It looks very much to me as if your tree is in the process of losing a branch. The stem where the foliage is dying, looks very dark, almost black. This is usually a sign with mimosa that this particular branch is failing. Keep an eye on the foliage further up the stem. If this too falls, i would advise removing the whole branch.
There are any number of reasons for this but this years weather has not been a Mimosa's friend. Its been too wet and not warm enough. Check your pot is adequately drained and that the compost dries out between watering.
Protecting indoors is sensible as they are not recognised as 100% hardy in this country, but reduce your watering whilst you over winter. It is difficult to see if the cause of the damage is viral or insect via photo but both are possible. Keeping a healthy one outdoors is always a matter of luck as opposed to science. You might manage it in central London or Cornwall where there are always a few degrees above average temperatures, anywhere else in the UK is a gamble.
Acacia can suffer from "red spider mite" or a virus may have got in through a pruning wound. You would need to have a piece looked at for proper diagnosis.
A light spray with general purpose fungacide or insecticide shouldnt hurt it but please do so with the plant outdoors. You dont want to end up gassing yourself.
Hope this helps.
Replied: Friday 5th of October 2012 12:20 PM
