Olive (Olea europaea L.) has great cultural and economic importance in the Mediterranean basin. Olive oil production is an important source of revenue for many countries in the region and contributes to the growth of their economies. The production of olives and olive oil depends on numerous factors, including the correct application of nutrients. When it comes to olive production and the environment, the type of fertilization used can make a significant difference. Nitrogen (N) and boron (B) fertilization, in particular, can have varying effects on olive production. Fertilization can greatly impact olive growth, nutrient uptake and distribution, phenolic content in olive, and olive tolerance to phytopathogenic fungus Verticillium dahliae Kleb. This study aimed to investigate the effects of N and B fertilization on their content in various parts of olive, including the roots, stem, and leaves. Additionally, this study aimed to evaluate the impact of these nutrients on olive growth traits. This study also examined the effect of N and B fertilization on the phenolic content (root, stem, leaves) and the antifungal activity of phenolic extracts on V. dahliae V-263. Fertilization experiments were conducted separately for N (experiment 1) and B (experiment 2). Seven different treatments were adopted in each experiment, representing seven molar concentrations of either N or B. The control treatments were N0 and B0. The study was conducted on one-year-old plantlets of the Istarska bjelica cultivar, which were grown in an inert substrate under controlled conditions. Measurements were taken periodically, including air temperature, substrate moisture, substrate pH, and olive growth traits. At the end of the experiment, samples of roots, stems, and leaves were collected and prepared for further analysis. The growth traits of olive roots were analyzed at the end of the experiment. Chemical analyses were conducted to evaluate the content of N (g/kg dry weight (DW)) or B (mg/kg DW), total and individual phenolic compounds (mg/100 g DW) in roots, stems, and leaves. In vitro antifungal activity of phenolic extracts, obtained from fertilization treatments with N or B, was assessed against V. dahliae V-263 (experiment 3 a,b; experiment 4). The growth traits of olive were positively affected by the applied N fertilization treatments (N1 – N6), resulting in a greater root and shoot growth compared to the growth resulting from control treatment N0. The uptake and translocation of N from roots to stem and leaves were significantly affected by the application of different N fertilization treatments (N0 – N6). The lowest content of N (g/kg DW) in olive (root, stem, leaves) was found in the control fertilization treatment N0. The content of total and individual phenolic compounds was negatively affected by the applied fertilization treatments (N4 – N6). This study reports the antifungal activity of phenolic extracts (EF), EF-N0 – EF-N6, (root, stem, leaves) modified by N fertilization treatments (N0 – N6). The application of phenolic extracts (treatments EF-N0 – EF-N6) on V. dahliae V-263 had no inhibitory effect on the conidial germination, therefore showing lack of antifungal activity. The application of higher molar concentrations of B (fertilization treatments B3 – B6) positively affected olive growth traits when compared to the control fertilization treatment B0. The lowest content of B (mg/kg DW) was observed in the control fertilization treatment B0 of all investigated parts of olive. The applied fertilization treatments B1 – B6 had varying effects on the content of total and individual phenolic compounds (mg/100 g DW) in the different parts of olive. The results of this study demonstrated that the phenolic extracts of roots and leaves had a significant antifungal effect on V. dahliae V-263. Specifically, phenolic extracts of roots (treatments EF-B0, EF-B3, and EF-B4), as well as phenolic extracts of stems (treatment EF-B2), significantly inhibited the conidial germination of V. dahliae V-263 therefore exhibiting antifungal effect. Additionally, significant inhibitory and antifungal effects were observed on the mycelium growth of V. dahliae V-263 when phenolic extracts from olive roots (treatments EF-B0 – EF-B6) and leaves (treatments EF-B0, EF-B2 – B5) were applied. The applied phenolic extracts (treatments EF-B0 – EF-B6) had different effects on the morphological characteristics of V. dahliae V-263 mycelium in vitro. The findings of this study contribute to a better understanding of the complex interaction between nutrient fertilization, phenolic compounds, and the relation between modified phenolic extracts and V. dahliae V-263 conidial germination and mycelium growth. Further studies are required to identify the effect of N or B fertilization on the content of phenolic compounds in the olive tree and their role in olive tolerance to V. dahliae V-263.