Alternaria leaf blight, is an important foliar disease of muskmelon (Cucumis melo L.) through the world. Disease observation indicated that leaf blight of melon, was widespread in El-Sharkia and El-Ismailia governorates during March to April under tunnel conditions in Egypt. Isolated fungi was identified as Alternaria cucumerina (Ellis & Everh J. A. Elliott) (Macrosprium cucumerinum), and its pathogenicity was confirmed on melon and cucumber. Dl-ß-amino-n-butyric acid (BABA) at high concentrations (1 and 0.5 mg ml-1) tested, had no antifungal effect on mycelial growth, spores production, and spores germination of A. cucumerina. Melon plants treated with BABA (10 mM) 7 days before inoculation resulted in significant disease protection. Treatment of leaf no. 2 and 3 (local), 7 days before inoculation induced resistance not only locally (treated leaves) but also systemically on leaf no. 4. Foliar application with BABA caused in treated tomato leaves small hyper sensitive (HR) like necrotic lesions which became visible 24 h after treatment, especially on adaxial leaf surfaces. These small lesions were characterized by cell death as demonstrated by irreversible uptake of Evans blue. Light and fluorescent microscopic examinations of leaf disks stained with trypan blue/Evans blue showed clusters of dead cells which were surrounded by cells exhibiting deposition of callose (1,3-β-glucan). In BABA treated melon leaves, O-2 generation was determined 12-36 h after treatment by nitroblue-tetrazolium staining and production of H2O2 was detected by 3,3-diaminbenzidine-4HCl staining 12-16 h after treatment. Furthermore BABA significantly increased free and total salicylic acid contents in treated melon leaf tissues. The studies with BABA against melon leaf blight suggested that BABA had no effect on mycelial growth, spores production and spores germination of A. cucumerina. Generation of O-2; H2O2, small hyper sensitive (HR) like necrotic lesions formation and increase free and total salicylic acid, leading to disease resistance mechanisms in melon leaf tissues to leaf blight