Abstract: Tomato stem canker caused by Altrnaria altrnata f.sp. lycopersici, hs one the most devastating diseases worldwide. In this study, possible effect of using Salicylic Acid (SA) in host resistance induction against tomato stem canker was investigated. Foliar applications of salicylic acid with two concentrations of 299 and 400 micro Mol. were tested against fungal pathogen. The results demonstrated that the use of 200 micro Mol. Dosage of SA was insufficient for inducing disease resistance. However, application of SA in concentration 400 micro Mol. significantly reduced disease index as compared with infected control. There were significant effect of plant cell reaction in term of percentage of penetration events that occurred at the site of attempted penetration. SA pre-treated plants were identified with higher numbers of single cell necrosis (Hypersensitive response), but lower blighted areas and discolorations, as compared with non treated plant (control). The results obtained from high performance thin layer chromatography (HPTLC) analysis indicated that there is highly correlation between increasing of the endogenous level of free SA in treated plants and reduction of the blighted leaf areas. SA pre-treatment plants caused an increase in the endogenous free SA levels in tomato leaves which resulted in systemic resistance induction.

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