Question
QUESTION: Hi James, I'm not sure if you can assist with this, but I am guessing my issue has something to do with soil, or what's in it.  I have three wiegala and several clusters of salvia growing in full sun in my backyard. Three years ago I planted rhododendron and azaleas; all of them have died. The salvia flourish to the point of being a bother; I repeatedly cut them back and dig them up, yet they return.  At least I know something can grow there.  This is the second year for my wiegala, and although they flowered in early spring, now they are slowly dying.  Their leaves are turning brown, but not getting crunchy; just falling off leaving long branches with nothing.  The browning is concentrated at the base of each, and then growing outward.  I have used miracle-gro with no results, so I am wondering if this might be indicative of grub worms. I saw some when I planted them, but didn't really give it a second thought as my lawn is seemingly healthy -- I thought grubs were always present.  Can you recommend anything?

ANSWER: Hi Susan, I don't think it's grub damage. Dig out a square foot of soil.  If you find 15 or more grubs in that area, grubs may be the problem.  If it's under 10, I think you have another problem.
Here are some possible causes...
Phytophora root rot.  Is the area moist?  If so, you may have a drainage issue and I would expect your rhodies (azaleas), salvia and your weigela to behave as you described.  Watch your watering.
Check your pH.  It may be out of range for your shrubs.
Look for signs of vole damage:  Dime sized holes or tunnels under your mulch.  These little varmints can eat the roots of a plant quickly.
Don't over-improve your soil when planting.  Use your existing soil and blend some coarse organic material with it to help separate your soil.
Always plant high.
These are the most common causes.  Look there first.  
Jim

---------- FOLLOW-UP ----------

QUESTION: Thank you so much for your response.  I gather my incessant watering to try to salvage them has only been accelerating the problem.  I have a weigala and some forsythia growing beautifully elsewhere in my yard and was trying to reproduce that in this other space.  I have not seen any voles this year, although I have in the past.  I suspect the issue is the first you suggested, root rot, after a quick google search.  It appears there is nothing I can do to save or reverse the affected plants, is that correct?

Answer
Usually an infected plant is not worth the cost of saving it, unless it has some sentimental value.  Agri-fos by Monterray will cure the problem, but you still need to move the plant or fix the water issue. Consider plants that like the water...Virginia sweetspire, buttonbush, summersweet, althea.  You'll have better luck. Jim