Question
Hi Tom,

I'm both a fan and owner of serveal Endless Summer Hydragneas.  
One of my bushes is going through hard times and I'm hoping you might be able to offer some feedback or a remedy.  Though they are producing flowers, a few of the stalks have suddently taken a turn. While the rest of the hydragnea looks healthy, those few stalks have started to wilt and die.  I don't see any bugs on the surface of the leaves or on the stalks. The plant is well watered.  I'm clueless.  Any feedback would be most appreciated.
Thank you.

Answer
Hi Caroline,
Thanx for your question.  It sounds like Armillaria root rot.  Check out the pictures at Alabama Cooperative Extension at the website shown below and let me know if that's not it.

http://www.aces.edu/pubs/docs/A/ANR-1212/ANR-1212.pdf

This website discusses Armillaria root rot and how to control it.  If only a few roots have been affected, you can remove some of the soil and expose the roots to air.  Don't put the diseased soil back on.  Dispose of it and get some clean soil.

http://ipm.illinois.edu/diseases/series600/rpd602/index.html

I hope this helps.
Tom