I'm trying to identify what is wrong with my lemon tree.
Previously it had aphids and I treated that and they're gone, but now it has some black stuff on the underside of the leafs and is causing them to fall off.
Here's some photos:
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It is hard to tell from the photos, but perhaps a fungus called black sooty mold
What Causes Black Sooty Mold?
Black sooty mold is actually an airborne fungi spore that latches on to the sweet sticky honeydew secreted by leaf and bark feeding insects.
On Crepe Myrtles and most other plants the culprit here is either sap sucking Aphids or Bark sucking Scale.
This is how it looks like on a lemon tree.