Eucalyptus leaf aqueous extract - a promising biopesticide for managing bacterial spot disease in tomato plants

Phytopathogen attacks threaten plant biodiversity and food production worldwide and are expected to worsen due to climate change. To reduce agricultural dependence on synthetic pesticides, eco-friendly alternatives must be developed. We have recently shown that aqueous extracts from dried leaves of young Eucalyptus globulus Labill. had promising in vitro antibacterial activity against phytopathogens. However, its in vivo effectiveness remains unclear. Thus, this study aimed to evaluate the efficacy of the extract against tomato seeds and plants infected with Xanthomonas euvesicatoria (Jones et al.) Constantin et al. (Xeu), as well as to assess the plant's physiological responses upon biopesticide application. For this, Solanum lycopersicum L. cv. Roma seeds were inoculated with a Xeu suspension, incubated with the extract (0, 15, and 30 g L^-1), and left for germination for 10 days.

The treatment of inoculated seeds with the extract increased radicle length and reduced the incidence of symptomatic cotyledons. Parallely, 1-month-old tomato plants were foliar-infected with a Xeu suspension and treated with various concentrations of the extract. Disease symptoms were monitored weekly over 3 weeks, showing a dose-dependent decrease following extract application. After 21 days, the extract at both concentrations equally reduced the leaf bacterial population compared to infected plants. The application of the extract at 15 g L^-1 to infected plants improved their physiological status by enhancing antioxidant enzyme activity and photosynthetic performance.

Overall, these findings suggest that the extract can be used as an effective tool against Xeu and contribute to reducing pesticide application. © 2025 Society of Chemical Industry.