Rice:Diagnostics

RICE PESTS AND NUTRITIONAL DISORDERS

ABIOTIC DISEASES & DISORDERS

‘‘‘Abiotic Diseases and Disorders: ‘‘‘ Rice plants may develop abiotic diseases with characteristic symptoms in response to nutrient deficiencies or when they are exposed to toxic concentrations of naturally occurring or synthetic chemicals in the soil and water environment.

AGGREGATE SHEATH SPOT (RHIZOCTONIA ORYZAE SATIVAE)

‘‘‘Aggregate Sheath Spot (Rhizoctonia oryzae sativae) ‘‘‘ The disease is characterized by oval lesions with a gray-green to straw- colored center surrounded by a distinct brown margin. Initially, lesions occur on the lower leaf sheaths at the waterline, range from about 0.5 to 4 cm in length and gradually spread vertically up the plant. Leaves of diseased sheaths generally turn yellow and die. The fungus survives between rice crops as sclerotia in soil or associated with infected debris. The disease is generally of minor importance, and control measures are not necessary.

AKEGARE

‘‘‘Akegare (Organic Acids and Toxic Gases): ‘‘‘ Akegare is a disorder that develops in the early stages of rice growth and is characterized by dark, green leaves that rapidly turn bright yellow. The roots of affected plants are shorter, larger in diameter, and have fewer root hairs than roots of unaffected plants. The disease frequently occurs in poorly drained, clay soils. Two methods used to control akegare are applications of nonsulfate fertilizers and surface drainage at late tillering to supply oxygen.

AKIOCHI

‘‘‘Akiochi (Hydrogen Sulfide Toxicity): ‘‘‘ The infected plant usually shows normal growth and color in early stages of development, with the first symptoms being a change in root color from white or light brown to black. The black root color of affected plants is a surface coating of iron that can be rubbed off in early stages of the toxicity, but later the whole root becomes black and begins to deteriorate. Grayish blotches first appear in irregular patterns along the margins of older leaves and later in younger leaves and affected leaves begin to roll and gradually turn brown from the tip down. In advanced stages of akiochi, soils emit an odor similar to that of rotten eggs. The best control measure is to drain fields to supply oxygen to affected plants.

BACTERIAL BLIGHT (XANTHOMONAS ORYZAE PV ORYZAE)

‘‘‘Bacterial Blight (Xanthomonas oryzae pv oryzae) ‘‘‘ Bacterial blight is one of the most serious diseases of rice worldwide and is found in both tropical and temperate regions. Leaf blight generally occurs from the maximum tillering stage onward. It begins as water-soaked stripes on the leaf blades that increase in length and width, becoming yellow and then white. Older infected leaves may appear grayish from the growth of saprophytic fungi. The wilt syndrome is the most destructive manifestation of the disease and leaves of infected plants wilt and roll up, turning grayish green. The leaves then turn yellow to straw-colored and wither, and the entire plant generally dies. A third syndrome associated with bacterial blight is called yellow leaf or pale in which the youngest leaf of the plant becomes uniformly pale yellow or has a broad yellow stripe. The disease can be effectively controlled with resistant cultivars. Cultural controls include avoiding excessive nitrogen fertilization, maintaining shallow water in nursery beds, providing good drainage during severe flooding, plowing under rice stubble and straw following harvest, and removing alternate hosts.

BACTERIAL BROWN STRIPE (PSEUDOMONAS SPP)

‘‘‘Bacterial Brown Stripe (Pseudomonas spp.) ‘‘‘ Bacterial brown stripe, also known as bacterial stripe, occurs in upland and wetland nurseries and also in nursery boxes. Symptoms consist of water-soaked stripes on the leaves and leaf sheaths, which turn brown. On the leaves, the stripes occur interveinally or along the midrib or leaf margins. The pathogen can also attack and rot young, unfolded leaves, which may result in stunting or death of the seedling.

BACTERIAL DISEASES

‘‘‘Bacterial Diseases: ‘‘‘ Bacterial diseases of rice are classified into three main groups: seedling, sheath, and grain diseases; foliar diseases; and culm and root disease. The principal diseases in each of these groups are caused by bacteria that belong to a single genus.

BACTERIAL LEAF STREAK (XANTHOMONAS ORYZAE PV ORYZICOLA)

‘‘‘Bacterial Leaf Streak (Xanthomonas oryzae pv oryzicola) ‘‘‘ The disease can occur at any growth stage and initially appears as small, interveinal, water-soaked streaks. The streaks are at first dark green and later become translucent and eventually become light brown. Finally, entire leaves turn brown and then grayish white and die. The disease can be controlled with the use of resistant cultivars and treated seed.

BACTERIAL PALEA BROWNING (ERWINIA HERBICOLA)

‘‘‘Bacterial Palea Browning (Erwinia herbicola) ‘‘‘ Symptoms usually first appear at early flowering as light brown, water-soaked lesions on the lemma or palea that eventually turn dark brown. No control for the disease is available.

BAKANAE (GIBBERELLA FUJIKOROI)

‘‘‘Bakanae (Gibberella fujikoroi) ‘‘‘ The classic syndrome associated with bakanae is elongated, slender, pale seedlings that may also be stunted and yellow, exhibiting root and crown rot. Plants that survive until maturity are sterile and produce no panicles or empty panicles. Also, the leaf sheaths of infected plants may turn blue-black with the production of perithecia. The best forms of control include using resistant cultivars, clean seed, and treating seeds with fungicides.

BLACK KERNEL (CURVULARIA SPP)

‘‘‘Black Kernel (Curvularia spp.) ‘‘‘ In general, these species cause a blackish discoloration, but some may cause leaf spots or seedling blights under certain circumstances. Curvularia species are characterized by conidia that are multicellular, ellipsoid, broadly fusiform, or obovoid, somewhat to strongly curved, with one or more of the center cells larger and darker than the others.

BLACK STREAKED DWARF (RBSDV)

‘‘‘Black-Streaked Dwarf (RBSDV) ‘‘‘ Early symptoms include stunting, dark green discoloration of the leaves, and occasional vein swellings on the sheaths. Infected plants may also show twisting of the leaf tips and splitting of the leaf margins. At later growth stages, prominent dark brown streaks consisting of white, swollen veins or galls form along the undersides of leaves and on the sheaths and culms. Dark brown blotches may form on the kernels.

BLAST (PYRICULARIA GRISEA)

‘‘‘Blast (Pyricularia grisea) ‘‘‘ Blast is most severe in irrigated rice grown in temperate regions or at high elevations in the tropics, and in rainfed, upland rice. The pathogen produces lesions on all parts of the shoot but rarely attacks the leaf sheath. When lesions first appear, they are often white to gray-green with darker green borders. Older lesions are generally whitish to gray with dry borders. Their shape varies, but lesions are characteristically diamond-shaped. Lesions on the leaf blades of plants at the reproductive stage are generally larger (2 cm long) than lesions on younger plants (less than 1 cm long). The pathogen remains in infested crop residue or in seed in temperate regions where only one rice crop is sown per year. In environments that are not highly conducive to blast, the disease can be controlled most easily by planting resistant cultivars. Other control measures include manipulating time of planting, fertilizer, and water management as well as the use of fungicides.

BORON TOXICITY

‘‘‘Boron Toxicity: ‘‘‘ Boron toxicity causes yellowing at the tips of older leaves along the margins and large brown elliptical spots. Severely affected plants are stunted and may die.

BROWN SPOT (COCHLIOBOLUS MIYABEANUS)

‘‘‘Brown Spot (Cochliobolus miyabeanus) ‘‘‘ Brown spot may be manifested as a seedling blight or as a foliar and glume disease of mature plants. On seedlings, the fungus produces small, circular, brown lesions, which may girdle the coleoptile and cause distortion of the primary and secondary leaves. On the leaves of older plants, the fungus produces circular to oval lesions that have a light brown to gray center surrounded by a reddish brown margin. The fungus also may infect the glumes, causing dark brown to black oval spots, and may also infect the grain, causing a black discoloration. The most important factors in control of brown spot are proper nutrients in the soil and prevention of water stress.

BUNCHY STUNT

‘‘‘Bunchy Stunt: ‘‘‘ Infected plants are stunted, tiller profusely, the leaves are short and narrow, and young leaves may show mottling. In severely affected cultivars, branches are formed from the upper nodes, and leaves are produced in bunches. Bunchy stunt is suspected to be caused by a virus.

CHLOROPHENOXY HERBICIDES

‘‘‘Chlorophenoxy Herbicides: ‘‘‘ In injury due to chlorophenoxy herbicides, such as 2,4-D, MCPA, and 2,4,5- T, the tips of older leaves turn yellow and become dry. Root growth is inhibited, and roots appear enlarged or stubby within 2 weeks after herbicide application.

CHLOROTIC STREAK

‘‘‘Chlorotic Streak: ‘‘‘ Symptoms of chlorotic streak include stunting, yellow streaking or spotting of young leaves, poor emergence of panicles with sterile or discolored spikelets, leaf distortion, vein swelling, excess tillering, and formation of aerial roots. The disease is associated with the rice mealy bug. This disease is usually manifested in India.

COLLAR ROT (PHOMOPSIS ORYZAE SATIVAE)

‘‘‘Collar Rot (Phomopsis oryzae sativae) ‘‘‘ The disease occurs at the late tillering stage and is characterized initially by small, dark brown lesions at the base of the uppermost, fully expanded leaves. The lesions enlarge and extend to the leaf sheath and leaf blade and the collar becomes rotted and the leaf blade drops off. No information on the control of this disease is available.

COLOR CHANGE YOUNG LEAVES: ‘‘‘

‘‘‘Nutrient Deficiencies Causing Color Change in Young Leaves’‘‘ Leaf symptoms caused by deficiencies of nutrients with immobile ions (Fe, Zn, Mn, B, and Cu) usually occur in young or upper leaves, causing older leaves to appear more normal in color and size during the early stages of these deficiencies.

COPPER DEFICIENCY

‘‘‘Copper Deficiency: ‘‘‘ Leaves of plants with copper deficiency turn a bluish green and become yellow along the midrib and near the tip. Newly emerging leaves may fail to unfold and appear needlelike.

CRINKLE

‘‘‘Crinkle: ‘‘‘ Leaves of emerging seedlings are crinkled or distorted and may develop pale yellow, lens-shaped lesions. Eventually, the entire leaves may turn yellow, and roots become stubby and bushy. This disease usually only appears in northern, coastal African countries.

CROWN SHEATH ROT (GAEUMANNOMYCES GRAMINIS)

‘‘‘Crown Sheath Rot (Gaeumannomyces graminis) ‘‘‘ The disease affects the leaf sheaths, causing a dark brown to black discoloration or lesion that extends upward from the crown and the leaves of heavily infected sheaths die. The culm and crown are also infected, which results in inhibition of tillering and contributes to lodging, incomplete grain filling, and premature ripening of the panicle.

CYST NEMATODES (HETERODERA SPP)

‘‘‘Cyst Nematodes (Heterodera spp.) ‘‘‘ No diagnostic symptoms are associated with rice cyst nematode infection. In severe cases, plants are stunted and have retarded growth, leaves are severely dry and yellow, and the number of tillers is reduced. Rice cyst nematodes are mainly associated with upland rice. Continuous flooding, fallow periods, or crop rotation with soybeans or sweet potatoes, and soil fumigation reduce populations of the disease.

DOWNY MILDEW (SCLEROPHTHORA MACROSPORA)

‘‘‘Downy Mildew (Sclerophthora macrospora) ‘‘‘ The leaves of infected plants may have yellow to white spots or patches, and the flag leaf and upper leaves may be curled, bent, or twisted. The symptoms are most evident on the panicle, which fails to emerge completely, is contorted, remains green, and fails to produce any grain. Control measures are not generally necessary; however, treating seed and covering nursery beds with a layer of fungicide- treated rice hulls has been reported to protect emerging rice seedlings from infection.

DOWNY MILDEW VIRUSES

‘‘‘Viruses Associated with Downy Mildew: ‘‘‘ Two morphologically distinct types of virus particles are found in plants infected with downy mildew. The viruses occur inside the mycelium and oospores of the fungus Sclerophthora macrospora (Sacc.) Thirumalachar, C. G. Shaw, & Narasimhan.

DWARF (RDV)

‘‘‘Dwarf (RDV) ‘‘‘ Rice dwarf was the first plant virus demonstrated to multiply in an insect and the first shown to be transmitted and passed transovarially by an insect. Infected plants show pronounced stunting and produce diminutive tillers or a reduced number of tillers. Root growth is severely retarded and irregular yellow specks are formed on the leaves and sometimes on the leaf sheaths. Applying insecticides and using cultural practices that eliminate or reduce overwintering sites for the vector, such as planting fallow rice fields with nonvector hosts or ploughing fallow paddy fields, can help lower virus incidence. Resistant cultivars should be used where available.

EAR BLIGHT

‘‘‘Ear Blight: ‘‘‘ Ear blight is characterized by a discoloration and/or blight of the entire panicle or portions of the panicle, including the neck, branches, internodes, nodes, and spikelets. The discoloration may be defined, brown to black streaks, or it may appear diffuse. The best form of control is to apply fungicides and plant before cool weather is likely to begin, and fields should have a proper balance of nutrients.

ECHINOCHLOA HOJA BLANCA (EHBV)

‘‘‘Echinochloa Hoja Blanca Virus (EHBV) ‘‘‘ Echinochloa hoja blanca virus can be transmitted to rice by the planthopper vector Sogatodes cubanus.

ECHINOCHLOA RAGGED STUNT (ERSV)

‘‘‘Echinochloa Ragged Stunt Virus (ERSV) ‘‘‘ Echinochloa ragged stunt virus is transmitted by the brown planthopper, Nilaparvata lugens, and is serologically related to rice ragged stunt virus.

EYESPOT (DRECHSLERA GIGANTEA)

‘‘‘Eyespot (Drechslera gigantea) ‘‘‘ Initially, the lesions appear as small, oval, water-soaked, gray-green spots, on leaves of all stages of growth. A yellow halo frequently surrounds young lesions and as the disease progresses, the lesions enlarge and become white to straw-colored and delimited by a well-defined, brown margin. The leaves of highly susceptible cultivars turn yellow and wither immediately after heading. Because the disease is of minor importance, there are no control measures available.

FALSE SMUT (USTILAGINOIDEA VIRENS)

‘‘‘False Smut(Ustilaginoidea virens) ‘‘‘ The rice kernels are replaced by globose, velvety spore balls up to 1 cm in diameter (occasionally up to 5 cm), which burst out from between the glumes. The balls consist of three spore-producing layers surrounding a hard core of tightly woven mycelium. The innermost and middle layers contain immature spores and are pale yellow and orange yellow, and the outer layer, which consists of mature spores, is olive to olivaceous-black. The best form of control is to use resistant cultivars where available and applying fungicides at heading.

FEEDER ROOT NECROSIS

‘‘‘Feeder Root Necrosis and Root Rot: ‘‘‘ The disease is difficult to diagnose. Infected plants do not wilt or collapse, and there is no obvious rot of the primary roots. Root symptoms are characterized by an overall lack of fine feeder roots, and there may be restricted, reddish brown lesions on the primary roots where the feeder roots were attached. In extreme cases, plants may be slightly stunted and appear unthrifty. The best form of control is to apply fungicides to the soil immediately prior to flooding and seeding.

FOLIAGE & GLUMES DISEASES

‘‘‘Obscure Fungal Diseases of the Foliage and Glumes: ‘‘‘ Many obscure fungal diseases of the leaves and glumes have been reported. Often infection was noted when the plants were weak, dying, or under adverse environmental conditions, and it is likely that some of the fungi involved are extremely weak pathogens or saprophytes.

FOOT ROT (ERWINIA CHRYSANTHEMI)

‘‘‘Foot Rot(Erwinia chrysanthemi) ‘‘‘ Leaf sheaths of infected plants typically exhibit a dark brown decay, and the attached leaves turn yellow and wilt. Infected culms and internodes turn black. Roots attached to infected nodes decay and fall off. Bacterial ooze may be present inside the culms, and infected plants have an unpleasant odor. In some cases, the young leaves of tillers that show no sheath browning may wilt as a result of systemic infection of the crown alone.

FUNGAL DISEASES

‘‘‘Fungal Diseases: ‘‘‘ Fungi have been reported to cause more diseases of rice than any other group of pathogens. Diseases caused by fungi are grouped into seedling, foliar, leaf sheath and culm, root and crown, and grain diseases. The most important of these, such as blast, sheath blight, stem rot, and seedling diseases, can devastate a crop when environmental and cultural conditions favor disease development.

GALL DWARF (RGDV)

‘‘‘Gall Dwarf(RGDV) ‘‘‘ Typical symptoms of the disease are pronounced stunting, dark green leaves, twisting of the leaf tips, and small galls on the underside of leaf blades and on the outer surface of leaf sheaths. The galls are at first light green and somewhat translucent, becoming white, but not dark brown like those caused by rice black-streaked dwarf virus.

GIALLUME

‘‘‘Giallume: ‘‘‘ Giallume is caused by a strain of barley yellow dwarf. Leaves of infected plants are yellow to orange, dry, smaller, erect, and may have serrated margins. Infected plants are stunted, have a reduced number of tillers, and produce blasted florets. Severely infected plants are killed. The use of resistant cultivars and insecticides are the best forms of control.

GLUME BLIGHT (PHOMA SORGHINA)

‘‘‘Glume Blight (Phoma sorghina) ‘‘‘ The disease may appear as brown, pinhead-sized lesions or as lens- shaped lesions with bleached centers and brown margins. When panicle emergence coincides with a rainy period, the disease appears as reddish brown blotches on the glumes when they become extremely wet. The use of clean seed and cultivars that show resistance to the fungus can help control glume blight.

GRAIN DISCOLORATION

‘‘‘Grain Discoloration: ‘‘‘ Grain discoloration occurs in most rice production regions of the world. Discolored grain is often chalky, brittle, and shrunken and has reduced viability. In addition, certain microorganisms associated with discolored grain are known to produce mycotoxins. These factors detract from the quality and appearance of the grain and make it less marketable. Proper insect control and cultural practices that prevent lodging will help minimize grain discoloration. In addition, it is crucial that grain be harvested at the proper moisture content or dried to prevent invasion of microorganisms in storage.

GRAIN ROT (PSEUDOMONAS GLUMAE)

‘‘‘Grain Rot (Pseudomonas glumae) ‘‘‘ The disease is manifested as a grain rot of mature plants in the field and also as a seedling rot. On seedlings, symptoms consist of a brown, water-soaked soft rot of the leaf sheaths accompanied by wilting or soft rot of the leaves. On the panicle, infected grains are shrunken and pale green, becoming dirty yellow to brown and dry. A brown margin between the infected and healthy parts of the grain is a diagnostic feature of the disease. The disease is favored by high night temperatures (28(C) and high humidity.

GRASSY STUNT (RGSV)

‘‘‘Grassy Stunt (RGSV) ‘‘‘ Infected plants show pronounced stunting, proliferation of tillers, and short, narrow, pale green to pale yellow leaves. Leaves also may have numerous small, irregular, dark brown or rust- colored spots. Infected plants produce no panicles or a few panicles with unfilled kernels. Symptoms become evident 10-20 days after infection. The best form of control is to use resistant cultivars.

HALO BLIGHT (PSEUDOMONAS SYRINGE PV ORYZAE)

‘‘‘Halo Blight (Pseudomonas syringae pv oryzae) ‘‘‘ Halo blight is characterized by circular, pale green to yellowish brown lesions, 2-10 mm in diameter, on leaf blades. The lesions are surrounded by a distinct halo and have a dark brown spot or stripe in the center.

HERBICIDE INJURY

‘‘‘Herbicide Injury: ‘‘‘ Herbicides may injure rice when they are improperly applied to the crop, when they remain as toxic residues from the previous crop, when they drift into rice from application to adjacent crops, when they contaminate irrigation water sources, and when insecticides predispose rice to herbicide injury.

HOJA BLANCA (RHBV)

‘‘‘Hoja Blanca (RHBV) ‘‘‘ Infected plants are stunted, and the leaves have yellow stripes or are mottled and in severe cases, the whole leaf blade may turn white. Also, the palea and lemma are distorted and turn brown. Infected plants also have fewer and smaller roots, and the roots may be dry. Plants infected at the seedling stage may be killed. The best form of control is to use resistant cultivars.

IRON DEFICIENCY

‘‘‘Iron Deficiency: ‘‘‘ Iron deficiency causes younger leaves to lose their green color, turn pale yellow to whitish, and die.

IRON TOXICITY

‘‘‘Iron Toxicity: ‘‘‘ Iron toxicity causes very small brown spots on lower leaves, appearing first on the leaf tip and spreading toward the base. Entire leaves may appear purplish brown in severe cases.

KERNEL SMUT (NEOVOSSIA SPP)

‘‘‘Kernel Smut (Neovossia spp.) ‘‘‘ Kernel smut is characterized by the replacement of the kernel endosperm with a black, sooty mass of spores. The disease is most noticeable early in the morning, when dew causes completely smutted grains to swell and erupt, releasing the spores. Infected grains may be completely or partially smutted. Heavy applications of nitrogenous fertilizers and excessive flood depths during flowering increase the incidence of disease in fields with a history of infection. Short-grained cultivars are most resistant.

LANCE NEMATODES (HOPLOLAIMUS INDICUS)

‘‘‘Lance Nematodes (Hoplolaimus indicus) ‘‘‘ Lance nematodes are associated with the fungus Sclerotium rolfsii Sacc. and usually appears in areas of India.

LEAF SCALD (GERLACHIA ORYZAE)

‘‘‘Leaf Scald (Gerlachia oryzae) ‘‘‘ The classic leaf symptom is zonate lesions of alternating light tan and dark brown that begin at the leaf edges or tips. As the lesions mature, the zonate pattern fades, and affected areas dry out, giving the leaf a scalded appearance. Other symptoms caused by the disease include small, reddish brown spots on the leaves and patches of brown lesions on the sheaths. The fungus survives on rice seed and in dry crop debris.

LEAF SHEATH & GRAIN DISEASES (PSEUDOMONAS SPP)

‘‘‘Leaf Sheath and Grain Diseases (Pseudomonas spp.) ‘‘‘ Leaf Sheath and Grain Diseases typically cause a severe sheath rot and/or extensive spikelet sterility and grain discoloration.

LEAF SMUT (ENTYLOMA ORYZAE)

‘‘‘Leaf Smut (Entyloma oryzae) ‘‘‘ Leaf smut is found in most rice-growing regions of the world and is considered to be a minor disease. The fungus produces small, slightly raised, black spots or streaks under the epidermis of leaves and sometimes on the leaf sheaths. Under heavy infestation, the leaves turn yellow, the tips dry out and die, and the leaves split lengthwise. Resistant cultivars provide the best method of control if disease control is necessary.

LESION NEMATODES (PRATYLENCHUS SPP)

‘‘‘Lesion Nematodes (Pratylenchus spp.) ‘‘‘ Lesion nematodes are common parasites in roots of upland rice in Zimbabwe and India.

MANGANESE DEFICIENCY

‘‘‘Manganese Deficiency: ‘‘‘ Manganese deficiency produces interveinal yellow streaks that form from the tip to the base of the leaf, later becoming dark brown and dry and plants often become stunted.

MINUTE LEAF & GRAIN SPOT (NIGROSPORA SPP)

‘‘‘Minute Leaf and Grain Spot (Nigrospora spp.) ‘‘‘ Nigrospora species are common saprophytes of rice worldwide, occurring on senescent plant parts. On plants weakened by disease, insect pests, or poor nutrition, species of Nigrospora may infect the leaves, leaf sheaths, and culms and have been reported to cause an ear blight and blackening of the rice kernels. Small, powdery, black pustules up to 5 mm in diameter occur on infected plant parts. The fungi enter the host through the stomates and generally grow in the outer parenchyma cell layers. No control measures are available.

MYROTHECIUM BLOTCH (MYROTHECIUM VERRUCARIA)

‘‘‘Myrothecium Blotch (Myrothecium verrucaria) ‘‘‘ Irregular to oblong lesions, 4-20 cm in length, with dark brown centers and a lighter brown margin, occur on the flag leaf sheaths. Brown lesions, sometimes with whitish centers, also occur on the glumes of affected plants. Little information on epidemiology or control is available.

NARROW BROWN LEAF SPOT (CERCOSPORA ORYZAE)

‘‘‘Narrow Brown Leaf Spot (Cercospora oryzae) ‘‘‘ Symptoms usually occur during the late growth stages and are characterized by short, elliptical to linear brown lesions on the leaf blades, 210 X 11.5 mm. The lesions tend to be narrow, short, and dark brown on resistant cultivars and wider and lighter brown with gray dry centers on susceptible cultivars. Symptoms similar to those on the leaves also occur on the leaf sheaths, pedicels, and glumes. A net blotch-like pattern often forms on leaf sheaths, where the cell walls turn dark brown and the intercellular areas are tan to yellow. The disease is controlled through the use of resistant cultivars.

NECROSIS MOSAIC (RNMV)

‘‘‘Necrosis Mosaic (RNMV) ‘‘‘ Infected plants have elongate or spindle-shaped, yellow flecks and streaks on the lower leaves and, sometimes, dry flecks on the basal portions of stems and sheaths. Infected plants also are moderately stunted, have a reduced number of tillers, and have a spreading growth habit. The use of flooded seedbeds or direct seeding into flooded paddies reduces the virus incidence along with applications of fungicides or chemical sterilants.

NEMATODE DISEASES

‘‘‘Diseases Caused by Nematodes: ‘‘‘ Nematodes are unsegmented roundworms that are aquatic. Plant-parasitic nematodes are typically microscopic, transparent, and vermiform. Plant-parasitic nematodes feed and reproduce on living plants and are disseminated by wind, irrigation and flood water, tools, machinery, animals, and infected plant propagation materials.

NITROGEN DEFICIENCY

‘‘‘Nitrogen Deficiency: ‘‘‘ Nitrogen deficiency causes older leaves to change from green to light green or yellow and new leaves are darker green. Leaves are also narrow, short, and erect, and in advanced stages of deficiency they die, turning straw-colored.

NUTRIENT DEFICIENCIES & TOXICITIES

‘‘‘Nutrient Deficiencies and Toxicities: ‘‘‘ Nutrient deficiencies and toxicities in rice are common in most rice production areas of the world. Affected plants develop distinctive symptoms, which usually occur in irregular patterns in the field. The irregular patterns may be caused by inherent soil differences, such as sandy streaks, variations in pH or salinity, and variable nutrient content or availability. They are also often associated with uneven fertilizer application, tillage practices, and management of residue of previous crops. When possible, moderate symptoms should be used in diagnosis, because they are usually more definitive than symptoms in severely affected plants.

NUTRIENT DEFICIENCY W/LEAF COLOR CHANGE

‘‘‘Nutrient Deficiencies and Toxicities Causing Color Change in Older Leaves: ‘‘‘ Symptoms caused by deficiencies of nutrients with mobile ions (such as N, K, P, and S) usually occur initially in the lower leaves, with new leaves showing no symptoms. These mobile nutrients are translocated from older leaves to younger leaves and prevent the development of symptoms in younger leaves, especially in cases of mild deficiency.

ORANGE LEAF

‘‘‘Orange Leaf: ‘‘‘ Initial symptoms of infected seedlings are short, malformed leaves. The edges of the leaves are serrated and yellow, and the leaf tips are twisted. As the infection progresses, the leaves turn yellow-orange, beginning at the tip. Eventually the leaves become entirely orange, roll inward, and dry out. No control practices are recommended because the disease is not economically important.

OTHER MINOR VIRUSES

‘‘‘Other Minor Viruses: ‘‘‘ Several cereal viruses have been transmitted to rice mechanically under experimental conditions, including barley stripe, brome mosaic, maize dwarf mosaic, ryegrass mosaic, sugarcane mosaic, wheat streak mosaic, and potato yellow dwarf viruses.

PHOSPHORUS DEFICIENCY

‘‘‘Phosphorus Deficiency: ‘‘‘ Leaves of plants with phosphorus deficiency initially turn yellow but then rapidly turn a grayish green. These plants are stunted, leaves are narrow, short, erect, rolled and brittle.

POTASSIUM DEFICIENCY

‘‘‘Potassium Deficiency: ‘‘‘ On plants with potassium deficiency, leaves are short, drooping, and dark green, with yellowing developing between veins near the leaf tip. Small brown spots may also develop near the leaf tip and group together to form blotches toward the leaf base as the deficiency progresses.

PROPANIL

‘‘‘Propanil: ‘‘‘ Yellowing develops on older leaves exposed to propanil at the time of application beginning at the leaf tip and spreading to the base. These leaves dry out very quickly and usually die.

RAGGED STUNT (RRSV)

‘‘‘Ragged Stunt (RRSV) ‘‘‘ Plants infected with the virus are stunted and develop short, dark green leaves that are serrated along one or both edges, giving them a ragged appearance. Leaf serrations and twisting of the leaves are most pronounced during the early growth stages and on the flag leaf. Vein swellings or galls appear on the underside of the leaf blade and on the outer surface of the leaf sheath near the collar. The galls are pale yellow to sometimes brown and up to 10 X 1 mm. The best form of control is to use resistant cultivars.

RED BLOTCH OF GRAINS (EPICOCCUM SPP)

‘‘‘Red Blotch of Grains (Epicoccum spp.) ‘‘‘ Red blotch is characterized by pink to rose to red discolorations of rice kernels. The disease occurs when rice plants lodge before harvest, and the panicles come in contact with the soil. The disease can be avoided by measures that prevent lodging.

RICE ROOT NEMATODES (HIRSCHMANNIELLA SPP)

‘‘‘Rice-Root Nematodes (Hirschmanniella spp.) ‘‘‘ Aboveground symptoms include retarded growth, reduction in the tiller number, stunting, yellowing, and late maturation in severe cases. The nematodes move within the roots, and a few days after infection, the females lay their eggs. Juveniles hatch and develop in the root cortex. The best form of control is to use resistant cultivars where available and apply nematicides as soil treatments or as root dips prior to transplanting.

RICE TARSONEMID MITE VIRUS (RTMV)

‘‘‘Rice Tarsonemid Mite Virus (RTMV) ‘‘‘ Disease caused by rice tarsonemid mite virus is associated with sheath browning and grain sterility usually located in Japan and the Philippines and is transmitted by the tarsonemid. The disease only occurs in the epidermal and mesophyll cells of the leaf sheaths and hulls of the kernels where the mites are present.

RING NEMATODES (CRICONEMELLA ONOENSIS)

‘‘‘Ring Nematodes (Criconemella cnoensis) ‘‘‘ This disease causes characteristic knotting of the roots.

ROOT KNOT NEMATODES (MELOIDOGYNE SPP)

‘‘‘Root-Knot Nematodes (Meloidogyne spp.) ‘‘‘ Aboveground symptoms include yellowing, wilting, retarded maturation, and reduction in growth and tillering. These transformations result in the formation of characteristic root galls and prevent root elongation. Between rice crops, the nematodes survive as eggs in the soil or continue to reproduce on other crops or on weeds. The females lay their eggs inside the galls, and juveniles hatch from the eggs and reinfect the same root. Several nematicidal compounds applied as soil drenches, foliar sprays, seedling root dips, or seed soaks or in irrigation water give good control of root-knot nematodes and result in increased yields.

RUSTS

‘‘‘Rusts: ‘‘‘ The disease can be described by black, oblong, or ellipsoidal teliospores, that are rounded and coronate at the apex occurring on leaves. At present, rust diseases are not important on rice.

SALT TOXICITY

‘‘‘Salt Toxicity: ‘‘‘ On plants exposed to excessive levels of salinity, the tips of older leaves become yellow and later turn brown to white. Whitish deposits of salt also accumulate on plants at the waterline and on soil at the edge of the field.

SCAB (GIBBERELLA ZEAE)

‘‘‘Scab (Gibberella zeae) ‘‘‘ The fungus initially causes bleached lesions or a bleached discoloration on the glumes. Affected areas become yellow to salmon to carmine with the production of sporodochia and conidial masses of the fungus. Infected grains are light, shrunken, and brittle and may be sterile. The fungus is cosmopolitan and can be found in the soil and on crop residues. No control measures are necessary.

SEED BOX DISEASES

‘‘‘Seed Box Diseases: ‘‘‘ Symptoms of damping-off and seedling blight in seed boxes range from poor germination and dark lesions on coleoptiles or mesocotyls to complete blighting of seedlings. Seedling blight may cause stunting and yellowing of seedlings and/or sudden blighting, with seedlings turning dry and straw-colored, brown, or gray. Roots are often discolored and rotted. The diseases tend to occur in circular areas within a box of seedlings. Cottony, white mycelium may be observed growing over the seeds, seedlings, and soil or planting medium. Infected seedlings are several inches taller than healthy seedlings and are thin, with long, narrow, yellow-green leaves. Seed treatment with an appropriate fungicide has been effective in controlling these diseases.

SEEDLING BLIGHT (PSEUDOMONAS PLANTARII)

‘‘‘Seedling Blight (Pseudomonas plantarii) ‘‘‘ The disease is caused by seedborne and soilborne fungi and is greatly influenced by environmental conditions. Preemergence seedling blight is characterized by a discoloration or rot of the embryo area of the seed, resulting in no germination or destruction of the germinating embryo. These lesions may prevent emergence of the seedling from soil, or the seedling may emerge but be stunted or weakened. In postemergence seedling blight, dark brown or reddish brown lesions are usually present on the coleoptile or on primary or secondary leaves through about the three-leaf stage. Above the lesions, the leaves become blighted and die, turning straw- colored, light brown, or gray. Seedling blight is mainly controlled by the use of seed-protectant fungicides.

SEEDLING DISEASE

‘‘‘Seedling Diseases: ‘‘‘ Seedling diseases can affect rice in water-seeded fields, in seedbeds grown for transplanting, and in seed boxes for mechanical transplanting. These diseases cause resistance reduction and irregularities in directly seeded rice and loss of healthy seedlings for transplanting in seedbeds and seedling boxes. Seed-rotting and seedling diseases can often be prevented by the use of pesticides or by cultural practices, which include eliminating infected seed, avoid overwatering, keeping a level soil surface, and using well drained soils.

SEEDLING SHEATH & GRAIN DISEASES (PSEUDOMONAS SPP)

‘‘‘Seedling, Sheath, and Grain Diseases (Pseudomonas spp.) ‘‘‘ Seedling, leaf sheath, and grain diseases of rice are seedborne and cause a drying or rot of the plant part affected. The various species may attack both seedlings and the grain and/or sheaths of mature plants.

SHEATH BLIGHT (RHIZOCTONIA SOLANI)

‘‘‘Sheath Blight (Rhizoctonia solani) ‘‘‘ Among the fungal diseases affecting rice, sheath blight is second in importance only to rice blast and often rivals it. The pathogen is soilborne and endemic in most rice production areas, and rice sheath blight can develop into a major production-limiting disease in a short time. Symptoms of sheath blight generally do not appear until plants are in the late tillering or early internode elongation growth stages. Initial symptoms consist of circular, oblong, or ellipsoid, green-gray, water-soaked spots about 1 cm long that occur on the leaf sheaths near the waterline. The lesions enlarge to approximately 2-3 cm in length and 1 cm in width, and the centers of the lesions become pale green or white and are surrounded by an irregular purple-brown border. Lesions on the upper portion of plants may group together to encompass entire leaf sheaths and stems. The best form of control is to use resistant cultivars and foliar applications of fungicides.

SHEATH BLOTCH (PYRENOCHAETA ORYZAE)

‘‘‘Sheath Blotch (Pyrenochaeta oryzae) ‘‘‘ Sheath blotch has been reported in many parts of the world. The disease usually occurs at the late tillering stage and is characterized by large, brown, oblong lesions, about 2.5-cm long, occurring on the leaf sheaths. As the disease progresses, the lesions lengthen up to 10 cm, and turn grayish at the center. Spores of the fungus are presumably dispersed by splashing water. There are no control measures available because of its minor importance.

SHEATH BROWN ROT (PSEUDOMONAS FUSCOVAGINAE)

‘‘‘Sheath Brown Rot (Pseudomonas puscovaginae) ‘‘‘ On seedlings, a systemic discoloration of the leaf sheath occurs, which may spread to the midrib or veins of the leaves. On the sheath, oblong to irregular dark green, water-soaked lesions occur, which become gray-brown or brown and may be surrounded by a dark brown margin. The sheath may also exhibit general water-soaking and drying without definable lesions. Grains of infected panicles are discolored, deformed, or empty. The best form of control is to use clean seed or seed treated with dry heat at 65(C.

SHEATH NET BLOTCH (CYLINDROCLADIUM SCOPARIUM)

‘‘‘Sheath Net Blotch (Cylindrocladium scoparium) ‘‘‘ Sheath net blotch has only been reported from Japan and is a minor disease. The fungus produces small, yellow, oval lesions, 1-2 cm long, which occur on leaf sheaths near the waterline. Over time the lesions expand, become whitish, and are covered by a brown, netlike pattern formed from horizontal and longitudinal streaks. The disease is of minor importance, and control practices are generally not necessary.

SHEATH ROT (SAROCLADIUM ORYZAE)

‘‘‘Sheath Rot (Sarocladium oryzae) ‘‘‘ Lesions occur on the upper leaf sheaths, especially the flag leaf sheath. Lesions are initially oblong, 5-15 mm in length, with gray to light brown centers surrounded by a dark reddish brown margin. Lesions may also consist of a diffuse reddish brown discoloration in the sheath. Sterile, shriveled, or partially filled grain is also associated with infection of the panicle. Certain cultivars have resistance to the disease.

SHEATH SPOT (RHIZOCTONIA ORYZAE)

‘‘‘Sheath Spot (Rhizoctonia oryzae) ‘‘‘ The lesions are usually found on leaf sheaths midway up the tiller and sometimes on the leaf blades. Lesions are oval, 0.5-2 cm long and 0.5-1 cm and may be pale green, cream, or white, with a broad, dark, reddish brown margin. The lesions are usually separate and do group together. The disease is of minor importance, and usually no attempts are made to control it.

STACKBURN

‘‘‘Stackburn: ‘‘‘ The causal fungus produces oval to circular spots on the leaves, 3-10 mm in diameter, which are initially tan, becoming gray to white, and are surrounded by a narrow, dark brown border. Small black dots are produced in the centers of older lesions. Infection of the kernels produces discolored, shriveled, or brittle grains. Dark brown lesions may also occur on the coleoptiles of germinating seeds or young seedlings. No specific control recommendations are available, but seed-protectant fungicides and hot-water treatment reduce the incidence and severity of seedling blight.

STEM ROT (MAGNAPORTHE SALVINII)

‘‘‘Stem Rot (Magnaporthe salvinii) ‘‘‘ The first symptoms are generally observed in the field after the mid tillering stage. Initially, the disease appears as irregular, black lesions on the outer leaf sheaths at the waterline and as the disease progresses, the lesions expand, and the fungus moves inward, forming lesions on the inner leaf sheaths. Numerous tiny, black sclerotia are embedded in diseased leaf sheath tissue, and sclerotia and mycelium of the fungus are generally present inside infected culms. Control of stem rot can be achieved through an integrated program of residue management, proper fertilization, and cultivar selection.

STRAIGHTHEAD

‘‘‘Straighthead: ‘‘‘ The characteristic symptoms of straighthead are not visible until the late boot to heading stages of rice. Plants in affected areas appear vigorous and dark green, with no discernible symptoms until heading. Affected plants may be darker green with more erect leaves than normal plants, but this symptom is not always apparent where the disorder occurs. The most characteristic symptom of straighthead is erect panicles that are partially to completely sterile and of insufficient weight to cause the panicle to tip over during the grain-filling stage as in normal rice. Affected plants usually remain green, and their senescence is greatly delayed compared with that of normal rice plants. Control is achieved by draining fields at the onset of the plants' reproductive stage. Cultivars with tolerance to straighthead are available in some areas.

STRIPE (RSV)

‘‘‘Stripe (RSV) ‘‘‘ Plants infected at an early stage develop yellow leaves, which remain folded and wilt and droop as they elongate. Also, plants infected early are considerably stunted and may die prematurely. Plants infected at later growth stages develop leaves with yellow stripes or spotting and dry streaks. Infected plants produce fewer tillers, and panicles are poorly developed or absent. Planting cultivars with resistance and applying insecticides are the best forms of control.

STRIPE NECROSIS (RSNV)

‘‘‘Stripe Necrosis (RSNV) ‘‘‘ Infected plants are stunted and have a reduced number of tillers. Leaves of infected plants have yellow stripes and exhibit drying. The disease is soilborne.

STUNT NEMATODES (TYLENCHORHYNCHUS SPP)

‘‘‘Stunt Nematodes (Tylenchorhynchus spp.) ‘‘‘ Stunt nematodes are not adapted to continuous flooding but may cause economic damage in rainfed rice. These nematodes suppress the length and dry weight of roots in inoculation experiments.

SULFONYLUREA HERBICIDES

‘‘‘Sulfonylurea Herbicides: ‘‘‘ Older leaves of some cultivars turn yellow to purplish green and rapidly become dry in areas affected by sulfonylurea injury. Younger leaves may remain green for some time in cases of slight injury.

SULFUR DEFICIENCY

‘‘‘Sulfur Deficiency: ‘‘‘ Leaves become pale green, and yellow, more slowly and less intensively than in nitrogen deficiency.

THIOCARBAMATE HERBICIDES

‘‘‘Thiocarbamate Herbicides: ‘‘‘ The thiocarbamates molinate and thiobencarb cause similar injury symptoms, but those of thiobencarb may be more pronounced. Older leaves of injured seedlings (at the two- to four-leaf stage) turn yellow and die rapidly. Seedlings become blue-green, deformed, twisted, and brittle, and a slight swelling occurs at the base of the plant. Roots remain healthy, with no apparent symptoms.

TRANSITORY YELLOWING

‘‘‘Transitory Yellowing: ‘‘‘ Infected rice plants are stunted and have a reduced number of tillers, and the lower leaves turn yellow to bright yellow or orange. Plants that exhibit leaf discoloration during early growth may produce normal leaves during subsequent growth stages and appear to recover. At heading, infected plants are noticeable because they produce no panicles or poorly formed panicles. The best forms of control include using resistant cultivars and applying insecticides.

TUNGRO & WAIKA (RTBV and RTSV)

‘‘‘Tungro and Waika (RTBV and RTSV) ‘‘‘ Typical symptoms are stunting and yellow or yellow-orange discoloration of the leaves. Infected plants also have a reduced number of tillers and may have rust-colored spots on the leaves. The degree of stunting and leaf discoloration is dependent on the rice cultivar, strains of the viruses, age of the plant when infected, and the environment. Tungro may produce a spotting, striping, or interveinal drying of the leaves. The best forms of control include using resistant cultivars and applying insecticides.

UDBATTA (BALANSIA ORYZAE SATIVAE)

‘‘‘Udbatta (Balansia oryzae sativae) ‘‘‘ Symptoms are usually not apparent until heading, when infected plants produce erect spikes covered by white mycelium instead of normal inflorescences. The flag leaf and sheath of infected tillers are sometimes slightly distorted, and the flag leaf and upper leaves may appear silvery. Infected plants may also be stunted. The use of clean seed or seed treated with hot water or fungicide has been reported to control the disease. Resistant cultivars should be used where available.

UFRA (DITYLENCHUS ANGUSTUS)

‘‘‘Ufra (Ditylenchus angustus) ‘‘‘ Plants show malformations and yellowing or mosaic discoloration of the upper leaves. Later, scattered dark stains appear on the leaves, and parts of the upper internodes of the stem turn dark brown. The disease is most typical and evident on the panicle where the ufra may be swollen or ripe. Swollen ufra occurs when the nematode damages the panicle at an early stage; the panicle coils and remains enclosed within the leaf sheath, and there is a strong tendency for the stem to branch. The ripe ufra occurs when the nematode damages the panicle at a later stage; the panicle emerges but is distorted, bears sterile or empty, distorted spikelets at the base, and produces normal grains only near the tip. Lengthening the time between crops by delaying sowing, transplanting after flooding, destroying old crop debris, and using early-maturing cultivars all decrease the severity of ufra.

VIRUS & MYCOPLASMALIKE DISEASES

‘‘‘Diseases Caused by Viruses and Mycoplasmalike Organisms: ‘‘‘ Rice necrosis mosaic virus and rice stripe necrosis virus are most soilborne fungi. Applications of insecticides are difficult to time and are often inadequate to reduce vector populations. Certain cultural practices may help reduce vector populations such as decreasing plant density, proper timing of planting, and synchronous planting of rice crops but may not be effective or feasible in all situations. Generally, plants that are infected during the early growth stages develop little grain, and total yield loss is common with severe infections. Plants infected at later growth stages often develop mild symptoms or no symptoms.

WATER MOLD

‘‘‘Water Mold: ‘‘‘ In water-seeded rice, water mold can be observed through clear water as a ball of fungal hyphae surrounding seeds on the soil surface. A circular copper brown or dark green spot about 25-35 mm in diameter eventually develops around the rotted seed or stunted seedling. The color is caused by bacteria and green algae that are mixed with the fungal hyphae. When seeds are attacked immediately after sowing, they do not germinate, because the endosperm is destroyed or the embryo is rotted. The fungi are present in cracks in the seed coat or where the radicle and plumules are emerging from the seed. Seed treatments with a recommended fungicide are effective in controlling water mold. Proper water management can also reduce water mold.

WHITE LEAF STREAK (MYCOVELLOSIELLA ORYZAE)

‘‘‘White Leaf Streak (Mycovellosiella oryzae) ‘‘‘ The disease is characterized by small, interveinal, oblong to linear lesions, 1-3 X 0.5 mm, visible on both leaf surfaces. The lesions are white to light gray and surrounded by a distinct, very narrow, brown margin. They may form long, uninterrupted white streaks on heavily infected plants. Young lesions may be white on the leaf surface and brown on the undersurface. No information on the epidemiology or control of this disease is available.

WHITE TIP (APHELENCHOIDES BESSEYI)

‘‘‘White Tip (Aphelenchoides besseyi) ‘‘‘ The common name white tip is based on the characteristic whitening of 3-5 cm of the leaf tip. The whitened tips eventually become dry and shred and the central part and the base of infected leaves are sometimes darker green than normal. The upper leaves are the most affected, and the flag leaf is often twisted, hindering the emergence of the panicle. Other symptoms include reduction in the length of the panicle, reduction in the number of grains, sterile flowers, misshapen grains, stunting of the plant, late ripening and maturation, and production of tillers from the upper nodes. Since the nematode is seedborne, hot-water treatment of seed is the best control method. Burning straw, weeds, and new growth, and seeding directly into standing water are also recommended for control.

WRINKLED STUNT & WITCHES BROOM

‘‘‘Wrinkled Stunt and Witches'- Broom: ‘‘‘ Both wrinkled stunt and witches'-broom are seed-transmitted. Symptoms include severe stunting with dark green, wrinkled, twisted leaves. They also show pronounced stunting and tillering producing up to 100 tillers.

YELLOW DWARF

‘‘‘Yellow Dwarf: ‘‘‘ Infected plants are stunted, tiller profusely, and show a general yellowing of the tissues, producing pale green or yellow leaves. Plants infected at the seedling stage develop symptoms 40-50 days after infection and often die prematurely. Plants that survive produce no panicles or a few defective panicles. Late planting of rice is recommended to avoid the migration of the vectors from the overwintering hosts, when their population is greatest. Applications of insecticides to reduce vector density in the nurseries and fields may also help reduce yellow dwarf incidence.

YELLOW MOTTLE (RYMV)

‘‘‘Yellow Mottle (RYMV) ‘‘‘ Infected plants are stunted and produce fewer tillers. Leaves are mottled or have yellowish streaks and are often twisted or crinkled. Panicles of infected plants show incomplete emergence and sterile kernels. The best form of control is to use resistant cultivars when available.

ZINC DEFICIENCY

‘‘‘Zinc Deficiency: ‘‘‘ In water-seeded, continuously flooded rice with zinc deficiency, the midribs of younger leaves turn yellow, especially at the base, and the yellowing spreads over the entire leaf. Affected leaves tend to be droopy and float on the water rather than grow erect. Brown blotches and streaks later appear on older leaves, plant growth is stunted, and plants may die in cases of severe deficiency. In dry-seeded rice, the characteristic symptom of zinc deficiency is small brownish spots that form on the leaf tip and eventually over the whole leaf surface, giving the field a bronzed appearance, soon after permanent flooding. New leaves of recovering plants may be green, with older leaves showing advanced symptoms.