Excessive use of heavy metals such as copper in agriculture biocides have raised environmental concerns thus warranting alternative strategies. Magnesium serves as one macronutrient for plants. Employing magnesium based biocidal material can act as a dual-purpose strategy to act both as a nutrient supplement and as a plants disease controlling agent. The low cost and environmental friendly magnesium hydroxide has been widely applied in wastewater purification, fire retardant material and in neutralizing stomach acidity. Recent studies suggest magnesium hydroxide as an effective antibacterial agent. This study focuses on synthesizing surface charge-tunable magnesium hydroxide particles and their potential application in agriculture. Magnesium hydroxide was synthesized by coprecipitation method. Various organic capping agents were used to control particle size. Positive and negative charged particle may have a different interaction preference towards negative charged bacteria surface. Different capping agent could change particle surface charge from negative to positive. The binding between capping agent and particles were confirmed by FT-IR. Zeta potential measurement was used to confirm the particle surface charge and the particle surface charge was variable with different capping agents. The hydrodynamic size was confirmed by DLS, the diameter of synthesized Mg(OH)2 with different capping agents were 200-300 nm. XRD was applied to study the crystal structure of as-synthesized Mg(OH)2. The morphology of synthesized material and the relationship between morphology and antibacterial effects are further studied by SEM. The antimicrobial activity was determined by performing a 96-well plate based MIC and CFU assays. The results showed that the minimum inhibitory concentration for Pseudomonas syringae and Xanthomonas alfalfae was about 250 μg/mL of metallic magnesium. This study introduces synthesis of a magnesium hydroxide based biocide that can be applied for plant disease control.