Phosphonic acid (PA) showed moderate inhibition of mycelial growth of Phytophthora cactorum, with an ED50 of 11.3 μg/mL. On the trunks of 8-year-old inoculated peach trees, previously injected with 4 g/tree, PA prevented the development of lesions for up to 12 months. Similar results were obtained when 20-year-old apricot trees were injected with 8 or 10 g/tree. When injected after successful inoculation, PA significantly retarded the development of lesions, causing them to dry up and heal. Furthermore, when sprayed on 3-year-old peach trees at 1 g/tree, there were indications that PA prevented lesion formation for up to 6 months.