I often receive queries about indoor bonsai tree pests. Sooty mold is an important one. This is one of those emails:
“I have a well established Fukien Tea tree but am battling with what appears to be black dirt on the leaves and branches. It is not dirt, as it continues to reappear even after spraying with a weak solution of soapy water. Is this a fungus and how do I treat it? ”
An especially bad case of is shown here. This "black dirt" is the growth of a black-colored fungus. It grows on the honeydew secretions caused by various sucking insects. The fungus itself will not damage your bonsai tree. (Unless, it is so extreme, it's blocking photosynthesis, as shown here.)
If you have this black fungus, it is not the real problem, but the result of another problem. Both need to be resolved. Usually and most likely, the problem is sucking insects.
Sucking insects extract what they need from the plant and deposit the rest as a sticky, sweet liquid onto the plant. Many
times it is not noticeable. These gooey secretions (often called honeydew) create a breeding ground for various fungi.
Black mold occurs when airborne spores land on the honeydew. At the first sight of sooty mold, look for the source of the problem.
This black fungus spreads quickly! Check your bonsai trees for aphids, scale and mealybugs frequently. Some of these plant pests are difficult to see. Be sure to look under leaves and in the crevices of the trunk.
If you see a black powdery substance, give your trees a spray with a strong stream of water (outdoors with a hose.) This may not totally remove the offending pest(s), but it is a good start.
If this does not work or is not practical, try horticultural oil, insecticidal soap or a systemic type pest control.
Remember: treat the source of the problem ... a fungicide is not the answer! Carefully follow the package directions for any treatment (natural or chemical.)
Once honeydew producing insects are eliminated, the fungus will die back. In more severe cases, you may still need to remove badly covered leaves and clean the trunk with soapy water and an old soft toothbrush!
If you are growing indoors, read the
indoor bonsai tree page.
Or check out another pest - Snails!