Miscanthus can be affected by a number of different leaf spotting fungi. One of the more noticeable is Miscanthus Blight, caused by the fungus Stagonospora. You will see purplish or rust colored spots and streaks on the leaves, especially the white sections of the variegated leaves. From a distance, with its rusty appearance, it is often mistaken for a rust disease. Treat the plant by cutting the diseased foliage out and completely remove it from the garden. Do not put it in a compost pile as that will spread the fungus through your garden. It’s unlikely this disease will affect anything in the vegetable garden, but no sense keeping spores anywhere nearby. Do not divide and transplant the grasses in the fall. If this turns out to be an ongoing problem, you don’t want to help spread it to new areas. See how the grass does next year. If we have a dry season you may not see the problem recur.