We have diagnosed it twice from Oregon's Willamette Valley in the last 60 years. Back to Brooke for more info.

I checked with the OSU plant pathologist and he indicates that this disease is present in the Willamette Valley (see previous reply).  Hard to know for certain that this is what is affecting your tree as there are other known problems that affect Mimosa trees (winter dieback, Nectria canker, etc.).

Do you have any photos that you can send in a reply to this message?  That way we can get closer to giving you the best possible diagnosis and some possible management options.  

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Mimosa tree was planted about 5 years ago.  It is one of the dark varieties; leaves coming on green, then darkening to deep purple.  No problems until now.  About ten percent of the tree was pruned off in the last month.  All significant leaf drop has been since then, primarily the last 7-10 days.

Pictures attached showing loss.  Leaves turn yellow-orange then drop.  I could see no evidence of so-called fruiting bodies anywhere on the tree bark.  I suppose they may be extremely small.

Thanks for your help.

Definitely appears to be a stressed tree.  The last few years have seen some relatively extreme weather (cold snaps in winter 2014-2015 and high temps last summer) which can be contributing to the dieback that you're seeing.  The best course of action is to make sure the tree has adequate irrigation this summer and keep eye on it.   Diagnostic testing (collecting samples from the tree and sending to a fee-based diagnostic lab) would be needed to confirm that the problem is indeed Fusarium wilt.  If the tree is diagnosed with Fusarium, there is unfortunately no treatment available.  If the tree dies or you remove it, then best to replant with a different tree species rather than replant another Mimosa into the area.  

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