Question
I have 2 beautiful mimosa trees on my property. They are about 100 ft. apart. This year the one at the back of the property is just a stick. Up to this spring it was just fine, I hadn't noticed any kind of change in the tree. it was fine last year. The one at the front of the property is just as beautiful as ever. I haven't seen any bugs or any sap on the tree that appears to be dead. They both are about 30 ft. tall and about 20 years old. Do you have any ideas ?.
   Thanks,
      Bob

Answer
The most common problem that causes rapid death of mimosa is mimosa wilt (aka Fusarium wilt of mimosa). You can visit this website and learn more about this disease:
http://pubs.ext.vt.edu/2811/2811-1020/2811-1020.html

In my experience, mimosa wilt is very rapid and seemingly happens over night.  Once the pathogen reaches the vascular (water conducting) tissue, it is a very rapid decline and death.

I would remove some of the branches from the dead tree and look for the vascular streaking that is discussed in the link.  If you see this browning, it will strongly suggest that this is the reason for the death.  Be sure to clean your pruning tools because you could risk spreading it to the healthy tree.  Dipping tools in a 10% bleach solution is recommended.

As for the other tree (still healthy), you are justified in your concern.  I would remove the dead tree since it may be harboring spores of the pathogen.  Additionally, the spores are in your soil.  They will be most concentrated around the dead tree, but you will want to use care not to move soil from the site.  This will mean cleaning shovels and other equipment.  When mowing or weed eating your yard, I would work from the healthy tree toward the dead tree in case any soil is carried on the equipment.  Runoff water can also move the soil.

There are a few mimosas that are resistant to wilt.  However, the varieties are not that common in the nursery trade and resistance is not immunity.  It means that they are less susceptible, but can still be attacked especially if there are other pathogens present or a stressful environment (ie. drought).  It is not known if your other mimosa is resistant.  But, if you decide to plant a replacement tree I would select a different type of tree or install one with resistance to mimosa wilt.

Good luck!