Major Diseases of Rice
Diseases of rice and their control
Diseases of rice may be grouped according to the organism that causes the disease: fungal, bacterial, viral, and those brought about by nematodes (Khush and Beachell 1972, IRRI 1968, Ou 1973). One other group, the physiological diseases, is not caused by any organism but is associated with nutritional disorders. The causal agents may be spread by water, wind, insects, animals, and man. Their interaction with plants are conditioned primarily by environmental factors, foremost of which are water relations, temperature variability, soil conditions, nutrition, light, wind, and susceptibility of plants.
Fungal diseases
Rice blast (Pyricularia oryzae)
This disease is caused by a fungus that attacks at any stage of growth of the rice plant, either producing lesions on leaves, nodes, parts of the panicles, or the grain. Typical leaf lesions are diamond-shaped and the center of the lesion is grayish; early infection and resistant reactions show small brown spots that are difficult to distinguish from other leaf diseases (Ling 1970).
Damage: The most severe damage occurs when the neck region of the panicles are attacked. The lesions often occur at the uppermost nodes. The neck is most vulnerable to the attack during early emergence of the panicle. It gradually becomes less susceptible as the panicle matures. When the attack takes place before the milk stage, the grains do not fill. In a later attack, the panicle breaks at the uppermost node and fall over. The grains may be partially filled. The kernels are usually chalky.
Control measures: Breeding for varietal resistance is fundamental and most practical. Varieties showing resistance in one locality may not be resistant in another place or vice versa because different pathogenic races are present in different locations (Moomaw et al 1967, IRRI 1968, Ou 1973).
Sheath blight (Rhizoctonia solani)
This disease is also known as Rhizoctoniasheath spot, which is very common in almost all rice-growing regions in Asia. The disease spreads rapidly and often undetected among densely populated plants. The damage starts at the lower part of the leaf sheath and spreads upward.
Damage: Sheath blight causes spots, mostly on the leaf sheath, but spots may also occur on the leaf blades if conditions are favorable. The disease is characterized by the presence of rather large, necrotic areas or spots that are greenish and ellipsoid. They quickly enlarge and may coalesce to form irregular discolored areas with blackish brown margin. Damage usually runs 25–30% if the disease develops at the booting stage. Factors that may increase the severity of sheath blight are high temperature and humidity, high levels of nitrogen application, and growing of HYVs.
Control measures: No variety has a high level of resistance to the disease. There are fungicides that can be used and should be applied on the leaf sheath when infection is at maximum tillering stage and again at booting stage (IRRI 1973).
Brown spot (Helminthosporium oryzae)
The disease is caused by a fungus that attacks seedlings, leaves, and developing grains. Spots on the leaves are oval or circular, uniform in size, dark brown, and evenly distributed. It is often difficult to separate the losses caused by brown leaf spot from those caused by soil deficiencies. The disease seldom can be found in rice crops grown on fertile soil.
Damage: The most common symptoms are seen on the leaf and glumes or grains in the panicle. This disease attacks the rice plant at all stages of growth. Seedling blight may occur in seedlings grown from heavily infected seeds and the disease may kill up to 50% of seedlings.
Control measures: Plants grown in poorly drained soils, which are lacking in silica, potassium, nitrogen, manganese, or magnesium, are easily attacked by the fungus. Cultural control is the most effective way of controlling brown spot. Grow plants in good soil and provides adequate fertilizer for normal growth. Planting of resistant varieties is another practical measure, but soil condition must be considered as it affects the severity of the disease (IRRI 1968).
Sheath rot (Acrocylindrium oryzae)
Little is known about crop losses caused by sheath rot, but it is not uncommon for 10–30% of the plants to be infected in fields where the disease occurs.
Damage: Spots develop on the uppermost leaf sheath enclosing the panicles. The young panicles remain inside the leaf sheath or merge only partially. Grains remain unfilled or are discolored. A whitish, powdery fungal growth occurs on the panicle inside the sheath. Eventually, the panicle may rot.
Control measures: Little is known about sheath rot control measures. This disease has not been a major problem and would be minimized by practicing farm sanitation and not using infected seeds for planting.
Narrow brown spot (Cercospora oryzae)
The narrow brown leaf spot disease would cause serious losses only on very susceptible varieties. The spot differs from Helminthosporium,which has oval-shaped lesions, while Cercosporaleaf spot lesions are narrow and linear in appearance.
Damage: Symptoms usually appear first on the flag leaf during the later growth stages. The disease produces linear spots, mostly on the blades, and spots may also be found on the leaf sheath and rice hull. The injury reduces the effective leaf area of the plant and predisposes the plant to lodging if the leaf sheaths are heavily infected.
Control measures: Apart from growing less susceptible varieties, little is known about the control of the disease and how the fungus survives from season to season and how it is disseminated.
Stem rot (Helminthosporium sigmoideum)
This disease occurs in almost every field where rice has been grown for many years. It lives in the soil as sclerotia for as long as 6 years on the straw. Infection of stems begins near the water line through wounds, as a black, irregular lesion that enlarges as the disease advances. The fungus produces sclerotia inside the leaf sheath and eventually penetrates the culms. One or two internodes are rotted, and tissues are covered with numerous, small, black sclerotia that are visible to the naked eyes. The upper leaves of infected stems frequently become yellowish and may die. Rotting stems lodge and yield losses can be high. Sclerotia are distributed in irrigation water. High levels of nitrogen and wounds caused by insects or other damage on the plant favor disease development.
Damage: During the later growth stages, the symptoms appear as small black spots on the leaf sheath near the water line, angular at times in shape and water-soaked. As the spot enlarges, the leaf sheath rots, and eventually the fungus attacks the stem, causing it to lodge and rot. If infected stems are split, they reveal dark gray masses of fungi and small black infection bodies called sclerotia.
Control measures: Stubbles and straws lying on the field must be quickly and thoroughly burned after harvest. It has been found useful to drain the field, allowing the soil to crack before irrigating again. The most practical control measure is to plant resistant varieties when available and suitable to the conditions. Chemical control is not effective. Burning of straw and stubbles reduces the level of sclerotia. The use of resistant and nonlodging varieties is the most effective control measure.
Bacterial diseases
Bacterial leaf blight (Xanthomonas oryzae)
Bacterial leaf blight is reported to have reduced Asia’s annual yield production by as much as 60%. The bacterium has races that differ in their ability to infect different resistant varieties. Losses are less in soils of low fertility and more serious where high rates of fertilizers are applied.
Damage: Bacterial leaf blight is said to be more severe in potassium-deficient soils or in those where excessive nitrogen is applied. Leaves of plants at seedling stage are more susceptible to the disease than those of maturing plants at booting stage. The early symptoms manifested are yellow, undulating lesions along the margins of the upper portion of the leaf blades. The lesions develop rapidly, parallel to the veins and extend laterally to the healthy leaf areas. Eventually, a large portion of the entire leaf blade is infected, turning yellow to dirty white. The bacterial leaf blight manifests three distinct types of symptoms: the leaf undulated blight, the kresek, and the pale yellow plant (IRRI 1978).
In the seedbed, bacterial blight first causes tiny water-soaked spots on the margin of the older lower leaves. The spots would enlarge, the leaves would turn yellow, then drying and wilting would follow.
Another race of bacterial blight is capable of causing kresek symptoms during the seedling and early tillering stages of plant growth. The bacterium enters the leaf through the cut surface of the leaves or roots during transplanting and systematically reaches the growing point of the plant (Ou 1973). When the infected stem is cut crosswise and squeezed between the fingers, yellowish bacterial ooze with a rotten smell appears.
Control measures: There seem to be no practical methods of control at present, except to plant resistant varieties and to get rid of ratoons and infected shoots.
Bacterial leaf streak (Xanthomonas translucens)
This disease infects different species of plants and attacks the rice plant from seedling to maturity stage. In the tropics, the bacteria are probably present throughout the year on other grasses and wild rice (Ou 1973, IRRI 1977).
Damage: The early symptoms are the appearance of transparent, linear lesions between the veins due to damaged tissues caused by the bacterium. Tiny ooze can be observed on the lesions, becoming longer as it covers the larger veins. The whole leaves of susceptible varieties may turn brown and die during later stages of the disease. At this point, the lesions extend and coalesce to form larger patches, making it difficult to distinguish from bacterial leaf blight symptoms.
Control measures: Resistant varieties are the best solution, although continuous selection and breeding for genetic resistance are difficult to establish ahead of the problem. Because of the pathogen’s variability, varieties do not remain resistant indefinitely and different sources must be constantly used in the breeding program.
Viral diseases
Viral diseases are very destructive in Asia where irrigated rice is grown. Aside from the virus infections, leafhoppers and planthoppers inflict complete physical damage on the rice crop (hopperburn). The important viral diseases are tungro, grassy stunt, ragged stunt, yellow dwarf, and orange leaf.
Tungro
The tungro virus manifested itself in the Philippines as early as 1970. The disease is transmitted by green leafhoppers. Periodic outbreaks have been reported all over Southeast Asia, affecting thousands of hectares in many countries (Ling 1972).
Damage: The virus-infected plants are stunted, the number of tillers is slightly reduced, leaves turn yellow or become mottled, and roots develop poorly. The plants take longer to mature because of delayed flowering. The panicles are often small, sterile, and panicles are not completely exerted. Yellowing, which ranges from light yellow to orange yellow or brownish yellow, usually starts from the tip of the leaves. Irregularly shaped dark brown blotches frequently develop in the yellow leaf, especially in young infected seedlings.
Grassy stunt virus
The brown planthopper transmits the virus, which was first noticed in the Philippines in 1962. Serious destruction was recorded from 1973 to 1979 throughout the country.
Damage: Diseased plants are severely stunted, have excessive tillering, and an erect growth habit. The leaves are short, narrow, pale green or pale yellow, and often with numerous small dark brown spots of various shapes or blotches. The infected plants usually live up to maturity but do not produce panicles; they may bear small panicles with dark brown and unfilled grains.
Rice ragged stunt
The disease, transmitted by the brown planthopper, was fist noticed in the Philippines in 1977. The same disease was observed in China, Sri Lanka, India, Thailand, and Indonesia after 1977. Because of the ragged leaf symptoms and stunting exhibited by infected plants, the name ragged stunt was adopted (Ling et al 1978, Heinrichs and Khush 1978).
Damage: Noted during the early plant growth stages are stunting, appearance of twisted and ragged leaves, and swelling along the veins. In the later growth stages, delayed flowering, nodal branching, incomplete panicle emergence, and unfilled grains are observed.
Yellow dwarf disease
The disease is widely distributed in Asia, but it occurs only occasionally (Ling 1972, Ou 1973. Considered serious in Taiwan and Japan, it is caused by a virus-like disease agent called mycoplasma. The green leafhoppers transmit the virus-like agent.
Damage: Plants are stunted, yellowish, and produce more tillers. Leaves are soft and slightly drooping. Infected plants may or may not produce panicles with unfilled grains.
Orange leaf
The disease is transmitted by the zigzag leafhopper, but it does not seem to cause serious problems in rice production.
Damage: The diseased plants have golden yellow to deep orange leaves, when they are about 4–6 weeks old in the field. The infected leaves gradually roll inward and dry out starting from the tips. The plants develop few tillers and often die quickly. When plants are infected at a later stage, panicles develop but these may not be completely exerted from the sheath. Grains are often unfilled.
Hoja blanca
This is the only virus disease in Latin America. It is cyclical in nature, causing severe economic losses for several years, followed by a period of relative unimportance. The easily recognizable symptoms include long yellowish white stripes and mottling on leaves, stunting of the plant, and small, deformed, highly sterile panicles with discolored spikelets. The disease rarely appears before plants are about 2 months old and begins in isolated patches that rapidly spread to cover the field. Hoja blanca is controlled at present through plant resistance to the insect vector. A few varieties are highly resistant to the virus. Insecticide control of the vector does not satisfactorily control hoja blanca.
Sogatodes oryzicola
This is the important vector of the virus that is transmitted by sucking sap from the rice plant. Fertilizer, density, and water have little effect on disease development and spread.
General control of viral diseases
At present, there are no practical ways of curing a plant after it becomes infected. The only way to control viral diseases is to prevent the plant from becoming infected during the early stages of plant growth, during which most damage is inflicted.
Resistant varieties
Planting resistant rice varieties are the simplest, most economical, and most effective way of controlling both the insect vector and the disease. A variety may be resistant to the feeding and development of the insect transmitter or resistant to the infection and development of the viral disease.
Control of vectors
Protective insecticide application to control leafhoppers and planthoppers is necessary during the early plant growth stages, especially when virus outbreaks have occurred. Do not wait for the economic threshold in this situation. The solution is not in the use of insecticides alone. An integrated management approach gives proper guidance and understanding of how insect control can be done through different disciplines and active participation of the community.
Eliminating host plants
It is essential that diseased rice plants and weeds be removed from the rice field and destroyed. Plowing under the rice stubble immediately after harvest will prevent ratoon growth, which could be a source of infection and a breeding place for insect vectors.