Answer: It's very possible that you are setting the stage for fungal diseases by watering overhead rather than watering just the soil and keeping moisture off the leaves of your plants. Phlox and other tall plants are subject to a number of what I call "high humidity" diseases?powdery and downy mildew, leaf spots and stem blights.
You can control these diseases by cutting back the frosted stems this fall and removing them from the garden. As new growth emerges next spring, keep a wary eye out for any development of the mildew and dust with sulfur or other appropriate fungicide. Sometimes increasing the spacing between the plants helps cut down on the spread of the disease, along with a reduction in overhead watering.
Good luck with your garden!
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