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Phosphonate-based products have demonstrated diverse abilities to protect crops against pests, with various modes of action proposed. In this article, we specifically investigated potassium phosphonate (KHP) on apple crops. Its performance to control three major apple bioagressors ( Venturia inaequalis , Erwinia amylovora , and Dysaphis plantaginea ) was evaluated under semicontrolled conditions. The product was able to confer significant protection rates (40 to 75% for apple scab, 40% for fire blight, and 30% for rosy aphid), which can be explained by its more or less efficient biocidal activity against the three pests, and by its ability to induce apple immunity (pathogenesis-related proteins and secondary metabolites genes). A cumulative effect of treatments as well as the systemic behavior of the product was demonstrated. Fields trials against apple scab and the postharvest disease bull's eyes rot ( Neofabraea vagabunda ) were performed on different apple varieties by applying KHP combined with light pest management programs either reducing (dessert orchards) or suppressing (cider orchards) fungicide applications. KHP was able to reduce apple scab by 70 to 90% on shoots and young and harvested fruit, and bull's eyes rot by 70 to 90% on harvested fruit. Overall, our results indicate that KHP is useful for the protection of apple trees against its major pests by direct effect and by triggering the host defense system.