Keep standing water off both your lawn and perennial plants.
2. Clip off and pick up off the ground any diseased leaves. Discard these entirely, do not compost them as composting will spread the disease.
3. It helps if you keep the problem plants as healthy as possible. You can do this by dosing with freshly made compost tea (like Moo Poo Tea), by keeping fresh compost around the base of the plant, and by removing old compost and mulch that has been exposed to the fungus from beneath the plant.
4. Treat with an organic anti-fungal treatment.