Abstract
Twelve rice varieties with and without a blast resistance gene, Pi-z, were tested for their field resistance in a paddy field in Takayama, Gifu Prefecture. It was attempted to evaluate the field resistance by estimating infection rate on each variety. A negative correlation was obtained between the number of lesions per plant, y, and the infection rate, r, calculated by the equation, r=(lny2-lny1)/(t2-t1), where y2 and y1 were the number of lesions at times t2 and t1, respectively. Taking this fact into consideration, corrected equations were proposed to estimate the infection rate: r=[ln{y2/(Y-y2)}-ln{y1/(Y-y1)}]/(t2-t1), and Y=y1/x1, where x1 was the proportion of diseased leaf-area at t1, and t2-t1 was the incubation period.
