The objective of the work was the evaluation of the effect of distilled water and very hard water (215.04 mval)on the effectiveness of Rubigan 12 EC (fenarimol) – 0.48 dm³ ha⁻¹ and Saprol 190 EC (triforyna) – 1.6 dm³ ha⁻¹ against powdery mildew on roses. Increased volume of tank mixture applied to 1 ha caused an increased effectiveness of the treatment. The water type did not exert any significant effect on fungicide effectiveness. Practically and economically justified in the protection of roses against powdery mildew is the use of 200 dm³ of spray mixture per 1 ha.