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?