This study was carried out to evaluate the effect of ammonium sulfate, calcium chloride, potassium dibasic phosphate and sodium carbonate on inhibiting mycelial growth of Alternaria alternate (Fr.) Keissler, Helminthosporium sp., Stemphylium botryosum Wallroth,and Curvularia lunata (Wakker) Boedijn isolated from lettuce (Lactuca sativa Folium.) leaves. The sodium carbonate and ammonium sulfate strongly inhibited mycelial growth and spore germination. Potassium dibasic phosphate and calcium chloride inhibited mycelial growth to a less extent. Sodium carbonate (25mM) decreased mycelial growth of A. alternate by 58.9%, Helminthosporium sp. by 58.8%, S. botryosum by 78.5% and C. lunata by 40.0%. The folpet fungicide used variably decreased growth of fungi in a range varied between 43.5% and 87.5% at 5ppm concentration. The effective concentration (EC50) figures indicated that sodium carbonate and ammonium sulfate caused more inhibition to A. alternata, Helminthosporium sp., S. botryosum and C. lunata. Lower concentration of potassium dibasic phosphate and calcium chloride showed lower effect. In general, the most effective chemicals used to control lettuce foliar diseases were sodium carbonate, ammonium sulfate, calcium chloride and potassium dibasic phosphate that decreased leaf spot severity. Sodium carbonate had a higher inhibiting activity against foliar diseases of lettuces. Expression of defense related enzymes involved in lettuce induced tolerance against infection was emphasized. The greater hydrolysis of plant cell walls by A. alternata, C. lunata, Helmithosporium sp. and S. botryosum was attributed to greater of polyglacturonase (PG) secretion.