This study conducted in the laboratories of General Commission for Agriculture Research( GCSAR) /Doma ,in order to determinate the sensitivity of some tomato varieties (KAMAR#4.F1 , VT60788.F1 , Orient. F1, Super red , Alegro and Yarmok) -inputted  and cultivated - in southern region of Syria to infection of local isolates of the pathogen Alternaria alteranta  ( D 2.3- R6.1a – Q12.4) the major agent of blight and leaf spot of tomato . The results shows that the severity of infection ranged between 20.3% to 74% . The cultivars ranged from sensitive  to high resistance , there is no significant differences between inputted varieties with each other for the isolate Q12.4(with probability level 1%) , and there is a significant differences with the rest of varieties , on the other hand ,the results showed that the significant differences existed between the  inputted varieties for the isolates R 6.1a and D2.3 , and with the cultivated varieties did not show a significant differences between the isolates Q12.4 and R6.1a , but the significant differences existed for the isolate Q12.4 . the inhibitory effect of sodium bicarbonate (NaHCO₃) in fungal linear growth tested for the fungus colonies in dishes by using a local isolate Q12.4 with four concentrations of NaHCo₃ (0.25  %    , 0.50 %     ,   1 %  and  2 % )  W/V in addition to the control . The results showed that concentrations ( 0.25% , 0.50% ) had a catalytic effect for fungal  growth by 68.97 % and 44.87 % respectively , while ,indicate concentrations ( 1% , 2%) had an inhibitory effect to the growth by 18.17% and 100% respectively , with the presence of significant differences between themselves ,and with the control at the level of probability 1% .