Fungicides-late blight interaction in the synthesis of phenolic compounds and defense enzyme activity in tomato

Due to the significant impact of late blight (LB) (Phytophthora infestans Mont.) on tomato (Solanum lycopersicum L.), we investigated the interaction between fungicides and this disease to understand how some plant defense mechanisms are affected over time. Following a randomized design, we evaluated the synthesis of phenolic compounds (PHE) and the activity of phenylalanine ammonium lyase (PAL), peroxidases (POX) and superoxide dismutase (SOD). The experiment involved the application of fosetyl-Al and fluoxastrobin (fungicides with dual modes of action) on healthy and infected tomato plants. LB severity was assessed weekly and leaf samples were collected at various intervals for biochemical analysis. The Kruskal-Wallis test (α = 0.05) analyzed main effects of infection, fungicide, and time on response variables, followed by Bonferroni post hoc for significant group differences and regression models to evaluate variable effects over time. The application of fungicides had no effect on enzymatic activity or PHE accumulation. While PAL and SOD activities were not significantly affected by infection, POX activity was significantly higher in healthy plants (4793.8 U g-1 fresh weight) compared to infected plants (1858.1 U g-1 fresh weight). A complex interaction between PHE accumulation in relation to LB severity and time was observed, with a notable increase in PHE levels at 50 days after transplant when disease severity was between 25 and 50%. Future studies should consider including a broader range of genotypes and isolates of P. infestans, a more extensive set of biochemical responses, and evaluations of the overexpression of genes related to plant defense.