Brown Spot: Helminthosporium oryzae
- Brown Spot is called as sesame leaf spot or Helminthosporiosis, attacks seedling in the nursery to the milky stage in the main field.
- The disease appears first as minute brown dots later becoming cylindrical or oval to circular (resemble sesame seed).
- Spots measure 0.5 to 2.0 mm in breadth – coalesce to form large patches leads to leaf dries up.
- Infection also occurs on panicle and neck with brown colour appearance
- Infected seeds show black or brown spots on glumes are covered by olivaceous velvety growth.
- Seedlings die and affected nurseries can be often recognised from a distance by their brownish scorched appearance.
- Dark brown or black spots also appear on glumes.
- Infected seed causes failure of seed germination, infection in nurseries leads to seedling mortality.
- Reduces the grain quality and weight at harvesting stage. Yield reduction is upto 50% in severe cases.
Mode of Spread and Survival:
Infected seeds are the most common source of primary infection. The fungus also survives on collateral hosts. Infected seeds, volunteer rice, infected rice debris and weeds are the major sources of inoculums in the field. The fungus can spread from plant to plant and in the field by airborne spores. The disease is common in nutrient-deficient soils.
The temperature of 25-30°C, Relative humidity above 80 per cent, excess of nitrogen aggravates the disease incidence.
Identification of pathogen:
- The somatic structures of the fungus consist of black velvety mycelial mats which are made up of prostrate hyphae and erect sporophores.
- The hyphae are abundant, branching and anastomosing. They are dark brown or olivaceous and measure 8-15 µm or more in diameter.
- The sporophores arise as lateral branches from the hyphae.
- The conidia measure 35-170 x 11-17 µm. Typical conidia are slightly curved, widest at the middle and tapering toward the hemispherical apex, where their width approximates half the median width. Mature conidia are brownish with a moderately thin peripheral wall.
Management: Cultural methods:
- As the disease is seed borne, Use disease free seeds.
- Removal of alternate & collateral hosts.
- Growing resistant varieties like ADT 44, PY 4, CORH1, CO44, Cauvery, Bhavani, TPS 4 and Dhan.
- Providing proper nutrition for optimum plant growth and prevention of water stress seem to be the most important factors in controlling brown spot.
- Soils known to be low in plant-available silicon should be amended with calcium silicate slag before planting and the field should be well irrigated to avoid water stress.
- Seed treatment with Pseudomonas fluorescens @ 10g/kg of seed followed by seedling dip (select 25 sq.m areas at one corner in the main field, stagnate water upto a depth of 2.5 cm and dissolve 2.5 kg of P. fluorescens. In this stagnated water, dip 20 cent nursery seedling for 30 minutes and transplant
- Since the fungus is seed transmitted, a hot water seed treatment (53-54°C) for 10-12 minutes may be effective before sowing.
- The above treatment controls primary infection at the seedling stage. Presoaking the seed in cold water for 8 hours increases effectivity of the treatment.
- Seed soak / seed treatment with Captan or Thiram at 4.0g /kg of seed or treat the seed with Agrosan or Ceresan 2.5 g/kg seed to ward off the appearance of seedling blight stage.
- Metominostrobin @ 500ml/ha or
- Spray Mancozeb (2.0g/lit) for 2 to 3 times at 10 – 15 day intervals. Spray preferably during early hours or afternoon at flowering and post – flowering stages.
- In addition to above seed treatment with captan or thiram or chitosan or carbendazim or mancozeb has been found to reduce seedling infection. Seed treatment with tricyclazole followed by spraying of mancozeb + tricyclazole at tillering and late booting stages gave good control of the disease.
- Application of edifenphos or chitosan or iprodione or carbendazim in the field is also advisable
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