternative hosts, planting early etc. Use light traps to attract adult flies and kill them. Conserve biocontrol agents in rice field.

Mole cricket
Gryllotalpa orientalis

Symptoms
The insect is very destructive because it feeds on seeds, tillers of mature plants and roots. It mainly cut the tillers at ground level and causes gap in rice field. Usually the symptoms appear in patches.
Cause
Insect
Comments
We can see burrows of insect at sides of rice field and usually burrows will have heap of soil at entrance.
Management
Use resistant varieties. Level the field properly and irrigate every 3-4 days after planting. Collect and kill the insects during land preparation. Avoid nurseries with raised bed to reduce cricket infestation during seedling stage. Improve biocontrol agents in field. If damage is large use food bait (commonly rice bran) mixed with insecticide and place along borders.

Rice bug
Leptocorisa oratorius F. and Leptocorisa acuta Thunberg

Symptoms
The insect attacks during spikelet stage of rice crop. Both nymphs and adults suck the content out of grains from pre-flowering spikelets to soft dough stage. This leads to unfilled, empty and discolored grains.
Cause
Insect
Comments
If the infestation is severe it may cause yield loss up to 30%.
Management
Keep the field and surrounding area free from weeds which serves as alternative host for insect during non cropping season. Equal distribution of fertilizers and water in rice field to encourage even crop growth. Collect and kill insects manually by using net during early morning and late after noon. Encourage biological control agents.

Grassy stunt
Rice grassy stunt virus (RGSV)

Symptoms
Stunted plants; short, narrow pale green or yellow leaves; mottled or striped pattern on newly unfolded leaves; irregular dark brown spots on leaves; few or no panicles produced;
Cause
Virus
Comments
Transmitted by leaf hoppers; disease widespread in rice growing regions of South and Southeast Asia, southern China, southern Japan and Taiwan
Management
Several varieties of rice resistant to the leaf hopper vectors have been developed but the insects have overcome the resistance in several countries; applications of appropriate insecticides can help to reduce populations of vectors in temperate regions

Tungro
Rice tungro bacilliform virus (RTBV)

Symptoms
Plants are stunted with a yellow-orange discoloration; plants may have a reduced number of tillers and rust colored spots on leaves; leaves may be mottled, striped or exhibit interveinal necrosis
Cause
Virus
Comments
Disease is the most severely damaging virus of rice in South and Southeast Asia; virus is transmitted by leafhoppers
Management
Rice varieties resistant to tungro virus have been developed and control the disease successfully; intense cultivation has led to the breakdown of the resistance by some virulent leafhopper strains; in Indonesia, the disease is successfully controlled by scheduling planting to obtain synchronous development and practicing crop rotation with resistant varieties