In Tunisia, the cereal sector plays an important socio-economic role. This work is carried out to study the effect of fungicide on water use efficiency and yield components of old and new varieties of bread wheat. The final yield components as well as the water use efficiencies were statistically analyzed at the end of the experimental campaign. The results showed that the Kodss and Maktarus varieties presented better performance even in the absence of the fungicide treatment. This confirms their ability to resist the most devastating stripe rust in bread wheat. Nevertheless, the used fungicide (ogame) has proven its effectiveness in protecting crops against biotic stress and improving yields, water use efficiencies, and production quality. Thus, it is recommended to avoid fungal treatments on new varieties.