Predisposing Factors - Rice Blast

Susceptibility to blast is inversely related to soil moisture. Plants grown under lowland condition (i.e., flooded soil or with high soil moisture) become more resistant, while plants grown under upland conditions become more susceptible. The pathogen requires free moisture for spore penetration. Dew formation, which often occurs in upland rice, provides free water favoring infection. Some studies showed that dew periods of 10 hours or more induces the disease. High relative humidity (90-92%) is also reported to be necessary for infection. Severe blast epidemics are usually associated with moist weather. Frequent periods of rain showers or drizzle are more favorable to infection than heavy rainfall of short duration. Low solar radiation and overcast skies also favor blast.

Many experiments in the past have shown that a high nitrogen supply is likely to favor heavy blast infection regardless of phosphorus or potassium supply. Reports suggest that the effect of nitrogen varies with soil, climatic conditions, and fertilizer application methods. Excess amount of quick-acting nitrogenous fertilizer like ammonium sulfate, and applying the whole season requirement only once has been found to cause severe blast disease.