Sensitivity of Alternaria radicina and A. tenuissima to the following antioxidant compounds ascorbic acid, benzoic acid, hydroquinone, salicylic acid and tannic acid was investigated in vitro. Mycelial dry weight of the tested fungi was completely inhibited at 10 mM benzoic acid and salicylic acid. Hydroquinone at 10 mM caused a complete inhibition to the mycelial growth of A.tenuissima. At the lowest used concentration (0.1mM), ascorbic acid and salicylic acid stimulated the growth of A.tenuissima and inhibited the growth of A.radicina. Tannic acid at 0.1mM stimulated mycelial growth of A.radicina and inhibited A.tenuissima. Spore formation was greatly affected by all the tested chemicals. The reduction in spore formation was enhanced by increasing the chemical concentration. Spore germination of A.radicina was more affected than of A. tenuissima particularly at 1.0 mM. Viability of conidiospores was greatly affected especially with of A.radicina in this respect.             Regarding polygalacturinase (PG) activity, all the tested chemicals reduced PG activity except tannic acid at 0.1 mM. Salicylic acid inhibited PG activity of A.radicina greater than of A.tenuissima. While, hydroquinone inhibited PG of A.tenuissima more than PG of A.radicina.             Resistance of 2-year-old pear plants var. Leconte against leaf-spot causing fungi i.e, A.radicina and A.tenuissima was induced by spraying pear plants with the antioxidant compounds. Salicylic acid caused the highest protection when it was applied 24h. before or after inoculation. Contrary, tannic acid increased the susceptibility of pear plants to both fungi when it was applied 48 hr. after inoculation It showed the least protection (0.7%) in case of A.radicina when applied 24h. after inoculation.