Efficacy of mangosteen peel extract combined with phosphonic acid for controlling Phytophthora palmivora in durian

The efficacy of mangosteen peel extract (MPE) combined with phosphonic acid (PA) for controlling Phytophthora palmivora, the causal agent of durian trunk canker and root rot, was evaluated in vivo using durian (Durio zibethinus) leaves and seedlings. Results showed that PA alone significantly reduced lesion size and achieved the highest disease inhibition on both leaves (0.61 cm, 71.58%) and seedlings (4.08 cm, 35.91%) at 14 days after inoculation. The combination of MPE with PA at 10,000 ppm showed efficacy on leaves (0.93 cm, 57.93%), with no significant difference from PA alone. Moderate inhibition levels were observed with MPE at 1,000 and 100 ppm combined with PA, whereas 10 ppm MPE showed lower efficacy. On seedlings, higher concentrations of MPE (≥1,000 ppm) did not improve disease control and were associated with larger lesion diameters, suggesting a possible concentration-related adverse effect. The commercial MPE product provided the lowest efficacy among the treatments on seedlings. Overall, these results indicated that PA was effective against P. palmivora under the condition tested, while the effect of combining MPE with PA varied with MPE concentration and plant tissue.