MAJOR DISEASES IN PADDY

Introduction:

      Paddy is cultivated globally.It is considered to be food of half of the worlds population.Paddy is cultivated on large scale in india and it is also major crop growing in Telangana,Andhrapradesh,also cultivated in several other states like West Bengal,Punjab and Haryana.One of the biggest problem faced by farmers cultivating paddy is due to the major diseases and pests affecting it.So,in this topic will give details about major diseases affecting paddy and proper management practices to control these diseases.

List of Diseases Affecting Paddy :

1. Rice Blast
2. Brown spot
3. Bacterial leaf blight
4. Rice Tungro
5. Sheath blight
6. Sheath rot
7. False smut
8. Bacterial leaf streak
9. Backanae
10. Rice yellow dwarf
11. Rice grassy stunt
12. Grain discolouration 

1. Rice blast:

    Casual organism: Pyricularia oryzae

Symptoms:

• This disease causes blue spots on leaves.
• It comes in various forms such as leaf blast,node blast and neck blast.
• The affected areas turn brownish-black and may rot, causing grain damage. 'Neck rot' refers to the rotting of the plant's neck due to this disease.
• 'Node rot' affects the nodes, leading to plant breakage.
• Additionally, creates it greyish- brown spots on the lower parts of the plant. Leaf blast results in ash-gray centers with brown-edged spots on the leaves and can even damage the flowering part.
• This disease weakens the plant's structure, leading to reduced grain production and potential crop loss.

Survival and mode of spread:

• The fungus survives on collateral hosts viz., Panicum repens, Digitaria magrginata, Brachiaria mutica, Leersia hexandra and Echinochloa crusgalli.
• Mycelium and conidia in the collateral hosts, straw and seeds are major sources of primary inoculurn.
• The secondary spread takes place through airborne conidia since spores of the fungus present throughout the year. Irrigation water may carry the conidia to different fields.

Favourable Conditions:

• Intermittent drizzles.
• High relative humidity (93-99 per cent).
• Cloudy weather.
• Low night temperature (between 15-20℃ or less than 26 ℃)
• Availability of collateral hosts.
• Application of excess nitrogen.

Management:

• Use of seeds from disease free crop.
• Grow resistant varieties like  Simhapuri,Tikkana,Sriranga,Phalguna,Swarnamukhi,Jaya,Vijaya,Ratna,MTU-1005,MTU-3&5.
• Remove and destroy the weed hosts in the field bunds and channels.
• Split application of nitrogen and judicious application of nitrogenous fertilizers.
• Treat the seeds with Captam or Thiram @2g/kg.
• Seed treatment with biocontrol agent Trichoderma viridae @4g/kg or Pseudomonas flourescens @10g/kg seed.
• Avoid close spacing of seedlings in the main field.

2. Rice Grassy stunt disease:

            Casual organism: Rice grassy stunt virus

            Transmitted by: Brown plant hopper

Symptoms:

• Diseased hills are severely stunted with excessive tillering and a very upright growth habit.
• Diseased hills has a grassy and rosette appearance.
• Leaves short, narrow and yellowish green with numerous small rusty spots or patches, which form blotches.
• Retention of green coloration of the leaves after application of sufficient nitrogenous fertilizers.
• Infected plants usually survive until maturity but produce no panicles.
• Symptom develops 10-20 days after infection.

Identification of pathogen:

 The virus exists in the vector and in the rice crop. Brown plant hopper nymphs and adults transmit it where rice is grown year round. RGSV is generally endemic. The  long winged adults of the insect are important in spreading the disease than the short winged forms. They feed on the diseased plant for at least 30 minutes to pick-up the virus. Higher infection is attained after prolonged inoculation feeding periods of up to 24 hours.

The availability of the vector encourages the damage. Rice grassy stunt virus (RGSV) is a member of the Tenuiviruses. It has fine filamentous particles, which are 6-8 nm in diameter. It has a nodal contour length of 950-1,350 nm. The particles have one capsid protein and the genome is made up of four single stranded RNA.

Management:

• Avoid close planting.
• Grow resistant varities like IR26 ,IR64 ,IR36,IR56 and IR72.
• Ploughing and harrowing the field to destroy stubbles right after harvest in order to eradicate other hosts.

3. Rice Tungro:

Caused by: Rice tungro bacilliform virus (RTBV) and Rice tungro spherical virus (RTSV)

Vector: Transmited by Green leaf hopper (Nephotettix virescens)

Symptoms:

• Plants affected by tungro exhibit stunting and reduced tillering, Leaves become yellow or orange-yellow, it may also have rust-colored spots 
• Yellowing starts from the tip of the leaf and may extend to the lower part of the leaf blade.
• Third leaf from the top most infected plants is taller than other leaves.
• Young leaves are often mottled with pale green to whitish interveinal stripes and the old leaves may have rusty streaks of various sizes.
• Delayed flowering panicles are small and not completely exerted
• Most panicles are sterile or partially filled grains.
• The plants may be killed if infected early.
• Tungro virus disease affects all growth stages of the rice plant specifically high at vegetative stage

Favourable conditions:

• Presence of the virus sources.
• Presence of the vector.
• All growth stages of rice plant specifically the vegetative stage.
• Synchronization of the three above factors.
• Age and susceptibility of host plants.

Mode of spread and survival:

• The pathogen survives in soil and in the infected stubbles and on collateral hosts Leersia spp., Plantago najor, Paspalum dictum, and Cyanodon dactylon.
• The pathogen spreads through irrigation water and also through rain storms.

Management:

Trap methods:

• Light traps are to be set up to attract and control the leaf hopper vectors as well as to monitor the population.
• Install light trap @ 5-6/ha
• Install yellow sticky trap 12/ha.
• In the early morning, the population of leafhopper alighting near the light trap should be killed by spraying/dusting the insecticides. This should be practiced every day

Cultural methods:

• Planting of resistant varieties against tungro virus disease is the most economical means of managing the disease.
• Grow disease tolerant cultivars like MTU 9992, 1002, 1003, 1005, Suraksha, Vikramarya, Bharani, IR 36, IET 2508, RP 4-14, IET 1444, IR50and Co45.
• Among the cultural management practices, adjusting the date of planting is recommended.
•  Likewise, observing a fallow period of at least a month to eliminate hosts and viruses and vectors of the disease.
• In epidemic areas follow rotation with pulses or oil seeds.
• Apply neem cake 12.5 kg/20 cent nursery as basal dose. 
• Summer deep ploughing and burning of stubbles.
• Eradicate of other tungro hosts are also advisable.
• Destruction of weed hosts on bunds.

4. Bacterial leaf blight:

Casual Organism: Xanthomonas oryzae pv.oryzae

Symptoms:

• Bacterial blight syndrome exhibits three types of symptoms: leaf blight, kresek  (the seedling blight or wilt phase) and the pale yellow leaf.
• The disease has been referred to as "bacterial leaf blight to indicate that the "leaf blight" phase of the syndrome is the most distinct and commonly observed symptom.
• Seedlings in the nursery show circular, yellow spots in the margin, that enlarge, coalesce leading to drying of foliage.
• "Kresek" symptom is seen in seedlings, 1-2 weeks after transplanting.
• The bacteria enter through the cut wounds in the leaf tips, become systemic and cause death of entire seedling. In grown up plants water soaked, translucent lesions appear near the leal margin.
• The lesions enlarge both in length and width with a wavy margin and turn straw yellow within a few days, covering the entire leaf.
• As the disease advances, the lesions cover the entire lamina which turns white or straw coloured.
• Milky or opaque dew drops containing bacterial masses are formed on young lesions in the early morning.
• They dry up on the surface leaving a white encrustation.
• If the cut end of leaf is dipped in water, it becomes turbid because of bacterial ooze.
• The affected grains have discoloured spots.

Pathogen Character:

• The bactenum is aerobic, gram negative, non spore forming, rod with size ranging from 1-2 x 0.8-1.0m with monotrichous polar flagellum.
• Bacterial colonies are circular, convex with entire margins, whitish yellow to straw yellow colored and opaque.

Favourable conditions/Epidemiology:

• Clipping of tip of the seedling at the time of transplanting.
• Heavy rain, heavy dew, flooding, deep irrigation water.
• Severe wind and terrmperature of 25-30°C.
• Application of excessive nitrogen, especially late top dressing

Mode of spread and survival:

• The pathogen survives in soil and in the infected stubbles and on collateral hosts Leersia spp., Plantago najor, Paspalum dictum, and Cyanodon dactylon.
• The pathogen spreads through irrigation water and also through rain storm.

Management:

Cultural methods:

• Burn the stubbles.
• Avoid clipping of tip of seedling at the time of transplanting.
• Use optimum dose of fertilizers.
• Remove weed hosts periodically.

Chemical Methods:

• Avoid clipping of leaf tips during transplantation.

FAQs:

1. What are the signs and symptoms of rice blast?

• Leaf lesions,which have dark edges and greyish centres.
• And causes neck rot in panicles.

2. What are some ways to avoid rice blast?

• Use resistant cultivars.
• Avoid applying too much nitrogen.
• Follow appropriate water management to prevent prolonged leaf moisture. 
• Using non-host crops in crop rotation.

3. Can vectors be controlled by natural predators?

Yes, Plant hopper population can be controlled by preserving predators like parastroids and spiders.

4. What are effective management techniques for controlling BLB?

• Grow resistant varieties.
• Avoid excessive application of nitrogen fertilizers.
•  Ensure proper field drainage to avoid waterlogging.

5. How is grassy stunt virus spread?

     It is a viral disease and it is transmtted by vector-Brown plant hopper.We can control this disease by using bioinsecticides like Dr.Eliminator.

Conclusion:

Controlling the major diseases namely Rice blast,Grassy stunt disease,Tungro disease and Bacterial leaf blight is important to reduce economic losses and to improve yield and quality.We should follow some effective management techniques like using resistant varieties,proper field and water management,balanced fertilization and preserving natural predators to control vectors.Along with these practices we can use biopesticide “Dr.Eliminator” for controlling vectors which are transmitting viral diseases.By following these measures farmers can reduce losses and improve productivity.