Mine did that and when I looked closely, there were brown spots under the leaves, some kind of scale I guess. I took a cloth and dipped it in some soapy dishwater and carefully wiped each leaf clean. This made my lemon tree very happy and it stopped crying and made new leaves and bloomed. Of course, my tree is small, so it wasn't so very hard to perform that task. I suppose it is the tree's defensive response to its attackers. (You would cry too, if it happened to you!)
Mine did that too, I thought it was sap - very sticky. Was thankful it was on ceramic tile - if it had been carpet - OH DEAR!
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