Four sprays of 25mM, 50mM, and 100mM solutions of NaH₂PO₄, Na₂HPO₄, KH₂PO₄, K₂HPO₄, NH₄H₂PO₄, and (NH₄)₂HPO₄ were used to compare their effects in controlling powdery mildew of flax. In this study, it was found that sodium monohydrogen phosphate Na₂HPO₄ was the only effective salt in suppressing the disease as it reduced powdery mildew severity by 44.75% when it was applied at the concentration of 50mM, other salts such as potassium monohydrogen phosphate significantly increased disease severity and they also caused significant reduction in seed and straw yield. The results of the present study indicated that concentration plays a decisive role in determining the effectiveness of phosphate salts in suppressing flax powdery mildew. Therefore, phosphate salts will be reevaluated in a future study by using lower concentrations as these lower concentrations could be effective in controlling the disease .