Foliar applied phosphite-based fungicides are an effective tool for managing pink rot, caused by the oomycete Phytophthora erythroseptica. Post-harvest applications have also been highly effective. Data on the control of Pythium leak have been difficult to obtain due to a lack of disease pressure and a lack of reliable inoculation methods. The logistics of the post-harvest application are difficult and some potato producers would like to use the foliar program if that would preclude the use of post-harvest applications. A field trial was conducted in 2019 on eight russet-type potato varieties (Burbank, Ranger, Norkotah, Umatilla, Alturas, Clearwater, Teton, and Dakota) to determine the effect of in-season and post-harvest phosphite fungicide application on pink rot and leak. Despite inoculation, pink rot pressure was low with little disease occurring in the field. Post-harvest inoculation with P. erythroseptica showed that the in-season application of phosphites was not effective in reducing infection at harvest for Ranger and Alturas but was effective in varying degrees on Burbank, Norkotah, Umatilla, Teton, Clearwater, and Dakota. Post-harvest phosphite application provided almost complete control of pink rot at harvest for all varieties. In-season phosphite fungicides were not effective in reducing Pythium leak as determined by disease incidence at harvest. Post-harvest treatment with phosphite was not conducted against leak based on the lack of efficacy shown with previous research. Research on different fungicides and methods of application for leak still needs to be determined. If pink rot develops in a potato field even with the use of in-season phosphite fungicides, a post-harvest application is advised in order to protect healthy tubers from becoming infected during the harvest and handling operations.