Investigations on the systemic acquired resistance induced by acibenzolar-S-methyl in tomato plants against Pseudomonas syringae pv. tomato

Growth chamber experiments showed that acibenzolar-S-methyl (ASM) (50divided by50 muM) treatments systemically protect tomato plants against Pseudomonas syringae pv. tomato. Evidence for this was the reduction in bacterial spot diameter and bacterial growth in planta. The efficacy of ASM was also found in open field conditions, where both leaves and fruit were protected from the disease. The best efficacy (about 80%) was obtained by spraying the plants 6divided by8 times with a mixture of ASM and copper hydroxide (2.5 + 80 g hl(-1) active ingredient). Open field experiments revealed that neither quality nor quantity of fruit yield were negatively affected by ASM treatments. By using the HPLC method of Scarponi et al. (2001), we showed that ASM rapidly translocates from treated to untreated tomato leaves, with the maximum translocation occurring 8 h after the treatment. ASM residues decreased as the time elapsed in both treated and untreated tomato leaves, reaching negligible values 72 h after treatment. We also demonstrated that the acidic derivative of ASM (CGA 210007), which systemically protected tomato plants against P. s. pv. tomato, was formed in tomato plants as early as 2 h after ASM treatment, albeit only in the treated leaves. CGA 210007 residues decreased in treated tomato leaves with a trend similar to that observed for ASM. Since neither ASM nor CGA 210007 inhibited bacterial growth in vitro and the protection against bacterial speck of tomato has been observed after the two compounds were completely degraded, we can conclude that the protection must be due to the activation of the plant's defence mechanisms.