It's a little hard to tell from your picture, but it looks like Miscanthus Blight, caused by the fungus Leptosphaeria sp. and its imperfect form Stagonospora spp..

Treatment:

Prune off and destroy affected leaves as soon as the spots appear.

Spray the remaining foliage with an organic copper-based fungicide. Follow label directions carefully.

In late fall, rake up and dispose of any withered leaves and other plant debris.