Question
Hi.  I hope you can help me discover why my beautiful and totally healthy "David" phlox just started to die for no reason.  One day I noticed that a couple of the stems were wilted (just the leaves) so I thought it needed to be watered.  That did not help so I cut those stalks to the ground.  Then every few days more leaves would wilt and I would cut those stalks to the ground and then the whole plant was gone.  I left the root ball in the ground to see if it would come back up but it never did.  And it was my favorite plant!  I love phlox and do battle with mildew a lot but have never had one so healthy just up and die.  Any ideas?  I have many, many perennials and have had no problems with any others doing this.  Thanks for any ideas you might have.  Doreen Thomson

Answer
Hi Doreen,

This may be the result of a root rot infection, usually caused by a bacterium or fungus.  Phlox are extremely susceptible to Cotton Root Rot caused by the fungus _Phymatotrichum omnivorum_.  The symptoms of this disease are exactly as you described: a small number of stems may yellow, wilt, and then die.  By the time these symptoms are present, the fungus is already well established, and there is little chance of the plant regaining vigor.

DISEASE SYMPTOMS - Symptoms are most likely to occur from June through September when soil temperatures reach 28 degrees C (82 degrees F). The first symptoms are slight yellowing or bronzing of the leaves. The upper-most leaves wilt within 24 to 48 hours after bronzing, followed by wilting of the lower leaves within 72 hours. Permanent wilt occurs by the third day, followed by death. The leaves remain firmly attached to the plant. Affected plants die suddenly, often after excellent growth.  

Roots are usually extensively invaded by the fungus by the time wilting occurs. Affected plants can be pulled from the soil with little effort. Root bark is decayed and brownish, and bronze colored wooly strands of the fungus are frequently apparent on the root surface.


This sounds like the progress of your afflicted Phlox plant.  For the time being, I would not try to grow another Phlox in that spot.  If you have options, plant a perennial monocot (such as daylily, iris, etc.) as these plants usually are not susceptible to the fungus.