| Controversy exists as to whether the primary mode of action of the fungicide fosetyl-aluminum (fosetyl-Al) in disease control is on the plant or the pathogen. Previously, researchers did not consider fosetyl-Al or its degradation product, phosphorous acid (H(,3)PO(,3)), to have antifungal properties. Data indicated that the mode of action of fosetyl-Al was through stimulation of host defense responses. In this study, several lines of evidence are presented indicating that fosetyl-Al and more specifically H(,3)PO(,3) have a direct mode of action against the fungus. Only species of Phytophthora were strongly inhibited by H(,3)PO(,3), although four Pythium species were moderately sensitive. Using chemical mutagenesis, isolates of P. capsici and P. parasitica var. nicotianae tolerant to fosetyl-Al and H(,3)PO(,3) were obtained. In vitro tolerance to fosetyl-Al and H(,3)PO(,3) was expressed in vivo. This provides the strongest evidence that the primary mode of action is directly against the pathogen. Fosetyl-Al is rapidly converted to H(,3)PO(,3) in tomato leaflets and in tobacco stems. With P. citricola and P. cinnamomi increasing phosphate levels in culture media greatly reduced the antifungal activity of ethyl phosphonate but had little effect on efficacy of H(,3)PO(,3). With P. capsici, increasing the phosphate concentration from 5 to 10 mM increased the EC(,50) value from 79 to 138 ug/ml H(,3)PO(,3). Comparing the concentrations of phosphate (2-9 mM) and H(,3)PO(,3) in plant tissue following treatment with fosetyl-Al or H(,3)PO(,3) to the in vitro efficacy of H(,3)PO(,3) in the presence of 5-45 mM phosphate suggests that H(,3)PO(,3) controls disease by a direct inhibition of the pathogen. Quantification of H(,3)PO(,3) in plant and fungal tissue by scintillation counting of tritiated H(,3)PO(,3) and high performance ion chromatography (HPIC) gave statistically equal results, indicating that both techniques are valid. After eight months, 95% of the tritium label of ('3)H -H(,3)PO(,3) was in the H(,3)PO(,3)-peak fraction as determined by HPIC.;Glyphosate had no effect on disease control while aminooxyacetic acid(AOA) only reduced the efficacy of fosetyl-Al and H(,3)PO(,3) at suboptimal fungicide concentrations. AOA (0.25 mM) inhibited uptake of H(,3)PO(,3) in P. capsici by 83%, which could explain the reduced efficacy of H(,3)PO(,3) in the presence of AOA. |
