Brown Spot Disease Of Rice

Brown spot of rice is caused by the pathogen named Bipolaris oryzae ( imperfect stage ) or ( Cochliobolusbolus miyabenus ( perfect ). This fungus can infect both mature rice plant and seedlings of rice. This disease has great economic importance because Bengal Famine was occurred in 1942 and 2 million people died in that area due to yield loss of 60- 90%.

There are so many collateral host of this fungus. They are Cynodon dactylon, Digitaria sanguinalis, Setaria italica, Leersia hexandra, Pennisetum tyhoides, Echinochloa colona etc.


If there are relative humidity more than 90%, it is a cloudy day, temperature range is between 25 degree to 35 degree, we must be careful in that day. Because those are favorable condition of this pathogen. In this time, if we apply high N- fertilizer, it will give extra advantage to the pathogen. Mainly this pathogen or fungus disseminated by wind. So must be careful, so that we can protect our rice plants.

Collateral host, stubble or straw of rice and seeds are the main source of inoculation of this fungus.

Symptoms:

Major infestation is seen on the leaf of plant. It also attacks glumes, coleoptiles, leaf sheath and seedling. It causes blight of seedling when it attack at seedling stage. Colors of the spots on the leaves is brown ( that is why the name of the disease is brown spot of rice ), shape is round to oval and size is 0.5 mm to 2 mm wide and 2 mm to 5 mm in length. At first spots are separate from each other but at later they may coalesce and become large. The coalesce spots form a large patches on leaves ( it is one of the characteristic symptoms ) of withered tissue. The middle or central part of the spots are gray or pale or dirty white in color and the margin of the spots are dark brown in color ( it is one of the characteristic symptoms ). In some leaves yellow halos are found in the center of the spot. At later whole leaf become yellow and dry. When grains are infected, grains become dark brown to black in color and later grains are shriveled.

Description of the Bipolaris oryzae:
Mycelium of this fungus grayish black in color. Mycelia are septate and branched. It's conidiophore is brown in color and remain as straight or slightly curved. Conidia are usually curved and tapering towards the end. Mycelium are germinate from two end of the conidia. So this fungus are belong to the Bipolaris Genus.

Disease cycle:
Though this disease disseminated by wind, it overwinters around the affected field. Different weeds are host collateral host of this pathogen. This fungus also overwinters into the infected seeds and stubble that remain in the field after harvesting crops. At first symptom is seen on the coleoptiles. On the collateral host conidia is produced by the fungus and spread through wind and this conidia cause secondary to the rice plant. At favorable condition conidia start to germinate and penetrate to the host tissue. After 3-4 days of infection symptoms are shown on leaves.

Systematic position:
Division: Eumycota
Sub-division: Deuteromycotina
Class: Hypomycetes
Order: Moniliales
Family: Dematiaceae
Genus: Bipolaris
Species: Bipolaris oryzae 

Management:
prevention measure
1. We must apply balance fertilizer for insuring the proper crop nutrition.
2. We should avoid water stress condition.
3. Also good drainage system is needed.
4. We must use healthy seeds for seedling.
5. We must avoid excessive use of N fertilizer.
6. Field should be weed free and stubble free.
7. We may cultivate brown spot disease resistant variety.
8. During preparation of seed bed we should prefer wet seed bed becacuse in dry seed bed pathogen is higher in dry seed bed than wet seed bed.

chemical control
Seed should be treated by dithane M-45 or Provex 200 at the rate of 2.5 g per kg of seeds.
Chemical control is done due to severe attack of the pathogen. But we must apply fungicide after taking the suggestion of the government agricultural officer. Cu fungicide like Cupravit is very effective and can control the disease.