One of my garden phlox plants loses it lower leaves ---they turn yellow , curl up, and fall off-- year after year. A plant pathologist was unable to identify any insect or fungus affecting the plant. I have divided it and transplanted a portion to another pot where it did the same thing. I have tried to limit watering, tried it in different areas of the garden (it's potted) with different degrees of sun exposure. I have sprayed it with insecticidal soap and insecticides/fungicides without apparent effect. The plant still blooms but on stems that are 3/4 leaf-free. Another phlox plant, under similar conditions, does fine. Any thoughts before I rip it out?

Deadheading the spent blossoms will help keep the leaves in production longer as well as extend the flowering time. Post a photo so we can get a look at it.

I have this problem with my garden phlox. I agree it's not a disease, I think that some varieties just do it more than others. I live with mine this way. I remove the spent leaves. I tolerate this because of its long bloom time and it's draw for butterflies.