Sooty mold

Sooty mold is a fungus that grows on honeydew, a sticky, sugary substance excreted by piercing-sucking insects such as aphids or scale insects. The fungus does not directly parasitize the plant but forms a black surface coating that blocks light. If untreated, this layer prevents photosynthesis, weakening the plant and causing yellowing and premature leaf drop.

Signs to check

• A black, powdery, soot-like coating that rubs off with a damp cloth.
• A sticky substance on leaves (honeydew), often visible before the black coating appears.
• Frequent occurrence beneath and around areas infested with aphids, scale insects, or whiteflies.
• Foliage beneath the black coating is often still green and intact, although weakened.

What you may see

• Black coating (sooty mold) · A dry, powdery black coating resembling soot forms on the surface of leaves and stems.
• Black, sooty deposit · A black, powdery, soot-like layer develops on the surface of stems and sometimes leaves.

What to do now

Clean off honeydew and address its cause

1. Prepare a cleaning solution. Mix 15 mL (about 1 measuring tablespoon) of liquid black soap into 1 liter of lukewarm water (about 1.06 US qt). Lukewarm water helps the soap dissolve completely.
2. Spray the solution generously over all sticky areas. Do not forget the undersides of leaves, a favorite refuge for pests.
3. Leave it on for 5 to 10 minutes. The soap will soften the honeydew and sooty mold, making them easier to clean off.
4. Gently wipe each leaf with a soft sponge or microfiber cloth. The aim is to physically remove honeydew, sooty mold, and some of the insects.
5. Rinse the plant with clean water to remove soap residue. You can place it in the shower or use a spray bottle filled with water.
6. Identify and treat the insect responsible. Once the plant is clean, apply a targeted treatment, such as neem oil, alcohol, or a suitable insecticide, to eliminate the source of the problem and prevent honeydew from returning.

Prevention

• Inspect the undersides of leaves and leaf axils every week for pests.
• Wipe dust off leaves with a damp cloth once a month.
• Quarantine every new plant for 2 to 3 weeks before placing it near the others.
• Ensure good air circulation around your plants to make the environment less welcoming to pests.