In vitro and in vivo antibacterial activity of copper alternative products against Xanthomonas euvesicatoria pv. perforans causing leaf spot and pith necrosis of tomato

In organic farming, copper-based compounds are widely used to control fungal and bacterial diseases affecting several crops, including bacterial pathogens on tomato plants. The copper accumulation in the soil, the development of copper-resistant strains and the potentially toxic effects on the soil microbial communities have prompted the search for alternative products able to reduce or gradually phasing out copper in agriculture, especially in organic cultivation. In addition, legal regulations in force within the European Union limit today the use of copper compounds in agriculture, imposing a maximum limit of 4 kg ha-1 year-1. In this work, 13 products including fertilizers with low copper content, resistance inducers, biocontrol agents and 20 essential oils were tested in vitro at different concentrations against Xanthomonas euvesicatoria pv. perforans. Comprehensively, the antibacterial effectiveness of all tested products was directly related to the concentration rate. Vitibiosap, Dentamet, Kiram, Kodens at 10% and four essential oils (Thymus vulgare, Origanum vulgare, Melaleuca alternifolia and Cinnamomum zeylanicum) showed the highest in vitro inhibitory activity. In in vivo tests, carried out on ‘Syr Elyan’ tomato plants, Dentamet and Kiram AT were effective in reducing disease incidence and disease severity of leaf spot and pith necrosis, with values comparable with Cu oxychloride and Cu hydroxide. Although further studies are needed under natural bacterial disease pressure conditions, these results highlight the possibility to replace or reduce the common Cu compounds used to control X. euvesicatoria pv. perforans mantaining a similar effectiveness of conventional treatments.