For many years management of Xanthomonas perforans, the causal agent of bacterial leaf spot (BLS) of tomato, with copper bactericides has been impractical because of the presence of only copper (Cu)-tolerant strains. Over the past decade, micron-sized particles (>100 nm size) and nano-sized particles (1-100 nm size) of magnesium oxide (MgO) particles were shown to have antimicrobial activity against several mammalian pathogens. In this study, three sizes of MgO were evaluated against X. perforans. The three different sized particles, 20nm, 0.3µm, and 0.6µm of MgO, all had high bactericidal activity against X. perforans in vitro. In field experiments, disease severity was significantly reduced by applications of the 20 nm sized particles at 200 µg/mL compared to the untreated control (UT), whereas the grower’s standard (Cu-EBDC), and the micron-sized (0.3µm, 0.6µm) particles of MgO treatmentswere not significantly different from UT (p = 0.05). In addition, fruits from MgO treated plots did not significantly alter levels of Mg, Cu, Ca, K, Mn, P and S compared to fruit in the UT. In current study, newly formulated Mg nano-materials significantly reduced disease severity to as low as 100 µg/mL compared to Cu bactericide treated plots in greenhouse and in the field (p = 0.05). Based on these findings, Mg nano-materials could be an effective alternative to the current grower standard.