Abstract Fosetyl-Al (aluminium tris-(ethyl phosphonate)) is a low-toxicity phosphonate commonly used to control oomycete-related diseases in crops. Trunk injection (endotherapy) delivers the fungicide directly into the vascular system, potentially improving efficacy while reducing environmental risks. This two-year study evaluated the efficacy and phytotoxicity of trunk-injected fosetyl-Al to manage ink disease caused by Phytophthora cinnamomi in Castanea sativa saplings. Fifty saplings received fosetyl-Al injections at 0–4% using pressurized devices, followed by stem inoculation with the pathogen; a 2% soil irrigation treatment was included for comparison. Trunk injections at 1%, 2% and 4% significantly suppressed stem necrosis and reduced root infections compared to irrigation and untreated control, with protection persisting for at least two years as reflected by reduced relative Area Under the Disease Progress Curve (rAUDPC) values. However, trunk injections also induced phytotoxic symptoms—including leaf burn, necrosis, and occasional plant death—with a clear dose-dependent relationship. While most plants recovered from moderate phytotoxicity, irreversible damage occurred at the highest doses, highlighting the critical need for precise dose adjustments, particularly in small-diameter trees. Logistic regression identified a Minimum Effective Dose (MED) range of 0.061–0.102 g/cm of stem circumference, corresponding to disease incidence targets of < 10% to < 1%, respectively, with phytotoxicity risks estimated between 10% and 25%. These results demonstrate that trunk injection of fosetyl-Al provides effective, targeted, and environmentally friendly control of ink disease, while emphasizing the need for dose optimization to minimize adverse effects.