Late blight disease caused by P. infestans of potato plants was controlled under field conditions using potassium or sodium bicarbonates alone or in combination with Citral. In laboratory experiments, results indicated that complete inhibition in linear growth of P. infestans was obtained with potassium or sodium bicarbonates at 2% and Citral at 0.5%. Moreover, in field experiments, results indicated that the most effective treatment was potassium bicarbonate at 2.0% plus Citral at 0.5% which reduced the disease severity by 84.4 and 82.4% during two growing seasons. The highest reduction was obtained with potassium or sodium bicarbonate at 1.0 or 2.0 respectively plus citral at 0.5% and Redomil which reduced the early blight incidence more than 61.8% as compared with untreated plants. As for potato yield the highest increased was obtained with potassium bicarbonate at 2.0% plus Citral at 0.5% which increased tuber yield by 76.0 and 67.9% during two growing seasons. All treatments significantly increased the chitinase and -1,3-glucanase activities. The great increased was obtained with potassium bicarbonate at 2.0 or 1.0% plus Citral which increased the chitinase and -1,3-glucanase activities more than 120.0 and 220.1% respectively. It could be suggested that combined treatments between potassium bicarbonate plus Citral might be used for controlling late blight disease of potato plants under field conditions.