AbstractThe main measure worldwide adopted to manage plant bacterial diseases is based on the application of copper compounds, which are often partially efficacious for the frequent appearance of copper-resistant bacterial strains and have raised concerns for their toxicity to the environment and humans. Therefore, there is an increasing need to develop new environmentally friendly, efficient, and reliable strategies for controlling plant bacterial diseases, and among them, the use of nanoparticles seems promising. The present study aimed to evaluate the feasibility of protecting plants against attacks of gram-negative and gram-positive phytopathogenic bacteria by using electrochemically synthesized silver ultra nanoclusters (ARGIRIUM‑SUNCs®) with an average size of 1.79 nm and characterized by rare oxidative states (Ag2+/3+). ARGIRIUM‑SUNCs strongly inhibited the in vitro growth (effective concentration, EC50, less than 1 ppm) and biofilm formation ofPseudomonas syringaepv.tomatoand of quarantine bacteriaXanthomonas vesicatoria,Xylella fastidiosasubsp.pauca, andClavibacter michiganensissubsp.michiganensis. In addition, treatments with ARGIRIUM‑SUNCs also provoked the eradication of biofilm forP. syringaepv.tomato,X. vesicatoria, andC. michiganensissubsp.michiganensis.Treatment of tomato plants via root absorption with ARGIRIUM‑SUNCs (10 ppm) is not phytotoxic and protected (80%) the plants againstP. syringaepv.tomatoattacks. ARGIRIUM‑SUNCs at low doses induced hormetic effects onP. syringaepv.tomato,X. vesicatoria, andC. michiganensissubsp.michiganensisas well as on tomato root growth. The use of ARGIRIUM‑SUNCs in protecting plants against phytopathogenic bacteria is a possible alternative control measure.Key points• ARGIRIUM‑SUNC has strong antimicrobial activities against phytopathogenic bacteria;• ARGIRIUM‑SUNC inhibits biofilm formation at low doses;• ARGIRIUM‑SUNC protects tomato plants against bacterial speck disease.