Important diseases of rice
|Brown spot or Sesame leaf spot
|Bipolaris oryzae (Helminthosporium oryzae)
|Narrow brown leaf spot
|Foot rot or Bakanae disease
|Bacterial leaf blight(BLB)
|Xanthomonas oryzae pv. oryzae
|Bacterial leaf strea
|Xanthomonas oryzae pv. oryzicola
|Rice tungro virus [RTV]
|Rice grassy stunt virus
|Rice dwarf virus
|Phytoplasma like organisms
1. Blast – Pyricularia oryzae
The pathogen produces three different symptoms viz., leaf blast, nodal blast and neck blast.
On the leaves, spindle shaped spots are formed with whitish grey centre and dark brown margin. These spots coalesce together and cause quick death of leaves.
On nodes, blackening of the tissues is noticed which eventually break up and all the plant parts above the infected nodes may die.
The infected neck region of the panicle show dark brown to black spots and shriveled. The affected plants develop chaffy grains. The fungus infects the grain also and the infected grains become dark brown.
Mode of spread
The secondary spread of the disease is through air borne. The pathogen also spreads through seed borne conidia.
The pathogen survives as dormant mycelium and conidia in the infected straw and seeds. Also, collateral hosts like Panicum repens, Leersia hexandra, Echinochloa crusgali also harbour the pathogen
Intermittent drizzles, cloudy weather, high RH (above 92 per cent), low temperature (around 200 C particularly during the nights, early morning dew, alternate with a day temperature around 300 C), long dew periods, excess dose of nitrogenous fertilizers favour the disease development.
- Seed treatment with [Captan / thiram /carbendazim @ 2g / kg (or) P.fluorescens @ 10g / kg of seed]. Seedling root dipping can also be followed with P.fluorescens.
- Avoid closer spacing of seedlings in the main field and excess dose of nitrogenous fertilizers.
- Grow resistant varieties like CR 1009, Co 43 & 44, ADT 36, ADT 39 and 40.
- Spray with Pseudomonas fluorescens @ 2% or Iprobenphos @ 500 ml/ha or Tricyclazole @ 400g/ ha can be followed.
2. Brown spot or Sesame leaf spot – Helminthosporium oryzae
Numerous brown, round to oval spots appear on the leaves and leaf sheaths. The spots appear like sesame and hence the name sesame leaf spot. The grains also become infected and dark brown spots can be seen on the glumes. The disease is responsible for the Bengal famine of 1942.
Mode of spread
The primary spread is through infected seeds and the secondary spread is through air borne conidia
The pathogen survives in the infected seeds and collateral hosts like Panicum repens, Leersia hexandra, Echinochloa crusgali, Cyanodan dactylon etc.
Cloudy days and excess of nitrogenous fertilizer application favours the disease development.
- Use disease free seeds.
- Seed treatment either with chemicals viz., Captan / thiram /carbendazim @ 2g / kg (or) bio control agent P.fluorescens @ 10g / kg of seed.
- Grow resistant varieties [Co 44, Bhavani]
- Spray the crop with Carbendazim 250 gm/ha or Mancozeb 1 kg/ha.
3. Sheath blight – Rhizoctonia solani
The infected areas just above the water level (Collar region) are greenish grey, oval, and become grayish white with brown irregular margin on the leaf sheath. In the advanced stages brown colour sclerotia (Irregular shaped) are formed in the infected tissues.
Mode of spread
The pathogen spreads through irrigation water
The pathogen can survive as sclerotia and mycelium in the diseased plant debris left in the field for about 20 months (depending upon the temperature and moisture conditions). It survives only for 5 to 8 months in moist soil. The pathogen also survives in collateral hosts like Panicum repens, Leersia hexandra, Echinochloa crusgali, Cyanodan dactylon etc.
High relative humidity, closer planting and warm temperature favours the disease development. Maximum infection occurs at 100 per cent and minimum at 85-88 % relative humidity.
- Avoid flow of irrigation water from infected to healthy fields.
- Summer ploughing helps to reduce soil borne sclerotia.
- Avoid excess dose of nitrogenous fertilizers.
- Spray with Carbendazim @ 250g / ha or chlorothalonil @ 1kg / ha or Propiconazole @ 0.1 % during tillering stage can be followed.
4. Sheath rot – Sarocladium oryzae
The disease affects the boot leaf covering the panicle. The affected areas shows grayish brown, oblong lesions on the upper most leaf sheath with a grey centre and brown margins. The infected young panicles remain in the sheath or emerge partially. In severe conditions the whitish mycelial growth can be observed in the panicle inside the sheath. The young panicles if affected remain inside the sheath or emerge partially.
Survival and Mode of spread
The pathogen survives in the infested seeds and spreads through air borne conidia. The infected seeds also spread the disease.
Closer planting, high humidity, a temperature range of 25o – 30oC favours the disease development. The pathogen easily attacks the plants through wounds. The panicle which does not emerge is normally attacked by this disease.
- Avoid excess dose of nitrogen fertilizers.
- Adopt optimum spacing.
- Spray with carbendazim @ 250g/ha or mancozeb @ 1kg / ha or chlorothalonil @ 1kg / ha or NSKE (5%), or Ipomoea/Prosophis leaf powder extract at booting stage controls the disease.
Differences between Sheath rot and Sheath blight diseases
|Sheath rot disease
|Sheath blight disease
|Caused by Sarocladium oryzae and its perfect stage is Acrocylindricum oryzae
|Caused by Rhizoctonia solani and its perfect stage is Thanetophorus curcumeris
|The disease affects the boot leaf covering the panicle or the upper most leaf sheath
|Affects the crop at the collar region or the base of the plants at the water surface
|The fungus produces conidia
|The fungus produces sclerotia
|The fungus spreads through air borne conidia and pathogen is also seed borne
|The pathogen is soil borne and spreads through irrigation water
|The disease is severe during boot leaf stage
|The disease is severe during maximum tillering stage
|The pathogen survives in the infected plant debris
|The pathogen survives as sclerotia in soil
5. Stem rot – Sclerotium oryzae
Small, black, irregular lesions appear on the outer leaf sheath. Theses spots enlarge and reach the inner leaf sheath. Finally the infected leaf sheath rots and sclerotia are formed in the host tissues.
Mode of spread
The pathogen spreads through irrigation water.
The pathogen survives as soil borne sclerotia in the infected stubbles and straw
Heavy application of nitrogen fertilizers favours the disease development.
- Summer ploughing and burning the stubbles left in the infested fields.
- Balanced fertilizer application.
- Avoid flow of irrigation water from infected field to healthy field.
6. Foot rot or Bakanae disease – Fusarium moniliforme
The infested seedlings produce tall, lanky tillers (Bakanae) much taller than healthy seedlings and forms flower earlier than the healthy ones. Above the ground level, adventitious roots are produced from the nodes.
Mode of spread
The pathogen is externally seed borne
The pathogen is also soil borne, but the fungus seems to be short-lived in soil in the tropics. The fungus survives in soil for about 4 months in the form of thick-walled hyphae or micro conidia. The fungus produces only micro and macro conidia and not chlamydospores.
A temperature range of 270-300 C and low R.H. is optimum for the fungal growth. Application of nitrogenous fertilizers encourages the disease development.
Seed treatment with captan or carbendazim or thiram @ 2g / kg of seed
7. False smut – Ustilaginoidea virens
It is also called as Green smut or Lakshmi disease as the farmers believe that the lakshmi disease affected fields normally results in a bumper harvest.
The disease appears on the ears and converts individual grains in to greenish smut balls. The infected ovaries are transformed in to large, velvety green masses. Usually only few grains are affected in a panicle.
Mode of spread
The chlamydospores are air borne spores but do not free themselves from the spore balls easily because of the presence of the sticking material.
The pathogens survives as sclerotia and chlamydospores in the soil
Rainfall and cloudy weather during the flowering and maturity periods favour the disease development.
- The disease usually does not require special management methods
- Spraying copper oxychloride reduces the incidence
8. Udbatta disease – Ephelis oryzae
The affected panicle emerges from the leaf sheath as a slender dirty grey coloured, cylindrical spike, rod like hard structures. They very much resemble as ‘’udbatta’’ hence the name is given as udbatta disease. No grains are formed in the affected panicles.
Mode of spread and Survival
The pathogen is externally seed borne and survives in the infested seeds.
A soil temperature of around 280 C and abundant soil moisture favour the disease.
- Hot water treatment 540 C for 10 minutes.
- Removal of collateral host.
- Grow resistant variety – CB 11.
9. Grain discolouration –Fungal complex disease
Dark brown to black spots appear on the grains. The infection may be external / internal causing discolouration [Red, yellow, orange, pink] of the glumes / kernels or both. Many fungi are found involved in the disease depending on the locality.
Mode of spread and Survival
The pathogens spreads through air borne conidia and survives in the infected plant debris, grains and collateral hosts.
High humidity and cloudy weather favour the disease development.
- Spray the crop at boot leaf stage with mancozeb 1kg/ha or captofal 250 g/ha or Carbendazim 250 g/ha.
- Store the grain with less than 11 % moisture content.
10. Bacterial leaf blight – Xanthomonas oryzae pv. oryzae
i. Leaf blight phase
Small water soaked streaks appear at the margin of the lamina near the tip which enlarges and become as straw coloured necrotic patch. The inner portion of the diseased leaves also shows wavy margin. Under humid condition, creamy white bacterial ooze characteristics of the disease can also be observed.
ii. ‘‘kresek’’ or wilt phase
kresek’’ or wilt phase is noticed at two week old transplanted crops and wilting of the plant results from systemic infection of the pathogen.
iii. Yellow leaf phase
The affected plants show pale yellow discolouration. The youngest leaf in a hill may turn yellow or white.
Mode of spread
The primary source of infection is through the seed borne inoculum. The secondary spread is through air and wind splashed rains.
The bacterium survives in the infected seeds, stubbles and collateral hosts.
The bacterium is rod shaped with round end, gram negative, non – spore forming and has single polar flagellum. Hydathodes, stomata, wounds are the portals of entry for the pathogen. Heavy rain, dew and severe wind favours the disease development
- Avoid clipping of tip of the seedling at the time of transplanting.
- Use optimum dose of fertilizers.
- Hot water treatment of seeds for 10 minutes at 52 – 540 C
- Grow resistant varieties like IR-20, IR-36, TKM-6
- Spray streptomycin sulphate and tetracycline combination @ 300g + copper oxy chloride @ 1.25kg/ha
OOZE OUT TEST
When affected leaves are cut and immersed in clear water in a test tube, a characteristic turbid ooze of the bacterium streaming from the vascular bundles can be observed.
11. Bacterial leaf streak – Xanthomonas oryzae pv. oryzicola
The infected leaves shows fine translucent streaks on the veins and the lesions enlarge lengthwise and turn brown. Bacterial exudates appear on the lesions at high humidity which later dry and remain on the lesions as particles.
Mode of spread
The bacterium spreads through air and wind splashed rains.
The bacterium survives in the infested seeds
Rain splash and irrigation water favour the disease development.
- Seed soaking with 0.25% streptocycline and hot water treatment at 520 C for 10 min.
- Spray with Agrimycin @ 100 ppm or streptocycline @100 ppm twice at 10 days intervals.
12. Tungro – Rice tungro virus [RTV]
The infected plants show yellow to orange discolouration of leaves and rusty blotches spreading downwards from the leaf tip. The infected plants become stunted. The infected plants produce few spikelets and panicles are small with discoloured grains.
The disease is spread through green leaf hopper Nephotettix virescens and N. nigropictus in a non-persistent manner
- Summer ploughing and burning diseased plants (Rogueing and Field sanitation).
- Control insect vectors by spraying systemic insecticide (Monocrotophos 0.25 %)
- Growing disease tolerant varieties like CO-45, IR-50.
Iodine test to identify RTV
Composition of iodine solution
- [Iodine – 2g, Potassium Iodide – 6g, Water – 100 ml]
- Dilute 10 ml of the commercial tincture iodine solution available in the medical shops with 150 ml of water
10 cm long leaf tip is cut before 6 a.m.
Dipped in Iodine solution for 30 minutes
RTV infected leaves show dark blue streaks.
13. Grassy stunt – Rice grassy stunt virus
Infected plants are stunted and produce excess tillers and have erect growth habit. Leaves are short, narrow, pale green / pale yellow and have numerous rusty brown spots of various shapes.
The disease is spread by brown plant hopper (Nilaparvata lugens) in a persistent manner.
- Rogueing and Field sanitation
- Use systemic insecticides to control the vector
- Grow resistant varieties like IR-28, IR-29, IR-30, IR-32, IR-34.
14. Dwarf – Rice dwarf virus
The infected plants show marked stunted growth with chlorotic or whitish specks on the leaves. The number of tillers may be reduced with retarded root growth.
- Leaf hopper Nephotettix nigropictus and Recilia dorsalis
- Also weeds like Echinochloa crusgalli harbour the pathogen during off-seasons
- Rogueing and Field sanitation to destroy the weed host
- Spraying with monocrotophos 500 ml / ha to control the insect vector
15. Yellow dwarf – Phytoplasma disease
The infected plants are stunted and have yellowish green to whitish green leaves with excessive tillering and leaves become soft and droop slightly. The affected plants are usually sterile.
- Green leaf hopper Nephotettix virescens and N. nigropictus transmit the disease
- Deep ploughing in summer and burning the stubbles.
- Grow resistant varieties like IR 62, IR 64.
- Control of insect vectors with systemic insecticides.
DIFFERENCES BETWEEN YELLOW DWARF AND TUNGRO
|Caused by Phytoplasma like organisms
|Caused by Virus
|Symptoms can be seen only in transplanted crop
|Symptoms can be seen in transplanted crop and nursery also
|Entire leaf become yellow
|Yellow / orange yellow lines alternative with Dark green lines appear on leaves
|Increased number of tillers
|Decreased number of tillers
|Leaves become soft and droop slightly
|Leaves stand erect
|Phytoplasma retained by the vector throughout its life
|Virus lost from the vector with in 2 days orafter moulting
|The vectors transmit Phytoplasma only 20-35 days after feeding
|The vectors transmit the virus immediately after feeding