In this study, the influence of different water volumes applied by the unmanned aerial vehicle (UAV) and back motor sprayer (BMS) in fungicide control of wheat diseases (Tan spot – Pyrenophora tritici-repentis and Septoria tritici leaf blotch – Zymoseptoria tritici) was researched in field conditions. During the whole experiment, no phytotoxicity of the used fungicide (solatenol 75 g/L + prothioconazole 150 g/L) was observed (and that´s why); the UAV application can be considered as safe for the wheat crop in all tested spray volumes. The trial results showed that the fungicide efficacy (FE) of UAV application depends on the used spray volume and air temperature during application. The best FE was achieved by using a BMS of 100 and 300 L/ha spray volume. It is proven that the lowest FE was recorded by using a UAV spray volume of 10 L/ha applied at high temperatures (28°C). The use of UAV ultra-low spray volume is not suitable for application in high air temperatures, which could dramatically decrease the FE of the applied fungicide. Suitable temperatures (around 20°C) for application did not negatively influence low UAV spray application volumes of 10 L/ha. The FE by using UAV spray volume of 10 L/ha was approximately 10% lower than the FE of 300 L/ha BMS spray volume. The BMS spray volume of 100 L/ha and UAV volume of 50 L/ha showed similar FE in suitable conditions for application. The results showed that for practical UAV fungicide applications, it is needed to use a spray volume of 50 L/ha or more. Further studies are needed to evaluate the effects of other pesticides to estimate how widely the results are applicable in agricultural practice.