Important diseases of rice, symptoms of damage and their management aspects are as follows:
1. Sheath Blight (Rhizoctonia Solani):
Symptom:
Initial lesions are water-soaked to greenish gray and later become grayish white with brown margin. Lesions on leaf sheaths near waterline. Presence of sclerotia. Lesions may coalesce death of the whole leaf. Partially filled or empty grains.
Management:
Apply FYM 12.5 t/ha or green manure 6.25 t/ha to promote antagonistic microflora. Soil application of P. fluorescens @ 2.5 kg/ha mixed with 50 kg FYM after 30 days of transplanting. Foliar spraying of P. fluorescens @0.2% at boot leaf stage and 10 days later. Avoid flow of irrigation water from infected to healthy field.
Carbendazim (1 g/lit), Propiconazole (1 ml/lit) may be applied. Spraying of infected plants with fungicides, such as Benomyl and Iprodione, and antibiotics, such as Validamycin and Polyoxin, is effective against the disease. Wider spacing with reduce nitrogen dosage and skip top dressing.
2. Sheath Rot (Sarocladium Oryzae):
Symptoms:
Irregular spots or lesions, with dark reddish brown margins and gray center, discoloration in the flag leaf sheath. Lesions enlarge and often coalesce and may cover the entire leaf sheath. Severe infection causes entire or parts of young panicles to remain within the sheath.
Unemerged panicles rot and florets turn red-brown to dark brown. Whitish powdery growth inside the affected sheaths and young panicles. Infected panicles sterile, shrivelled, or with partially filled grain.
Management:
Apply Ipomea or Prosophis leaf powder extract 5 %. Spraying of Mancozeb at 2.5 g or Carbendazim at 1.0 g/lit or Benomyl 0.5 g/lit of water at flowering stage. Destruction of the infected plant debris by burning.
3. Blast (Pyricularia Oryzae):
Blast is the most serious disease of rice which may infest the crop at any stage of growth. Foliar infestation is common in medium and long duration varieties during September- October and next infestation in early duration varieties. Minimum temperature with high relative humidity of 85-90 % and large differences between day and night temperatures are favourable environment for its occurrence. Close spacing and high doses of N also favour the diseases because of the high humidity on the microclimate.
Symptoms:
Typical leaf spots are elliptical and somewhat pointed at both ends and the margins are usually brown to reddish brown. The spot begin as small, water soaked whitish, grayish or bluish dots. They enlarge quickly under humid environment.
Management:
Grow resistant varieties like Asha, Deepa, Govind, Triveni, Rasi, Chaitanya, Pant dhan 6, Pusa 205, Annada, Jaya, IR 36 etc. Burn affected straw and stubbles. Treat the seed with Tricyclozole at 0.4 % (4 g/kg) before sowing. Spraying Tricyclozole at 0.06 % (6 g/lit) or Edifenphos at 0.1 % (1 ml/lit) when the symptoms first appear is also equally effective.
4. Brown Leaf Spot (Helminthosporium Oryzae):
Symptoms:
Occur in nursery as well as main field. Causes blight of seedlings. Leaf spotting is very common. Isolated brown, round to oval (resemble sesame seed). Spots measures 0.5 to 2.0mm in breadth – coalesce to form large patches. Seed also infected (black or brown spots on glumes spots are covered by olivaceous velvety growth). Infection also occurs on panicle neck with brown colour appearance. About 50% yield reduction in severe cases.
Management:
Use disease free seeds. Since seed soak/seed treatment with Captan or Thiram at 2.0g Ag of seed. Spray Mancozeb (2.0g/lit) or Edifenphos (1ml/lit) – 2 to 3 times at 10 – 15 day intervals. Spray preferably during early hours or afternoon at flowering and post – flowering stages.
5. Bacterial Leaf Blight-BLB (Zanthomonas Oyzae):
BLB occurs in all the tropical countries and yield losses range from 20-30 percent. Higher doses of N, P and K deficiency increase the disease incidence.
Symptoms:
Lesions usually start as water soaked strips along leaf margins of the upper part of leaf blades, either at one edge or both edges. The lesions enlarge and turn yellow within a few days. Lesions may extend to the leaf sheath and may reach its lower ends. In severe cases, grains may also be infested.
Management:
Grow tolerant varieties like Swarna, MTU 9992, IR 20, Asha, Deepti, Govind, Radhe, Usha, etc. Use seed from disease free crop. Treat the seed by soaking for 12 hrs in a mixed solution of Streptocycline (0.015 %) and Wettable Cereson (0.05 %) followed by hot water treatment at 52-54°C for 30 minutes. Drain the water from fields as frequently as possible.
6. False Smut (Ustilaginoidea Virens):
Symptoms:
Only few grains in a panicle are usually infected and the rest are normal Individual rice grain transformed into a mass of yellow fruiting bodies. Growth of velvety spores that enclose floral part. Immature spores slightly flattened, smooth, yellow, and covered by a membrane. Growth of spores results to broken membrane. Mature spores orange and turn yellowish green or greenish black.
Management:
Use disease free certified seeds. Collect the diseased-grains and destroy them as soon as possible. At tillering and pre-flowering stages, spray Hexaconazole @ 1ml/lit or Chlorothalonil 2g/lit.
7. Leaf Streak (Xanthomonas Oryzae pv. Oryzicola):
Symptoms:
Initially, small, dark-green, water-soaked translucent streaks on veins from tillering to booting stage. Lesions turn brown and bacteria ooze out under humid weather.
Management:
Use disease free seed. Grow nurseries preferably in isolated upland conditions. Avoid clipping of seedlings during transplanting. Balanced fertilization, avoid excess N application. Skip N application at booting (if disease is moderate). Drain the field (except at flowering stage of the crop). Destruction of wild collateral hosts. Avoid flow of water from affected fields. Grow tolerant varieties (IR 20, etc.). Spray streptomycin sulphate and tetracycline combinations @ 300g + copper oxychloride @ 1.25 kg/ha.
8. Tungro Virus:
Tungro virus is most important disease of rice in tropical Asia. It is transmitted by green leafhopper.
Symptoms:
Stunting, yellowing of leaves and slight reduction in tiller number in susceptible varieties. Shortening the leaf sheath, leaf blade and internode causes stunting. Yellowing usually starts from tip of leaves and the colour may vary from light yellow to orange yellow or brownish yellow. Young leaves are, usually mottled with pale green to whitish spots. The grains are covered with dark brown blotches.
Management:
Grow resistant varieties like Annapurna, Ratna, Janaki, Triveni, Udaya, Tara, Suraksha, IR 36, etc. The vector can be controlled effectively by spraying phosphamidon 85 SL @ 0.5 ml/litre of water. In all, 5 sprays should be done including one in nursery after 10 days of sowing followed by 15,30,45 and 60 days after transplanting in the main field. Rogue out diseased plants.
9. Khaira:
The disease is caused by zinc deficiency. Usually appears in nursery but may appear, in patches, after 10-15 days of transplanting, Growth of the diseased plant is stunted. High yielding varieties show chlorosis between the veins of new leaves, where brown spots are formed.
Management:
Spray a mixture of 5kg zinc sulphate and 2.5 kg lime in 1000 litres of water after 10 days of sowing in nursery. Give second spray as above on 20th days of sowing in the nursery. Give third spray as above in the field after 15-20 days of transplanting if symptoms appear in the field.