The effect of five antioxidants (citric acid, salicylic acid, benzoic acid, ascorbic acid, and sodium citrate) on the resistance of tomato plants (Lycopersicon esculentum Mill) to early blight disease incited by Alternaria solani was investigated in vitro and in vivo. The results indicated that all of the antioxidants tested markedly inhibited the mycelial growth of A. solani in vitro. The degree of inhibition was directly proportional with the antioxidant concentrations. The mycelial growth was greatly inhibited at the highest concentration (10.0 mM) of all antioxidants tested. Salicylic acid, ascorbic acid, and citric acid were more effective throughout the concentration ranges in inhibiting the mycelial growth than sodium citrate and benzoic acid respectively. Soaking tomato seeds in the antioxidants prior to sowing markedly increased tomato resistance to early blight disease by A. solani. All antioxidantse employed at different concentrations, caused a marked reduction in the appearance of early blight disease on tomato plants and the disease incidence was completely inhibited at the highest concentrations of every antioxidants tested. Salicylic acid, ascorbic acid, and citric acid were the most effective in controlling the disease; followed by sodium citrate, and benzoic acid. Also data indicated that the increasing in stem average length and leaves number per plant was corresponded with the reduction of disease severity. Phytoalexin (tomatine) production was greatly increased in antioxidant-treated inoculated tomato plants. In all treatments, the level of tomatine was higher in the leaf than stem of inoculated tomato plants compared with untreated inoculated control and treated uninoculated plants. Maximum production of tomatine was achieved in tomato plants treated with salicylic acid, ascorbic acid, or citric acid, and inoculated with fungal spores where it was not detected in leaf and stem of uninoculated control plants. These results suggested that antioxidants may play an important role in controlling the early blight disease through inducing production of phytoalexin from cells of tomato plants. Some antioxidants are used as food grade so they are safe for use as antifungal without any risk to human, animal, and environment.
Key words: Antioxidants, Tomato, Early blight, Alternaria solani, Phytoalexin tomatine.
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