Is it the bottom parts of the flowers, or the bottom parts of the plant in general? A photo would be useful.

The lower leaves of Michaelmas daisies often turn brown as they start to bloom. Many gardening books recommend planting them behind lower plants to hide the brown.

yes this is normal if you mean the leaves lover down the stem. I hide them with shorter plants and if I cant I run my hand down the stem and pull them off.