Rice Brown Spot is a fungal disease caused by the pathogen Bipolaris oryzae that affects rice. It damages rice plant leaves, stems, and grains and severely limits global rice production. The disease causes round to elongated brown lesions on the leaves and stems, which can cause premature defoliation and lower grain production. Both seedlings and fully grown plants are susceptible to the fungal disease known as brown spots.
This disease manifests in a more severe form in places with inadequate management. The loss of yield of between 50 and 90 percent was the primary cause of the “Bengal Famine” in 1942, which resulted in the deaths of 2 million people. In India, epidemics were responsible for a loss of between 14 and 41% of HYVs. It is widespread in inadequate and poor soils (Poor man’s illness).
Rice Brown Spot management in Paddy
The causal organism of Rice Brown Spot
Brown leaf spot, caused by the fungus Helminthosporium oryzae, develops from seedling to maturity and can result in up to 50-90% yield loss in extreme situations. Digitaria sanguinalis, Leersia hexandra, Echinochloa colona, Pennisetum typhoides, Setaria italica, and Cynodon dactylon are all collateral hosts for the illness. The pathogen is transmitted through wind and can be the source of infection from seed, collateral hosts, rice straw, or stubble.
The disease cycle of Brown Spot of Rice
- Infected seeds and stubbles are the primary sources of infection for Helminthosporiose, also known as the fungal blight that causes brown leaf spots in rice.
- The fungus may live on them for two to three years, and it can spread its conidia through the air. Conidia move to their greatest extent at wind speeds of 4.0 to 8.8 hours under ideal conditions of low temperatures of 27 to 28 C, relative humidity of 90 to 99%, and rainfall of 0.4 to 14.4 mm.
- Additionally, the fungus can survive on unrelated hosts, including Leersia hexandra and Echinochlora colonum. It emphasizes how crucial it is to get rid of contaminated stubbles and prevent the development of collateral hosts to lower the chance of infection.
Favorable conditions for the spread of Brown Spot disease in the Paddy field
- Temperatures range from 25 to 30 degrees Celsius
- (>90%) relative humidity
- The arrival of Heavy north-east monsoon rains in the late season.
- Cloudy and humid weather
- Excess nitrogen enhances disease incidence.
- For infection to occur, the leaves must be damp for 8-24 hours.
Symptoms of Rice Brown Spot disease in Paddy
- The disease begins as minute brown specks and progresses to cylindrical, oval, or circular shapes. (similar to sesame seed)
- Spots with a width of 0.5 to 2.0mm congregate to create huge patches.
- The leaf then dries up as many spot areas congregate.
- Infection also appears on the panicle and throat in a brown color.
- Seeds are also infected (black or brown patches on glumes are covered by olivaceous velvety growth). Seedlings perish, and damaged nurseries may frequently be identified from afar by their brownish-burned look.
- Glumes might also develop dark brown or black patches.
- Plant infection causes seed failure, seedling death, and grain quality and weight loss.
- In extreme circumstances, the yield is reduced by 50%.