I doubt that there is any disease involved. It is a very common characteristic of a number of taller growing perennials that the lower, basal foliage will dry out and turn brown the later it gets in the season. Asters, garden phlox and mums will all display this trait, among others.

Cutting the plant back several times during the growing season (but no later than the beginning of August) will help to keep the plant shorter and denser with less lower die-off as well as promoting more flower buds. And infrequent but deep watering at the base of the plant during any hot, dry periods in summer should help as well.

Thanks for your response. I'm familiar with perennials drying out and that's not what happened here. This was a cushion mum and one that I had pinched back until July 1. It was definitely either disease or maybe someone said nematodes. The leaves were not just brown but very brittle. It looked like a dead plant with flowers on the tips of each branch. I wish I had saved a picture.

Cushion mums are bred to be pot plants and are forced into bloom in greenhouses. They tend to be quite root bound and often don't thrive when planted in the garden.

At the public garden where I volunteer they are treated like annuals. Once the bloom fades they are pulled from the ground and discarded. Of course, their space is needed for winter color, and the gardeners really don't have time to do all the pinching anyway.