Effect of Sheath Blight on Yield in Tropical, Intensive Rice Production System. R. M. Cu, Entomology and Plant Pathology Division (EPPD), and Agronomy, Plant Physiology, and Agroecology Division (APPA), International Rice Research Institute, Los Bafnos, Philippines. T. W. Mew, K. G. Cassman, and P. S. Teng, Entomology and Plant Pathology Division (EPPD), and Agronomy, Plant Physiology, and Agroecology Division (APPA), International Rice Research Institute, Los Bafnos, Philippines. Plant Dis. 80:1103-1108. Accepted for publication 22 May 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1103.

Sheath blight (Rhizoctonia solani) of rice (Oryza sativa) is associated with intensive and high-input production systems. The effect of sheath blight on yield, the effect of high nitrogen (N) rate on sheath blight 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 to an increase in yield, but at the highest N level, a reduction in yield was observed. Sheath blight incidence also increased with increasing N level. The estimated yield reduction from sheath blight in plots receiving the highest N rate ranged from 20 to 42% in artificially inoculated plots. The highest sclerotial population recorded was only 2.02 sclerotia per 500 g of oven-dried soil or about 1.23 sclerotia per liter 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 pathogen may play an important role in sheath blight epidemics in this intensive rice production system. Screenhouse and field experiments indicated significant yield losses when sheath blight infection started at panicle initiation, booting, or flowering. The effect of sheath blight on yield resulted primarily from a reduction in mean seed weight and a lower percentage of filled spikelets. No yield loss or decrease in yield components was observed when infection started at tillering or grain filling.