Abstract
Rice leaf blast disease incidence was investigated in mixed plantings of susceptible and resistant rice cultivars in the nursery and paddy field. In the blast nursery experiment, increase of rice leaf blast disease could be described by the equation dx/dt=rx(1-x), where x is the proportion of diseased area per plant of susceptible cultivar, t is the time in days and r is the apparent infection rate. The r value in mixed plantings of susceptible and resistant cultivars (rm) was in accordance with the equation rm=rs+clogem, where rs is r in single planting of susceptible cultivar, m is the proportion of susceptible cultivar in mixed plantings and c is a constant. In the paddy field experiment, the gradient of leaf blast disease incidence from inoculum sources which were placed in the center of plots was given by the equation log10y=A+Blog10x, where y is the number of acute-type lesions per hill of susceptible cultivar, x is the number of hills from inoculum source and A and B are parameters. The B values of plots, which represented leaf blast disease gradient from inoculum source, increased with both increasing ratio of susceptible cultivar in mixed plantings and with time. The presumed total number of acute-type lesions in plots was calculated by the equation log10y=A+Blog10x, using the A and B values for each plot. In order to compare rice leaf blast increase of each plot directly, The recalculated total number of acute-type lesions was obtained by dividing the presumed total number of acute-type lesions by the ratio of susceptible cultivar in the mixture. The recalculated total number of acute-type lesions in mixed plantings was smaller than that of single planting of susceptible cultivar. Infection rates between each investigated date which was calculated by the exponential growth model using the recalculated total number of acute-type lesions approximated Leonard's equation rm=rs+clogem.
