Yes, I am also wondering if one gets more direct sunlight than the other.

debra

They get very similar sunlight. There is usually a tree canopy near the dying one, but that tree has been stressed and this year put out exceedingly few leaves. Thankfully we had a wet summer so the hostas, etc., under it are faring well. But, the dying one is positioned such that it really didn’t get much shade from the tree, and the healthy one is certainly in full sun. So, if anything, the unhealthy one would now be in a more similar sun situation to the heathy one.

It’s always looked great every year. I would think it is more related somehow to how wet we’ve been than the sun. I do think it’s very possible the dying one has more clay in its soil than the healthy one. I’ve spent siginificant time ammending my soil, but I’ve noticed stubborn clay still persists near the dying one. Does that sound more reasonalbe—root rot? If so, how can I tell?

It's quite tricky to diagnose what one of the many fungal problems might be to blame but at the very least, I would suggest carefully lifting the affected plant and examining the roots and crown. Some fungal problems such as crown rot or honey fungus can be diagnosed by the appearance of sclerotina while phytoptheras are microscopic and not easily seen., although the feeder roots will be very diminished and there could be blackening or general decay. However, the continuing presence of pathogens in the soil could be an ongoing problem which can be difficult to cure. The easiest method (imo) is solarisation under black plastic but at this time of year, it isn't really an option available. I think hydrogen peroxide has been used as a root drench but I have never tried it. The most problematic issue is containment: pathogens can spread to uninfected plants and can persist in the soil for a long time. You need to lift the affected plants very carefully, disposing of them and any soil around the roots. At the very least, I would not replant with dianthus in that area. I am not any sort of expert in plant pathogens but I do always worry about these sorts of fungal problems.

Excellent advice, rosaprimula. I’ve got signs of other potential problems that could be from moisture on other plants, so this bed may be more of a chore in the future. I’ll probably create a second post with other photos for more insight.

Is there any chance that a gardener accidentally sprayed glyphosate into this area? It would also help to explain why the tree is also not doing well.

I think that Deanna is the only gardener.

@NHBabs z4b-5a NH Still have the same question. The fact that the Dianthus went downhill together with a nearby tree seems like an important clue to a bigger story.