I have one cultivar that every year ends up with brittle leaves with a silver cast. It is Sundown. The plant comes up normal. And flowering is not affected. After many years it never seems to have spread to any other plants.

Does anyone know what causes this?

t looks like powdery mildew to me. Some plants are very susceptible to it while other plants growing next to them are without any symptoms. It can be controlled with a fungicide but usually they do better if they have better air circulation around them. Many garden phlox and monardas are afflicted with powdery mildew. Plants with tougher leaves like echinaceas, do not get it very often. Lilacs too are very prone to it.

Big Sky Echinacea have tended to have more problems for me then other series.