Predisposing Factors - Brown Spot

Brown spot disease is particularly noticeable when the crop is produced in nutritionally deficient or otherwise unfavorable soil conditions. Brown spot serves as an index or indicator of poor soil conditions for rice production hence, it is sometimes called poor man's disease. Brown spot commonly occurs when silica, potassium, manganese, magnesium, iron, or calcium is deficient. It is reported that fine grade silicon fertilizer (calcium silicate slag) reduces the disease to as much as 48%-80% and increases yields of rice by 20-26%. Rice grown in unflooded soil is also more susceptible to brown spot.

Susceptibility is reported to increase with age; rice is most susceptible to infection at flowering and grain-filling stages.

Disease development is favored by high relative humidity (86-100 %) and the  optimum temperature for growth is between 16 and 36°C. Leaves must be wet for 8-24 hours for infection to occur. Yield losses due to a 1942 brown spot epidemic in Bengal was attributed to continuous temperatures of 20-30°C for two months, unusually cloudy weather, and higher-than-normal temperatures and rainfall at the time of flowering and grain-filling stages.