The study hypothesis proposes that the use of Trichoderma, associated with fertilization with 100% of the recommended phosphorus, may mitigate saline stress and maximize the productivity and quality of the tuberous root. This study aims to evaluate the mitigating effects of phosphate fertilization and Trichoderma harzianum in beet plants under salt stress, by measuring the initial growth, leaf gas exchange, productivity and quality of the beet. The experimental design used was entirely randomized, in a 3 × 2 × 2 factorial scheme, referring to three doses of phosphate fertilization (25%, 50% and 100%), with and without the use of Trichoderma-based inoculation, and two levels of electrical conductivity of the irrigation water (0.5 and 6.2 dS m−1). Salt stress negatively affected the leaf area of the beet. The shoots’ dry mass was reduced as the electrical conductivity of the irrigation water increased, especially in the treatment with the 25% P2O5 dose. Salt stress reduced photosynthesis to a greater extent at the 25% P2O5 dose and in the absence of Trichoderma harzianum. Increasing the electrical conductivity of the irrigation water reduced transpiration and increased leaf temperature at the 25% P2O5 dose and in the presence of Trichoderma harzianum. The 25% P2O5 dose increased the stomatal conductance of the beet. The higher electrical conductivity of the irrigation water negatively affected water use efficiency, most significantly at the 25% P2O5 dose. Our data showed that the doses of 50% and 100% P2O5 were more efficient at increasing the productivity and quality of the beet, with the tuberous root diameter being higher under the lower electrical conductivity of the water and the absence of Trichoderma harzianum. The pH was high under the lowest electrical conductivity of the water, with a dose of 25% P2O5 and the absence of Trichoderma harzianum.
