Question: What is this condition? What is causing it and how do I fix the problem? There seems to be a black material at the stem of each leaf.
Certified GKH Gardening Expert
You may have a few issues on your plants.
The black on the leaves is likely Sooty Mold. This is the residue from pests.
You may also have fungus disease that could be caused by excess moisture or poorly draining soil.
Treat the plants with Neem Oil, Neem Oil works as both an insecticide and fungicide.
Correct the watering if needed.