Technical Abstract: The effects of potassium nitrate (KNO3) application on Phytophthora stem rot disease reduction of Glycine max (L.) Merr. cvs. Chusei-Hikarikuro and Sachiyutaka, and fungal growth and zoospore release of a Phytophthora sojae isolate were investigated under laboratory conditions. The application of 4-30 mM KNO3 prior to inoculation greatly reduced incidence of disease in the two soybean cultivars. Although a concentration of 20–30 mM KNO3 led to a slight decrease in the growth rate of the PJ-H30 isolate on PDA medium, no significant relationship was observed between inhibition of the growth rate and disease reduction on application of 0.4-10 mM KNO3. Disease suppression recorded in laboratory experiments using pathogen mycelium was due to the response of plant tissues rather than a direct inhibition of pathogen hyphal growth by the application of KNO3. The extent of disease reduction was related to increased potassium concentration in plants of the two cultivars (except for some cases involving cv. Sachiyutaka), suggesting that differences existed between the two cultivars in terms of the effect of KNO3 application on disease suppression. Scanning electron microscope observation with fresh samples indicated marked accumulation of potassium at the penetration-stopping sites of P. sojae in the cortex layer of soybean plants treated with 30 mM KNO3, compared with the non-treated control plants. The presence of 0.4-30 mM KNO3 decreased the release of zoospores. These results suggest the possibility of applying a solution containing 20-30 mM of KNO3 to decrease the incidence of disease in agricultural fields by the response of plant tissues to KNO3.