Antifungal activity of monopotassium phosphate (KH2PO4) and dipotassium phosphate (K2HPO4) was evaluated as possible alternatives to synthetic fungicides for the control of 10 phytopathogenic fungi in this study. The percentage of mycelial growth inhibition ranged from 0 to 52.2% and 0 to 100% for KH2PO4 and K2HPO4, respectively. The percentage of inhibition in spore germination and germ tube elongation ranged from 0 to 100% for both compounds. K2HPO4 at 2% completely inhibited the mycelial growth of Sclerotinia sclerotiorum (Lib.) de Bary in soil tests. In pot experiments conducted under controlled conditions, pre-inoculation (protective) treatments of KH2PO4 and K2HPO4 in all the concentrations tested except 0.25% were effective on the control of rust fungi (Puccinia triticina Eriks., and Uromyces appendiculatus (Pers.:Pers.) Unger). In these tests, foliar applications of KH2PO4 at the highest concentration (1%) tested provided 66.1-72.0% and 70.3-80.2% disease control against P. triticina and U. appendiculatus, respectively. At the same concentration, foliar applications of K2HPO4 provided 64.0-70.8% and 67.1-73.6% disease control against P. triticina and U. appendiculatus, respectively. However, post-inoculation (curative) treatments of both potassium phosphate compounds were ineffective on the control of P. triticina and U appendiculatus. These data suggest that KH2PO4 and K2HPO4 have no post-inoculation activity against both rust fungi and they should be used as a protective fungicide. The results of this study showed that both potassium phosphate compounds tested could become natural alternatives to synthetic fungicides for control of phytopathogenic fungi.