Abstract
In order to know the effect of the degree of resistance or susceptibility of rice plants to fungal growth on infection rate of blast fungus, various rice varieties were grown in the field where various amounts of fertilizer were top-dressed. The disease increase was measured by lesion number and the degree of resistance or susceptibility of rice plants to blast fungus was estimated by measuring degree of hyphal growth in the leaf sheath cell. Infection rate was calculated by the equation y=y0er(t-t2/2T) or rs=2.3/(t2-t1).·log10y2/y1. The average degree of hyphal growth measured after the first day showing a minimum temperature over 18°C increased by top-dressing or late transplanting. This indicates that the degree of resistance or suscpetibility of rice plant to blast is increased by these treatments. There was found a varietal difference of average degree of hyphal growth. The regression of infection rate (r) on the average degree of hyphal growth was positive and statistically significant. Regression of infection rate in short period (rs) on degree of hyphal growth in plant cell at the presumed infection time of newly formed lesions was also positive and significant. The extent to which the degree of resistance or susceptibility of rice plants contributes to variation of both infection rates is about 40%. The effects of the degree of resistance or susceptibility of plant on both infection rates, r and rs were different between years, showing that the influence of climatic factors on infection rate is very large.
