Q: I hope you can help. The leaves on my orange and lemon trees are turning black. The black material does not wash off easily, but it wipes off if I scrub the leaf with water and a paper towel, even though it comes back in a few weeks. The leaves seem to be producing the black powder. I thought they were trying to protect themselves from too much sunlight, but it grows on shady leaves, too. Why would the leaf do this? Won’t it block the sunlight and prevent photosynthesis?
A: You are right that the black powder will block too much sunlight. But it is not being produced by the plant. It is a fungus called sooty mold. It grows on any surface that is being coated with sugar water. The sugar water is coming from the sap of your trees or a tree growing above them. The sap is getting out of the tree as the droppings of insects.
