Answer: Sudden yellowing sounds like either a heat or a water stress problem. As a rule mimosa trees are relatively disease free although insects such as aphids can feed on the leaves, causing them to yellow and drop. Without inspecting the tree it's difficult for me to make an accurate diagnosis. Make sure your tree is getting adequate water and see if you can find aphids or other insect pest feeding on the remaining leaves. If so, collect a few of them to take into your local cooperative extension office or well staffed nursery for positive identification. Once you know what you are dealing with you can use the proper control measures. Best wishes with your tree.
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