Metalaxyl is an effective approach to control Phytophthora palmivora infection in durian plantation. However, inappropriate metalaxyl usage may increase production cost, pathogen with fungicide resistance, and environmental toxicity. This study established and validated a simple and reproducible procedure to measure metalaxyl concentration in the durian leaf using HPLC. Linearity of the detection ranged from 1–100 µg/mL. The limits of detection (LOD) and quantification (LOQ) were 0.27 and 0.91 µg/mL, respectively. The extraction method gave recovery rates ranging from 88% to 103%. Durian seedlings were treated with 4 g/L metalaxyl either by foliar spray or soil drench. The highest metalaxyl accumulation in durian leaf was found between 6–24 h after treatment and persisted above its effective concentration at least 60 days after foliar application. The dissipation pattern fit to a first-order kinetics equation showed a half-life of 16.50 days. Soil drenching led to eight times higher metalaxyl concentrations in plants than foliar spraying and caused plant death within 15 days after application. These results suggest that foliar spraying of 4 g/L metalaxyl or soil drenching at a lower concentration every two months is sufficient in controlling P. palmivora infection in durian seedlings.