I wish I knew! Mine have the same problem.

It's a common problem with BESs. There have been a number of other threads on this issue. Since BESs tend to seed around everywhere after a few seasons in the garden, it seems to me that the best thing to do is to remove any old ones that are having problems & just keep the newer seedlings, especially those that are at a distance from the mother plants. The new ones in my garden have generally been very clean, at least for the next few years. Toss any seedlings that are not clean. I usually don't let any of them stay around for more than a few years & this seems to keep all the plants clean & with a much longer bloomtime. I have been really surprised by the length & quality of bloom on my BES seedlings over my older diseased plants - must have forgotten what it was like to have healthy BESs!

IME other kinds of plants don't seem to be bothered by whatever fungus, mildew, or mold this is that causes the blackening & eventually reduces bloom of the BESs, so I've never bothered to clean the soil, just planted different perennials in their old space. Others here may have had a different experience however.

If you are worried about not having enough seedlings if you remove all the mother plants, keep them around (ugly as they are) until the seedheads are ripe & shake the seeds around where you want them to come up next year. I never had heavy seeding until the year I removed the 6 foot long patch of original plants on a pretty day in late fall - the seeds must have accidentally spread every which way - because I had new plants many yards away in at least 6 or 8 spots the next summer without even trying to seed them. LOL.

Great advice! Thanks. I will remove the "mother plants", leave any healthy seedlings and give the seed heads a good shake! Better luck next year, I hope. Liz

I've attached an informative article below which suggests removing the infected leaves and increasing air circulation would help (depending on what it is you actually have). My blooms aren't affected at all - just the foliage, so perhaps you have something else going on. I plan to cut the entire plants down to the ground in the fall instead of just removing the flower stalks as is normally done since most of the leaves are affected. I constantly thin them anyway, but I'll see what happens next year with this method.

Here is a link that might be useful: Diseases of Rudbeckia