Symptoms of Rice Brown Spot Disease

Rice brown spot disease affects various parts of the rice plant, including leaves, leaf sheaths, stems, and grains.

1. Leaves: In the early stages, small brown spots appear on the leaves, gradually enlarging into circular or oval lesions, typically 1-2 millimeters in diameter. The spots have brown edges and grayish-white to yellow-brown centers, often surrounded by a yellow halo. In humid conditions, black mold may develop on the spots. In severe cases, lesions may merge, causing the leaves to wither and turn yellow.
2. Leaf Sheaths: Lesions on the leaf sheaths are similar to those on the leaves but are usually larger and darker in color. The spots can spread to cover the entire sheath, causing it to turn brown and rot.
3. Stems: Brown, elongated lesions appear on the stems, and in severe cases, the stems may break, affecting the plant’s growth and development.
4. Grains: Brown spots develop on the grain husks, and in severe cases, the entire husk may be covered, affecting the quality and yield of the rice.

Disease Cycle

1. Pathogen Source: The causative agent of rice brown spot disease is Bipolaris oryzae. This fungus overwinters as mycelium or conidia in infected rice straw, seeds, and other host plant residues. Under favorable conditions, the pathogen produces conidia that act as the primary infection source.
2. Environmental Factors:
   • Temperature and Humidity: The disease is most likely to occur and spread in warm, humid conditions. The ideal temperature for the pathogen’s growth is between 25°C and 30°C, with a relative humidity above 90%. Continuous rainy, hot, and humid weather during the rice-growing season encourages the disease.
   • Light: Insufficient sunlight weakens the rice plants and increases the risk of infection. For example, cloudy weather and low light in the later stages of rice growth can reduce photosynthesis, weakening the plants and making them more susceptible to the pathogen.
3. Cultivation and Management Factors:
   • Improper Fertilization: Rice plants lacking nitrogen or potassium are more susceptible to brown spot disease, while over-fertilization with nitrogen can disturb the balance of nutrients and exacerbate the disease.
   • Irrigation Practices: Prolonged deep water irrigation reduces soil aeration, inhibits root growth, and weakens the plants, increasing the likelihood of infection. Poor drainage also leads to high field humidity, creating favorable conditions for pathogen growth and spread.
   • Planting Density: High planting densities reduce airflow and light penetration, weakening plant growth and increasing disease risk.
   • Continuous Cropping: Continuous rice cropping allows the pathogen to accumulate in the soil, worsening the disease with each successive year.

Control Methods

1. Agricultural Practices:
   • Resistant Varieties: Planting rice varieties resistant to brown spot disease can significantly reduce the incidence of the disease.
   • Crop Rotation: Rotating rice with non-grass crops like soybeans, peanuts, or vegetables can reduce pathogen buildup in the soil and lower the risk of infection.
   • Seed Treatment: Before sowing, treat seeds by disinfecting them. Methods include soaking seeds in hot water at around 50°C for 30 minutes, followed by immediate cooling, or using fungicides like prochloraz for seed treatment.
   • Field Management:
     • Apply balanced fertilizers, avoiding excessive nitrogen. A recommended ratio is nitrogen:phosphorus

       at 2:1:1.5, with adjustments based on soil fertility and crop growth. Organic fertilizers can also be applied, with 1000-1500 kg per hectare being a suitable amount.

     • Use proper irrigation methods, like shallow and frequent watering, and avoid prolonged deep water irrigation. Allow the fields to dry at appropriate times, especially during the tillering stage and before harvesting.
     • Adjust planting density based on soil fertility and variety characteristics to ensure good air circulation and light exposure in the field.
2. Chemical Control:
   • Preventive Spraying: During the tillering to booting stages, spray fungicides such as tricyclazole, propiconazole, or prochloraz every 7-10 days, repeating 2-3 times for best results.
   • Curative Treatment: If disease outbreaks are detected, apply fungicides like azoxystrobin-propiconazole or tebuconazole, increasing the dosage and spraying frequency. Spraying should be done every 5-7 days, repeating 2-3 times to contain the spread.