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As someone learning about lawn fungus also, I've learned that the fungicides don't actually "repair" the grass, they just stop the disease from getting worse, and eventually the grass recovers on its own.

This year I used a combo of disease ex and Bayer as recommended by lawniac.

I applied on May 9th and haven't seen any further problems. I'll wait it out hopefully until it starts to get really hot, humid, and wet and then maybe hit it again as a preventative.

I believe azoxystrobin (DiseaseEx) is not effective on rust, but propiconazole is. If you've had fungus issues already I'd suggest sticking with a fungicide program through summer, it will attack the grass again as soon as you stop.