QUANTIFICATION OF THE RELATIONSHIP BETWEEN SHEATH BLIGHT SEVERITY AND YIELD LOSS IN RICE

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.