AbstractInvestigations on the influence of aluminium ethyl phosphite on the plant phenolic metabolism in the pathogen‐host‐interactionsPhytophthora fragariae ‐strawberry andBremia lactucae ‐lettuceAluminium ethyl phosphite exhibited pronounced preventive and very good curative activities. In strawberries, 4000 ppm of the compound affected an eminent protection againstPhytophthora fragariaeafter root and leaf applications; the earlier the fungicide treatment was carried out, the more distinct was the control effect. Similar relations were established in the pathogen‐host‐interactionBremia lactucae, in which 5000 ppm of the fungicide proved to be highly active following pre‐ and postinfectional applications.In both parasite‐host‐interactions, an enrichment of phenolics in consequence of ade‐novo‐synthesis in diseased plant tissue was observed only very late and exclusively, when fungicide and parasite had come together; inoculation on one hand or fungicide treatment on the other resulted in no remarkable effect.With great certainty, however, the augmentation of phenolics was not the cause, but rather the consequence of fungicidal efficiency which resulted in killing of the fungus and partial destruction of host tissue after direct influence of aluminium ethyl phosphite on the parasite itself. A clear proof for a main and causal evidence of aluminium ethyl phosphite‐induced phenolics for protection of strawberries againstPhytophthora fragariaeand of lettuce againstBremia lactuca