Quantification of the Relationship Between Sheath Blight Severity and Yield Loss in Rice. M. A. Marchetti, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Texas A&M University Agricultural Research and Extension Center, Beaumont 77713. C. N. Bollich, Research Agronomist, Agricultural Research Service, U.S. Department of Agriculture, Texas A&M University Agricultural Research and Extension Center, Beaumont 77713. Plant Dis. 75:773-775. Accepted for publication 16 January 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991. DOI: 10.1094/PD-75-0773.

Breeding rice with resistance to sheath blight is difficult, because the resistance is expressed quantitatively, and the relationship between symptom expression and disease-induced yield losses is not clear. A 5-yr study to quantify this relationship showed a high correlation between a disease index (DI) and percent yield loss, r² = 0.74. The DI is calculated from visual estimates of the percent harvest area at prescribed disease severity levels. DIs across seven cultivars and 5 yr ranged from 1.7 to the maximum 9. A regression line is described by y (loss) = –1.8 + 5.1 x (DI). Evaluation of breeding lines for sheath blight resistance is based upon symptom expression. Two F₇ lines, selected solely on disease symptoms, from progeny of resistant x-susceptible crosses showed superior sheath blight resistance as regards effects of disease on yield.