EFFECT OF SHEATH BLIGHT ON YIELD IN TROPICAL, INTENSIVE RICE PRODUCTION SYSTEM

Sheath blight (Rhizoctonia solani) of rice (Oryza sativa) is associate d with intensive and high-input production systems. The effect of shea th blight on yield, the effect of high nitrogen (N) rate on sheath bli ght incidence, and the stages of crop that are most susceptible to the disease and vulnerable to yield losses were investigated. Grain yield data from a long-term experiment showed a quadratic polynomial curve in response to N input. An initial increase in N supply corresponded t o an increase in yield, but at the highest N level, a reduction in yie ld was observed. Sheath blight incidence also increased with increasin g N level. The estimated yield reduction from sheath blight in plots r eceiving the highest N rate ranged from 20 to 42% in artificially inoc ulated plots. The highest sclerotial population recorded was only 2.02 sclerotia per 500 g of oven-dried soil or about 1.23 sclerotia per li ter of puddled paddy soil. This low sclerotial density in our studies suggested that sclerotia may not be the primary source of inoculum in a tropical lowland rice system. Crop residues colonized by the pathoge n may play an important role in sheath blight epidemics in this intens ive rice production system. Screenhouse and field experiments indicate d significant yield losses when sheath blight infection started at pan icle initiation, booting, or flowering. The effect of sheath blight on yield resulted primarily from a reduction in mean seed weight and a l ower percentage of filled spikelets. No yield loss or decrease in yiel d components was observed when infection started at tillering or grain filling.