PlantPalA sticky black film on leaves
If the dark coating wipes away, look for honeydew from insects above it.
What to check first
Wipe a small area with a damp cloth. If the coating lifts and the surface is sticky, look under leaves and along stems above it for aphids, mealybugs, soft scale or whiteflies.
What to do now
Deal with the insects you find first. Wipe washable leaves gently with water and a soft cloth. A black spot that is part of the tissue needs a different check.
What to watch next
If fresh stickiness or black film returns, look higher up and check nearby plants. New coating often means insects are still feeding.