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The use of plant-based preparations to replace chemical pesticides is a challenge for sustainable agriculture. Preparations from nettle (Urtica dioica L.) are good candidates, as the treatment of common bean plants (Phaseolus vulgaris L.) with aqueous suspensions of nettle reduced symptoms of halo blight disease caused by Pseudomonas syringae pv. phaseolicola (Pph). The aim of the present work was to test if nettle infusions (Ui) have similar effects and find out what activity is behind this: antimicrobial, promotion of the natural defenses of plants, and/or antioxidant. To achieve this, Pph growth was tested in the presence of infusions of nettle leaves collected in two different years (Ui18 and Ui22), and we found that it was only weakly inhibited at high concentrations of Ui18. Interestingly, Ui22 promoted bacteria growth at all concentrations. Second, we estimated the production of reactive oxygen species (ROS) in response to flagellin22 (flg22) in common bean leaf discs, since recognition of this bacterial peptide usually leads to ROS accumulation in tissues as a plant immune response. However, leaf discs that were preincubated in Ui showed no accumulation of ROS after flg22 treatment, suggesting that Ui can neutralize ROS production. Finally, in a Pph inoculation experiment of common bean plants grown in vitro, we observed that pretreatment of plants with Ui drastically reduced foliar oxidative damage and disease symptoms 6 h after inoculation. This effect was more noticeable for Ui22, which was related to the higher antioxidant activity found in this extract in comparison with Ui18. These results suggest that the protective properties of Ui are mainly due to the content of antioxidant bioactive compounds.