Several web sites recommend rotating between a contact fungicide and a systemic fungicide to avoid creating a rust resistant rust. I believe they said Heritage was prone to this problem and should not be used more than three times in succession before using a fungicide with a different mode of action like a contact fungicide..

If the rust is present within the leaf rotating to a contact fungicide would not seem to work and the rotation should be to a systemic with a different mode of action.
Does anyone have more information on this.

Also I believe one site said Heritage was not a vertical rising systemic but moved more transversly like from the top of a leaf to the underside.