This article is part of the search for alternatives to the use of pesticides (especially elicitors of resistance) within the context of integrated pest management (IPM). For this reason, resistance triggers based on potassium phosphite were used in the form of oak stem spraying. The aim of the study was to assess the influence of phosphites applied as a commercial product called Kalex. The changes in the immediate environment and the development of root systems and tree crowns (based on defoliation and health indicators) were assessed. The treatment of oak stems with potassium phosphite had no negative effects on the forest environment. The acidity (pH) of the soil did not change nor did the content of Mg and Ca. Phosphorus and potassium from the treatments were taken up by the living part of the bark and transported to the fine roots. However, some of the treatments certainly entered the soil via the logs when it rained. Thus, these elements also reached the vicinity of the roots, where they were present in larger quantities. As a result of leaf loss (defoliation), the crowns of the control trees were more thinned out than those of the trees treated with the phosphite preparation. Over several years, this led to a decrease in the health of the control trees, i.e. a change in the crown architecture (deformation due to the formation of short shoots) expressed in vitality grades. The calculated synthetic damage index (Syn), which takes into account the degree of defoliation and health, was also higher in the control trees than in the treated trees indicating the effectiveness of the treatment with the commercial product Kalex. The fine roots of the treated oaks had more favourable development parameters than the corresponding roots of the control trees especially in terms of length, number and surface area. This increased the ability of the treated oaks to take up the water needed to increase photosynthetic efficiency in order to feed the roots. However, in a situation of extreme drought in 2015 resulted in the death of the fine roots of the oaks (independent of the treatment) and continued the following year. Only in the 2017 season, when soil moisture improved significantly, did the oaks return to the state of root development before the severe drought. As a result, the percentage of dying trees in the treated tree group was statistically lower than in the control group. The treated oaks (especially those that were up to 30% defoliated) survived better during the five−year observation period (2013−2017) and were able to effectively absorb nutrients and water from the soil due to the better condition of their fine roots which was reflected in better shoot development in the crowns (assessed by defoliation, vigour and Syn−index).

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