Please look at the attached photos. Do you think this is powdery mildew? It is affecting a well established patch of creeping phlox and is killing a year old caryopteris. Other plants in the bed are fine. What are my options? Do I need to remove the plants and treat the soul? Remove all the soil?
Montgomery County Maryland
While powdery mildew can be common this time of year, we do not see symptoms on the Phlox subulata or Caryopteris, though close-up images with more detail might be needed. Additionally, these two plants rarely, if ever, contract this disease. Caryopteris foliage is normally quite white/silver due to its fine hairy coating, and moss phlox foliage can whiten when it dies.
Both of these species are quite sensitive to poor drainage and excessive soil wetness, however. If the soil here is compacted or otherwise watered often or retains too much water, that can kill roots either through direct insufficient oxygen or though infection by opportunistic pathogens. Dying roots result in dying top growth because there is no longer an adequate supply of moisture and nutrients to keep it all alive. While the process isn't reversible with fungicides, if the soil is allowed to dry out more the plants may be able to re-grow any lost roots and recover, albeit gradually. Similarly, make sure any mulch used does not contact the base of the main stems to promote good air circulation.
Root loss can be masked by mild weather, only becoming evident and more dramatically symptomatic once plant stress ramps-up in summer due to heat and intermittent rainfall. This past July was one of the hottest on record, for instance, and this would have severely stressed plants with roots that may have started ailing months earlier from different weather patterns or conditions.
Trim out any dead stems or stem tips on the Phlox. For the Caryopteris, if you are willing to wait to see if it improves, trim next spring instead as they normally can experience stem dieback in winter and would need a clean-up trimming in spring anyway. They can be treated like a perennial if you wish, removing just about all of the top growth; it won't impact flowering. If the Phlox still looks a bit tired in spring, you can enjoy the flowers and then also give it a severe cutting-back to stimulate more regrowth.
You do not need to treat or remove soil*, but if drainage needs improving and the plants were being replaced, you can take the opportunity to mix-in some organic matter (like compost). Use just enough to give the soil a crumblier texture (rather than chunky clay); too much of a good thing is possible with soil amendments and creates its own problems.
* The exception for removing soil would be if the plants are succumbing to Southern Blight, a common pathogen (which probably would have made itself evident earlier in the year than now) you can look for on the soil surface at the base of these plants: https://extension.umd.edu/hgic/topics/southern-blight
Thanks. The phlox is very dense and perhaps that is part of the problem. Maybe I'll thin it out and give it a hair cut and see how it does in the spring.
The caryopteris leaves are withering so perhaps I'll just dig it up, cut it back and move it somewhere else for the winter and see how it does. The other one I have that's about 8 feet away is thriving.
Thanks for your quick response.