The reason we're advised to rotate between different fungicide groups is to reduce the chances of the fungus becoming resistant to any particular fungicide. Alternating with another fungicide with a different mode of action makes it harder for that to happen because the fungus isn't always being exposed to the same fungicide and therefore potentially being genetically selected for any ability to resist its effects.
The strobilurin fungicides (Group 11) like the one you're using are considered high risk for fungicide resistance happening, and if it happens with one strobilurin then the fungus will be resistant to ALL other strobilurins and those fungicides will also no longer work, or work as well. From what I understand though, while resistance to strobilurins has developed rapidly with some fungal pathogens, it hasn't so far with rusts. Let's hope it stays that way because they're so far the most effective for daylily rust.
