What sooty mould, sap-sucking insect and ants have in common? Well, lots of it. They are actually a part of a feeding chain. When you see this ugly black coat of sooty mould it is a sign that your plant is infected by sap sucking insects as well. The sooty mould doesn't feed on the sap but feeds on the sweet product of sap-feeding insects that usually occupy the underside of leaves.
The sooty mould looks really ugly but also blocks the sunlight and makes the photosynthesis difficult. New foliage wasn't affected so much, but there was lots of mould on the old leaves.
I spent one hour and a half cleaning my lemon tree! Firstly, I used an old toothbrush to remove the shield aphids. They settled along the main vein of the leaves and the stems as well.
When using chemicals to kill them one have to use an oil-based insecticide which will allow the chemical to come under the insect's shield.
The aphids produce sweet and sticky secret that serves the sooty moulds as food. As the secret drops on the leaf surface, sooty moulds start to appear.
After the brushing, I gave my lemon tree a good shower to wash away the remaining aphids and moulds.
They too feed on the honeydew produced by aphids. They don't make any damage on the plant itself but may attack ladybugs and other beneficial insects that feed on aphids.