Abstract
Infection rate of blast fungus was estimated under various field conditions. It was calculated by the two equations, y=Y/(1+ke-rt) (logistic curve) and y=y0er(t-t2/2T), where y is the number of lesions, Y is the final number of lesions in the year, y0 is the number of lesions at the initial time of infection, r is infection rate, t is the time in days and T is the time when the increase of lesion number stopes. When the disease increase plotted with lesion numbers, the latter equation fitted better than the former one. Infection rate calculated by the latter equation showed a significant negative regression against the number of lesions at the first recording time, indicating the influence of density effect of pathogen multiplication.
Infection rate was influenced more greatly by the amount of top dressed fertilizer and annual changes in climatic conditions than the other given factors, that is, transplanting time and variety of rice.
