Question
Hi Jennifer,

I've got an Indian Bean Tree in my garden for the last 5 years or so. It has been doing well until about 6 weeks ago about half of the branches started to wilt.  Today I pruned the dead branches off and discovered that in some of them the core is rotten.  It's black in colour and very soft.  This black core is about 3-4mm in diameter.  What is the likely cause of this?  Is there anything I can do about it?  Will the disease spread to other trees nearby?  The most valuable one is an acer about 15 ft away.

Your help will be greatly appreciated.

Kind regards,
Terence

Answer
Hello Terence,

The most common disease that I find on catalpa is Verticillium wilt.  It will cause a wilt of the branches and in most cases the tree will die.  You have done a great job by removing the dead wood.  I would suggest that if any additional branches are wilting, that they should be removed immediately.  Hopefully, you can prevent the pathogen from reaching the main trunk where it could quickly spread to other branches.

In trees with Verticillium wilt, there is often streaking of the sapwood just under the bark.  I will paste links below t0 several websites that provide much more information about this disease.  On one of these webpages, there is an image of the streaking.  The blackening you observed in the center of the wood may actually be due to another wood decay pathogen that has entered the tree.  

If your problem is Verticillium wilt, unfortunately maple (Acer) trees are highly susceptible.  The pathogen survives in the soil, so you should avoid disturbing the soil around the affected tree.  In time, the propagules of the pathogen will break down, but it could take many years.  You would want to avoid planting a susceptible tree in the same location as the catalpa.

Here are a few links that will provide you with more information about this disease.
http://nac.unl.edu/diseasetrees/chap44.pdf
http://pnwhandbooks.org/plantdisease/host-disease-descriptions/catalpa-verticill
http://aces.nmsu.edu/pubs/_circulars/circ554.html

Good luck with your trees.
--Jen