Particle-size dependent bactericidal activity of magnesium oxide against Xanthomonas perforans and bacterial spot of tomato.

Bacterial spot, caused by Xanthomonas spp., is a highly destructive disease of tomatoes worldwide. Copper (Cu) bactericides are often ineffective due to the presence of Cu-tolerant strains. Magnesium oxide (MgO) is an effective alternative to Cu bactericides against Xanthomonas spp. However, the effects of particle size on bactericidal activity and fruit elemental levels are unknown. In this study, nano (20 nm) and micron (0.3 and 0.6 µm) size MgO particles were compared for efficacy. Nano MgO had significantly greater in vitro bactericidal activity against Cu-tolerant X. perforans than micron MgO at 25-50 µg/ml. In field experiments nano and micron MgO applied at 200 and 1,000 µg/ml were evaluated for disease control. Nano MgO at 200 µg/ml was the only treatment that consistently reduced disease severity compared to the untreated control. Inductively Coupled Plasma Optical Emission Spectroscopy revealed that nano MgO applications did not significantly alter Mg, Cu, Ca, K, Mn, P and S accumulation compared to fruits from the untreated plots. We demonstrated that although both nano MgO and micron MgO had bactericidal activity against Cu-tolerant strains in vitro, only nano MgO was effective in bacterial spot disease management under field conditions.

Nano-Magnesium Oxide: A Novel Bactericide Against Copper-Tolerant Xanthomonas perforans Causing Tomato Bacterial Spot.

Bacterial spot caused by Xanthomonas perforans causes significant damage on tomato in Florida. Due to the presence of copper (Cu)-tolerant X. perforans strains, Cu bactericides are not effective in disease management. Hence, there is a critical need to find alternatives for Cu. Antibacterial activity of magnesium oxide (Nano-MgO), and other metal oxide nanoparticles, were evaluated against a Cu-tolerant and -sensitive X. perforans strain. In vitro experiments demonstrated high antibacterial activity of Nano-MgO against both strains compared with the commercial Cu. The minimum inhibitory concentration of Nano-MgO is 25 µg/ml and the minimum bactericidal concentration is 100 µg/ml against a Cu-tolerant X. perforans strain after 4 h of exposure. Structural changes in the bacterial membrane following exposure to Nano-MgO treatments compared with the controls were observed using transmission electron microscopy. In two greenhouse experiments with a Cu-tolerant strain, bacterial spot severity was significantly reduced by Nano-MgO at 200 µg/ml compared with Cu-ethylene bis-dithiocarbamate (grower standard), and the untreated control (P = 0.05). In three field experiments, Nano-MgO at 200 µg/ml significantly reduced disease severity with no negative impact on yield compared with the untreated control. Inductively coupled plasma mass spectrometric analysis of the fruit confirmed that Nano-MgO application did not lead to the accumulation of Mg, Cu, Ca, K, Mn, P, and S. This study is the first to demonstrate the potential of Nano-MgO against bacterial spot of tomato.