Test and treat your plants before bringing them in
A friend told me the leaves of her Ficus benjamina are covered with black soot. I suspected that the plant had been infected with a soft-scale insect that exudes a honeydew substance that breeds sooty mold. However, upon examining the plant, I saw it was severely infested with spider mites.
If you moved your houseplants outside for summer, there is a good possibility that they are infested with spider mites.