My 86-year-old mother has a 30-plus-year-old dwarf tangerine tree in San Lorenzo, CA that started getting a black residue under most of the leaves. It's very hard to rub off and now there's starting to also get a white residue with the black that's very hard to rub off too. The tree seems healthy, leaves are green on top and the fruit is very tasty with no leave drop. What can I do to correct this, or will this work itself out in time and not need anything?
Black Residue Under Some Leaves
Certified GKH Gardening Expert
It sounds like the tree has sooty mold, which is actually an indicator of a pest infestation. Many times the pests that cause sooty mold are too small to see, but they can be damaging. I would recommend treating the tree with neem oil. This will treat both the pests and the sooty mold. Here is more information that may help: