Blast (Leaf and Neck)

Signs and symptoms

• initial symptoms are white to gray-green lesions or spots with dark green borders
• older lesions elliptical or spindle-shaped and whitish to gray with necrotic borders
• lesions wide in the center and pointed toward either end
• lesions may enlarge and coalesce to kill the entire leaves
• symptoms also observed on leaf collar, culm, culm nodes, and panicle neck node
• internodal infection of the culm occurs in a banded pattern
• nodal infection causes the culm to break at the infected node
• few, no seeds, or whiteheads when neck is infected or rotten
  
  
  
  
  
  
  
  
  
  

• presence of the blast spores in the air throughout the year
• upland rice environment and high elevation in the tropics
• cloudy skies, frequent rain, and drizzles
• high nitrogen levels like ammonium sulfate
• high relative humidity (90% and higher) and wet leaves
• temperature from 25-28°C
  Pyricularia grisea (Cooke) Saccardo (anamorph)

  P. oryzae Cavara (anamorph)

  Magnaporthe grisea (T. T. Hebert) Yaegashi & Udagawa (teleomorph)

  cultural practices:

  There are some cultural practices that are recommended against the rice blast. Early sowing of seeds after the onset of the rainy season is more advisable than late-sown crops. Early sowing allows escape from the build-up of inoculum originating from neighboring farms. Excessive use of fertilizer should be avoided as it increases the intensity of blast, indirectly by promoting luxuriant crop growth which in turn increases the relative humidity and leaf wetness of the canopy. Excessive application of nitrogen at the start of the cropping season should be minimized to avoid luxuriant crop growth which favors the disease. Split application of nitrogen based on the actual requirements of the crop is recommended to manage the disease.

  Soils known to be low in plant-available silicon should be amended with calcium silicate slag.

  Planting resistant varieties against the rice blast is the most practical and economical way of controlling rice blast.

  Systemic fungicides, such as probenazole, tricyclazole, fthalide, pyroquilon and tricyclazole, are effective against the disease.