Pathogen
         Pyricularia oryzae (Teleomorph: Magnaporthe oryzae)

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Identification

Initial symptoms on the leaves appear as white to gray-green lesions or spots, with dark green borders. Older lesions are elliptical or spindle-shaped having whitish to gray centers and red to brownish or necrotic borders. Some lesions are diamond shaped,
wide in the center and pointed toward either both ends. For beginners, blast lesions can commonly be confused with
brown spot lesions; however, the latter tend to be more round, brown in color and have a yellow halo surrounding the lesion. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Blast lesions on the node are blackish to grayish-brown covering around the node, causing the culm to break easily. Lesions on the neck are grayish brown and can cause girdling. If infection of the neck occurs before milky stage, no grain is formed, but if infection occurs later, lower weight and poor quality grains are formed. On susceptible varieties lesions can enlarge and coalesce, growing together, to kill the entire leaves. Neck and node blast can also cause whiteheads or white panicles, similar to stem borer infection. Whiteheads caused by stem borers can be pulled apart from the plant as the stem will separate at the point where the insect bored into it. With neck and node blast, tugging on the stem will not result in removal.

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Favorable Conditions
• Intermittent drizzles, cloudy weather, more of rainy days, longer duration of dew
high relative humidity (93-99 percent).
• Low night temperature (between 15-20°C or less than 26°C).
• Availability of collateral hosts and the excess dose of nitrogen.

 

Disease Cycle
The disease spreads primarily through airborne conidia since spores of the fungus present throughout the year. Mycelium and conidia in the infected straw and seeds are major sources of inoculum. Irrigation water may carry the conidia to different fields. The fungus also survives on collateral hosts viz., Panicum repens, Digitaria marginata, Brachiaria mutica, Leersia hexandra, and Echinochloa crusgalli.  Spores land on leaves, germinate, penetrate the leaf, and cause a lesion 4 days later; more spores are produced in as little as 6 days. Infections from spores arriving from a distance are termed primary infections. Primary infections generally result in a few widely scattered spots on leaves. Spores arising from the primary infections are capable of causing many more infections. This cycling is called secondary spread. Secondary spread is responsible for the severe epidemics of the blast in fields and localized areas.

 

 

 

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Control management

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• Grow resistant to moderately resistant varieties CO47, IR 20, ADT36, ADT39, ASD 18 and IR64. Avoid cultivation of highly susceptible varieties viz., IR50 and TKM6 in disease favorable season.

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• Remove and destroy the weed hosts in the field bunds and channels.

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• Treat the seeds with Captan or Thiram or Carbendazim or Tricyclazole at 2 g/kg. or Pseudomonas fluorescens @ 10g/kg of seed. Spray the nursery with carbendazim 500mg/L or tricyclazole 300mg/L.

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• Spray the main field with Edifenphos 500 ml or Carbendazim 500 g or Tricyclazole 500g or Iprobenphos (IBP) 500 ml       /ha.

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