Rice

Disease Management

Introduction Bacterial Blight Bacterial Leaf Streak Blast Brown Spot Sheath Blight Sheath Rot Tungro Virusus Grain discolouration Diseases at different stages & Control measures

Introduction

  • In rice, diseases are caused by fungi, bacteria and viruses.
  • Losses due to the diseases in rice is very heavy and famines have been reported in the history due to the crop diseases.
  • Our country faced a severe famine in 1943 in Bengal due to epidemic out break of brown leaf spot (Helminthosporium oryzae).
  • Blast disease was a severe epidemic in 1919 in Thanjavur district.
  • It also occurred in severe form in 1985-88 in Tamilnadu.
  • Bacterial blight was an epidemic in 1963 in Bihar.
  • Rice Tungro diseaseout break was severe during 1985-87 in Tamilnadu.


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Bacterial blight: Xanthomonas oryzae p.v. oryzae

Symptom


  • The disease is usually noticed at the time of heading but in severe cases occur earlier also.
  • Seedlings in the nursery show circular, yellow spots in the margin,which later enlarge, coalesce and cause drying of foliage.
  • "Kresek" symptom is seen in seedlings, 1-2 weeks after transplanting.
  • The bacterium enters through the cut wounds in the leaf tips, becomes systemic and causes death of entire seedling.
  • In grown up plants water soaked, translucent lesions appear usually near the leaf margin.
  • They coalesce together forming long lesions with wavy margins and turn yellow.
  • As the disease progresses, the lesions cover the entire leaf blade which may turn white or straw coloured.
  • Lesions may also be seen on leaf sheaths in susceptible varieties.
  • Milky opaque dew drops containing bacterial masses are formed on young lesions in the early morning.
  • They dry upon the surface leaving a white encrustation.
  • The affected grains have discoloured spots surrounded by water soaked areas.
  • If the cut end of leaf is dipped in water, bacterial ooze makes the water turbid.

Favourable conditions

  • Clipping of tip of the seedling at the time of transplanting,
  • Heavy rain
  • Heavy dew
  • Flooding
  • Deep irrigation water
  • Severe wind
  • Temperature of 25-30°C
  • Application of excessive nitrogen
  • Late top dressing

Mode of spread

  • The pathogen survives in soil and in the infected stubbles and on collateral hosts.
  • The pathogen spreads through irrigation water in dry season and also through typhoons and rain storms.

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Bacterial leaf streak: Xanthomonas oryzae pv. Oryzicola

  • Bacterial leaf streak is another important disease preventing the realisation of high yield potential offered by intensive cultivation.
  • Although it is less devastating than bacterial blight, it causes substantial yield losses.

Economic importance of the disease

The susceptible cultivars sustain considerable losses because entire leaf areas turn yellow and dry up prematurely. Losses ranging from 5-30 per cent depending on the cultivars have been reported.

Symptoms


  • Infection in the florets and seeds results in brown or black discoloration and death of ovary, stamens and endosperm and browning of glumes.
  • Seeds fail to mature but the rachis do not show any symptom.
  • The first noticeable and characteristic symptoms of the disease are the water soaked ,translucent interveinal streaks running parallel to the veins which may be 1-10 cm long.
  • The progress of the streaks is longitudinal, limited by the veins and soon turn yellow or orange brown.
  • All along the streaks bacterial exudates could be observed as tiny yellow or amber colored droplets.
  • These streaks may coalesce to form large patches and cover the entire leaf surface.
  • Symptoms are more conspicuous in indicas than in japonicas.

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Blast: Pyricularia oryzae: (Sexual stage: Megnaporthe grisea)

Symptoms

  • The fungus attacks the crop at all stages from seedlings in nursery to heading in main field.
  • The symptom can be seen on leaves, nodes, neck of panicle and on grains.
  • On the leaves, the lesions start as small water soaked bluish green flecks, soon enlarge and form characteristic spindle shaped spots with grey centre and dark brown margin (Leaf blast).
  • The spots join together as the disease progresses and large areas of the leaves dry up and wither.
  • Similar spots are also formed on the sheath.
  • Severely infected nursery and field present a burnt appearance.
  • The nodes are affected later and irregular black areas encircle the nodes.
  • The affected nodes may break up and all the plant parts above the infected nodes may die (Node blast).
  • The grains may be poorly filled or become chaffy.
  • Dark spots may be formed on the grains.

Favourable conditions

  • Application of excessive doses of nitrogenous fertilizers
  • intermittent drizzles
  • cloudy weather
  • high relative humidity (93-99%)
  • long dew periods
  • low night temperature (below 20°C)
  • availability of collateral hosts

Mode of spread

  • The disease spreads primarily through airborne conidia since spores of the fungus are present throughout the year.
  • Mycelium and conidia in the infected straw and seeds are important sources of inoculum.
  • Irrigation water may carry the conidia to different fields.
  • The fungus also survives on collateral hosts.

Scouting

  • Leaf blast, bacterial leaf blight and brown spot
  • Select 10 hills from each microplot and grade the disease intensity.
  • Intensity Grade
  • Less than 1% leaf area affected 1
  • 1-5% of leaf area affected 3
  • 6-25% of leaf area affected 5
  • 26-50% of leaf area affected 7
  • 50% of leaf area affected 9

Neck blast

Select 5 hills in each microplot and the percentage of panicles infested is calculated to assess the grade.

Intensity Grade

  • Less than 1% 1
  • 1-5% 3
  • 6-25% 5
  • 26-50% 7
  • 50% 9

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Brown spot:Helminthosporium oryzae (Syn: Drechslera oryzae) (Sexual stage : Cochliobolus miyabeanus)

Symptom

  • The fungus attacks the crop from seedling in nursery to milk stage in main field.
  • The disease appears first as minute brown dots, later becoming cylindrical or oval to circular.
  • Then,, several spots coalesce and the leaf dries up.

  • The seedlings die and affected nurseries can be often recognized from a distance by their brownish scorched appearance.
  • Dark brown or black spots also appear on glumes.
  • The infection of the seed causes failure of seed germination, seedling mortality and reduces the grain quality and weight.

Favourable conditions

  • Temperature of 25-30°C
  • Relative humidity above 80 per cent.
  • Excess of nitrogen aggravates the disease incidence.

Mode of spread and survival

  • The infected seeds are the most common source of primary infection.
  • The fungus also survives on collateral hosts.

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Sheath blight: Rhizoctonia solanii (Sexual stages: Thanetophorus cucumeris)

Symptoms

  • The fungus affects the crop from tillering stage.
  • Initial symptoms are noticed on leaf sheaths near water level.
  • On the leaf sheath oval/elliptical or irregular greenish grey spots are formed.
  • As the spots enlarge, the centre becomes greyish white with brown margins.
  • The spots coalesce and the leaf sheaths die showing rotting symptoms.
  • The infection extends to the inner sheaths resulting in death of the entire plant.
  • Older plants are highly susceptible. Five to six week old leaf sheaths are highly susceptible.

Favourable conditions

  • High relative humidity (96-97%)
  • High temperature (30-32°C)
  • Closer planting
  • Heavy doses of nitrogenous fertilizers

Mode of spread and survival

  • The pathogen can survive as Sclerotia or mycelium in dry soil for about 20 months and for 5-8 months in moist soil.
  • Sclerotia spread through irrigation water.

Sheath blight

Select 10 hills in each microplot and assess the grade as follows:

  • Spots limited to lower ¼ of leaf sheath area 1
  • Spots present on lower ½ of leaf sheath area 3
  • Spots present more than ½ of leaf sheath 5
  • Spots present on more than ¾ of the leaf sheath area severe infection on upper leaves 7
  • Spots upto the top of tillers. Severe infection on all leaves and death of plants 9

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Sheath rot Sarocladium oryzae (Syn: Acrocylindrium oryzae)

Symptoms


  • They enlarge and develop grey centre and brown margins covering major portions of the leaf sheath.
  • The young panicles may remain within the sheath or emerge partially.
  • The panicles rot and abundant whitish powdery fungal growth is formed inside the leaf sheath.

Favourable conditions

  • Close planting
  • High doses of nitrogen
  • High humidity and temperature around 25-30°C
  • Injuries made by leaf folder, brown plant hopper and mites increases infection.
  • Host nutrition is one of the important factors that influence the disease development.
  • The susceptibility increases with the increase in the rate of N fertilizers.

Mode of spread and survival

Mainly through air-borne conidia and also seed-borne.

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Rice tungro disease

Identification /detection of Rice Tungro Disease

  • Rice tungro disease occurs most frequently in irrigated areas where rice is grown continuously.
  • In some seasons it may develop into an outbreak and affect other rice areas causing severe yield loss.
  • Tungro is a disease of the rice plant caused by a virus.
  • The virus causes the rice plant to become sick and show symptoms of disease.
  • Disease symptoms usually appear between one and two weeks after the plant has become infected with the virus.
  • Tungro disease can be identified by the symptoms on affected rice plants.

Symptoms

  • Rice plants affected by tungro at an early growth stage have stunted growth, yellow or yellow orange leaves and a slightly reduced number of tillers.
  • If plants are infected at a late growth stage, there may be yellowing of the leaves but no stunting or reduced tillering is noticed.

Spread of the disease

  • Tungro disease is spread by two kinds of insects: Green leafhopper and the Zig Zag leafhopper.
  • It cannot spread through seed, soil, air, water, wound in the plant or in any other way.
  • There are two species of green leafhopper commonly found in rice fields; they are Nephotettix virescens and N. nigropictus.

About the virus particles

  • There are two different kinds of rice tungro viruses.
  • One kind of virus has a round shape and is called rice tungro spherical virus, or RTSV.
  • The other kind has a bullet or bacilliform shape and is called rice tungro Bacilliform virus, or RTBV.
  • Rice plants can become stressed and show symptoms which look similar to tungro.
  • Symptoms caused by other diseases, viz., diseases caused by other rice viruses like rice yellow dwarf and grassy stunt also resemble tungro.
  • Some diseases caused by fungi or bacteria may at first sight look similar to tungro.
  • Lack of nutrients can stress rice plants and cause conditions which may appear a little similar to tungro, like nitrogen and zinc deficiency.
  • Sometimes feeding damage by rice stem borer at the vegetative stage of plant growth may, at first sight be confused with tungro.

Detection of rice tungro virus

Earlier two tests were employed for detecting rice tungro virus. They are

1. Iodine test

2. Transmission test

  • The iodine test is not 100 per cent reliable for tungro identification, as plants infected with some other rice virus also may sometimes show a dark color reaction.
  • The results of the transmission test will not be available until more than three weeks after the initial observation.
  • Under these circumstances one reliable method is the TAS-ELISA.
  • The virus can be detected by using the antiserum.
  • Scouting of Virus and mycoplasma diseases

Select 10 hills in each microplot and calculate the percentage of hills infected.

  • Intensity Grade
  • Less than 1% 1
  • 1-10% 3
  • 11-30% 5
  • 31-60% 7
  • 61-100% 9

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Grain discolouration

Drechslera oryzae, D. rostratum, Curvularia lunata, Trichoconis padwickii, Sarocladium oryzae, Fusarium moniliformi.

Symptom

  • The grains may be infected by various organisms before or after harvesting causing discolouration, the extent of which varies according to season and locality.
  • The infection may be external or internal causing discolouration of the glumes, kernels or both.
  • Dark brown or black spots appear on the grains.
  • Under humid conditions, the fungal growth may be prominently seen.
  • The discolouration may be red, yellow, orange, pink or black, depending upon the organism involved and the degree of infection.
  • This disease is responsible for quantitative losses of grains.

Favourable conditions

High humidity and cloudy weather during heading stage.

Mode of spread and survival

The disease spread is mainly through air-borne conidia. The fungi survives as parasite and saprophyte in the infected grains, plant debris / other crop debris respectively.

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Diseases at different stages & Control measures

Crop Stage Diseases
Seedling Blast, Brown spot, Tungro
Tillering Blast, Sheath rot, Sheath blight, Brown spot, Bacterial leaf Blight
Vegetative / Reproductive Blast, Brown spot, Tungro, Bacterial leaf Blight
Maturity Blast, Grain discolouration, Bacterial blight, Tungro

Control Measures for Disease Management

  • Avoid raising nurseries near lamp post
  • Seed Treatment with Pseudomonas fluorscens @ 10 g talc based formulation in 400 ml water/kg of seed (reduces sheath blight, blast and RTV)
  • Nursery application (2 days before pulling of seedlings) of talc based Pseudomonas fluorescens @ 200 g/20 cents
  • Examination of nurseries frequently for monitoring the diseases
  • Spray the nursery with Carbendazim 25 g or Edifenphos 25 ml or Mancozeb 32 g for 8 cent nursery.
  • In areas where tungro is a problem, 1.4 kg of Carbofuran can be applied on 10th day after sowing to control the vector green leafhopper.
  • If Carbofuran is not applied, any one of the following chemicals like Monocrotophos or Fenthion @ 16 ml/16 lit of water can be sprayed between 10-20 DAS.

Main field disease management

  • Avoid cultivation of highly susceptible varieties
  • Remove and destroy the weed hosts in the field bunds and channels
  • Avoid close spacing of seedlings in the main field.
  • Application of nitrogen through ammonium chloride
  • Apply optimum dose of fertilizers
  • Apply organic amendments
  • Burn the stubbles
  • Avoid flow of irrigation water from infected to healthy fields.
  • Deep ploughing in summer and burning of stubbles.
  • Crop rotation, adjustment of planting time and proper fertilization.
  • Avoid clipping of tip of seedling at the time of transplanting.
  • Avoid flooded conditions.
  • Use disease free seeds.
  • Treat the seeds (wet seed treatment) with Carbendazim or Tricyclozole at 2 g/kg
  • Spray the crop at boot leaf stage with mancozeb 1 kg or Iprobenphos 500 ml or Carbendazim 250 g/ha for management of grain discoloration
  • Store the grains with 13.5-14.0% moisture for management of grain discoloration.
  • Pseudomonas fluorescens @ 0.2% (2 g) can be applied on 60, 70 and 90 DAP.
  • Neem oil 3% (30 ml/lit) or 5% (50 g/lit) neem seed kernel extract can be applied (for management of Sheath blight, sheath rot and grain discoloration).
  • Botanical formulations against sheath blight disease like Neem Azal and Neem Gold are effective.
  • For bacterial leaf streak foliar spray of fresh cow dung water extract at 20 % is recommended.
  • For grain discoloration Neem oil + Pungam oil 60 EC © @ 3% (30 ml/lit) can be applied. Application of carbendazim + mancozeb (1:1) at 0.2 % is also recommended.
  • Spray the main field with Edifenphos 250 g or Iprobenphos 500 ml or Carbendazim 250 g or Tricyclazole 400 g or Thiophanate Methyl 500 g/ha (for management of blast)
  • Spray Carbendazim 250 g or Chlorothalonil 1 kg or Edifenphos 1 lit/ha (for management of sheath blight)
  • Spray Streptomycin sulphate and tetracycline combination 300 g + Copper oxychloride 1.25 kg/ha (for management of bacterial leaf blight and bacterial leaf streak).

Fungicides commonly recommended and spray schedule/ha

Diseases
Ediphenphos (ml)
Carbendazim (g)
IBP (ml)
Mancozeb (g)
Blast
500
250
500
-
Brown spot
500
-
500
1000
Sheath rot
750
250
-
1000
Sheath blight
-
250
-
-
Grain discoloration
-
250
500
1000

  • The above recommended fungicides are to be applied only on the appearance of diseases.
  • The fungicides must be applied to main crop and also to the field bunds.

Resistant varieties

Diseases Varieties
Blast ADT 36, ADT 37, CO 43, CO 47, CR 1009
Tungro ADT 37, PAIYUR 1, CR 1009
Bacterial leaf blight CR 10


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